Patent Application
20130002157
The present invention relates to light emitting devices and, more particularly, to semiconductor light emitting devices that include multiple different types of light emitting devices.
A wide variety of light emitting devices are known in the art including, for example, incandescent light bulbs, fluorescent lights and semiconductor light emitting devices such as light emitting diodes (“LEDs”). LEDs have the potential to exhibit very high efficiencies relative to conventional incandescent or fluorescent lights. However, significant challenges remain in providing LED lamps that simultaneously achieve high efficiencies, high luminous flux, good color reproduction and acceptable color stability.
LEDs generally include a series of semiconductor layers that may be epitaxially grown on a substrate such as, for example, a sapphire, silicon, silicon carbide, gallium nitride or gallium arsenide substrate. One or more semiconductor p-n junctions are formed in these epitaxial layers. When a sufficient voltage is applied across the p-n junction, electrons in the n-type semiconductor layers and holes in the p-type semiconductor layers flow toward the p-n junction. As the electrons and holes flow toward each other, some of the electrons will “collide” with corresponding holes and recombine. Each time this occurs, a photon of light is emitted, which is how LEDs generate light. The wavelength distribution of the light generated by an LED generally depends on the semiconductor materials used and the structure of the thin epitaxial layers that make up the “active region” of the device (i.e., the area where the light is generated).
Most LEDs are nearly monochromatic light sources that appear to emit light having a single color. Thus, the spectral power distribution of the light emitted by most LEDs is tightly centered about a “peak” wavelength, which is the single wavelength where the spectral power distribution or “emission spectrum” of the LED reaches its maximum as detected by a photo-detector. The “width” of the spectral power distribution of most LEDs is between about 10 nm and 30 nm, where the width is measured at half the maximum illumination on each side of the emission spectrum (this width is referred to as the full-width-half-maximum or “FWHM” width). LEDs are often identified by their “peak” wavelength or, alternatively, by their “dominant” wavelength. The dominant wavelength of an LED is the wavelength of monochromatic light that has the same apparent color as the light emitted by the LED as perceived by the human eye. Because the human eye does not perceive all wavelengths equally (it perceives yellow and green better than red and blue), and because the light emitted by most LEDs is actually a range of wavelengths, the color perceived (i.e., the dominant wavelength) may differ from the peak wavelength.
In order to use LEDs to generate white light, LED lamps have been provided that include several LEDs that each emit a light of a different color. The different colors combine to produce a desired intensity and/or color of white light. For example, by simultaneously energizing red, green and blue LEDs, the resulting combined light may appear white, or nearly white, depending on, for example, the relative intensities, peak wavelengths and spectral power distributions of the source red, green and blue LEDs.
White light may also be produced by partially or fully surrounding a blue, purple or ultraviolet LED with one or more luminescent materials such as phosphors that convert some of the light emitted by the LED to light of one or more other colors. The combination of the light emitted by the LED that is not converted by the luminescent material(s) (if any) and the light of other colors that are emitted by the luminescent material(s) may produce a white or near-white light.
As one example, a white LED lamp may be formed by coating a gallium nitride-based blue LED with a yellow luminescent material such as a cerium-doped yttrium aluminum garnet phosphor (which has the chemical formula Y3Al5O12:Ce, and is commonly referred to as YAG:Ce). The blue LED produces an emission with a peak wavelength of, for example, about 460 nm. Some of blue light emitted by the LED passes between and/or through the YAG:Ce phosphor particles without being down-converted, while other of the blue light emitted by the LED is absorbed by the YAG:Ce phosphor, which becomes excited and emits yellow fluorescence with a peak wavelength of about 550 nm (i.e., the blue light is down-converted to yellow light). A viewer will perceive the combination of blue light and yellow light that is emitted by the coated LED as white light. This light is typically perceived as being cool white in color, as it primarily includes light on the lower half (shorter wavelength side) of the visible emission spectrum. To make the emitted white light appear more “warm” and/or exhibit better color rendering properties, red-light emitting luminescent materials such as CaAlSiN3 based phosphor particles may be added to the coating. Alternatively, the cool white emissions from the combination of the blue LED and the YAG:Ce phosphor may be supplemented with a red LED (e.g., comprising AlInGaP, having a dominant wavelength of approximately 619 nm) to provide warmer light.
Phosphors are the luminescent materials that are most widely used to convert a single-color (typically blue or violet) LED into a white LED. Herein, the term “phosphor” may refer to any material that absorbs light at one wavelength and re-emits light at a different wavelength in the visible spectrum, regardless of the delay between absorption and re-emission and regardless of the wavelengths involved. Thus, the term “phosphor” encompasses materials that are sometimes called fluorescent and/or phosphorescent. In general, phosphors may absorb light having first wavelengths and re-emit light having second wavelengths that are different from the first wavelengths. For example, “down-conversion” phosphors may absorb light having shorter wavelengths and re-emit light having longer wavelengths. In addition to phosphors, other luminescent materials include scintillators, day glow tapes, nanophosphors, quantum dots, and inks that glow in the visible spectrum upon illumination with (e.g., ultraviolet) light.
A medium that includes one or more luminescent materials that is positioned to receive light that is emitted by an LED or other semiconductor light emitting device is referred to herein as a “recipient luminophoric medium.” Exemplary recipient luminophoric mediums include layers having luminescent materials that are coated or sprayed directly onto, for example, a semiconductor light emitting device or on surfaces of a lens or other elements of the packaging thereof, and clear encapsulents (e.g., epoxy-based or silicone-based curable resin) that include luminescent materials that are arranged to partially or fully cover a semiconductor light emitting device. A recipient luminophoric medium may include one medium, layer or the like in which one or more luminescent materials are mixed, multiple stacked layers or mediums, each of which may include one or more of the same or different luminescent materials, and/or multiple spaced apart layers or mediums, each of which may include the same or different luminescent materials.
Pursuant to some embodiments of the present invention, semiconductor light emitting devices are provided which include a first string of first light emitting diodes (“LED”) that emit unsaturated light having a color point that is within at least eight MacAdam ellipses from one or more points within a first region on the 1931 CIE Chromaticity Diagram defined by x, y chromaticity coordinates (0.32, 0.40), (0.36, 0.48), (0.43 0.45), (0.36, 0.38), (0.32, 0.40), a second string of second LEDs that emit unsaturated light having color point that is within at least eight MacAdam ellipses from one or more points within a second region on the 1931 CIE Chromaticity Diagram defined by x, y chromaticity coordinates (0.35, 0.48), (0.26, 0.50), (0.13 0.26), (0.15, 0.20), (0.26, 0.28), (0.35, 0.48), and a third light source that emits radiation having a dominant wavelength between 600 and 720 nm. These semiconductor light emitting devices further include an associated drive circuit that is configured to provide a first drive current to the first string of LEDs, a second drive current to the second string of LEDs and a third drive current to the third light source, where at least two of the first, second and third drive currents can be independently controlled.
In some embodiments, at least one of the first LEDs may have a recipient luminophoric medium that includes a first luminophoric material that emits green light in response to light emitted by the first LED and a second luminophoric material that emits yellow light in response to light emitted by the first LED. In other embodiments, at least one of the second LEDs may have a recipient luminophoric medium that includes a first luminophoric material that emits green light in response to light emitted by the second LED and a second luminophoric material that emits yellow light in response to light emitted by the second LED. In still other embodiments, at least one of the second LEDs may have a recipient luminophoric medium that includes a first luminophoric material that emits green light in response to light emitted by the second LED and a second luminophoric material that emits yellow light in response to light emitted by the second LED and at least one of the first LEDs may have a recipient luminophoric medium that includes a third luminophoric material that emits green light in response to light emitted by the first LED and a fourth luminophoric material that emits yellow light in response to light emitted by the first LED.
The semiconductor light emitting device may be designed to emit a warm white light having a correlated color temperature between about 2500K and about 4100K, a CRI Ra value of at least 90 and an r9 value of at least 90, and/or may have a luminous efficiency of at least 130 lumens/watt. The third light source may comprise, for example, at least one organic LED. In some embodiments, the semiconductor light emitting device may also include a fourth LED that emits radiation having a dominant wavelength between 490 and 515 nm. In some embodiments, the first, second and third drive currents can be adjusted so that the light output by the semiconductor light emitting device has a correlated color temperature anywhere between 2500K and 6500K while providing a CRI Ra value of at least 90 and/or a luminous efficiency of at least 130 lumens/watt.
Pursuant to further embodiments of the present invention, light emitting devices are provided that include a first LED string that includes a first LED that has a first recipient luminophoric medium that includes a first luminescent material that emits light having a peak wavelength within the green color range in response to radiation emitted by the first LED, a second LED string that includes a second LED that has a second recipient luminophoric medium that includes a second luminescent material that emits light having a peak wavelength within the yellow color range in response to radiation emitted by the second LED, and a third LED string that includes a third LED that emits light having a peak wavelength within the red or orange color range. These light emitting devices further include a drive circuit that is configured to provide first, second and third drive currents to the respective first, second and third LED strings, where at least two of the first, second and third drive currents are independent of each other.
In some embodiments, the drive circuit is configured to provide first, second and third drive currents that are independent of each other. The third LED may be an organic LED that emits red light. The drive circuit may be configured to set the first, second and third drive currents at values that will drive the respective first, second and third LED strings so that they generate a combined light output having a color point that is within three MacAdam ellipses from a selected color point on the black-body locus.
In some embodiments, the first recipient luminophoric medium may further include a luminophoric material that emits yellow light in response to light emitted by the first LED. In other embodiments, the second recipient luminophoric medium may further include a luminophoric material that emits green light in response to light emitted by the second LED. In still other embodiments, the first recipient luminophoric medium may further include a luminophoric material that emits yellow light in response to light emitted by the first LED and the second recipient luminophoric medium may further include a luminophoric material that emits green light in response to light emitted by the second LED.
In some embodiments, the light emitted by the first luminescent material in response to radiation emitted by the first LED may have a full-width-half-maximum emission bandwidth that extends into the cyan color range. The light emitting device may emit a warm white light having a correlated color temperature between about 2500K and about 4100K, a CRI Ra value of at least 90, an r9 value of at least 90, and a luminous efficiency of at least 130 lumens/watt. The first, second and third drive currents can be adjusted so that the light output by the semiconductor light emitting device has a correlated color temperature that is between 2500K and 6500K while providing a CRI Ra value of at least 90.
Pursuant to further embodiments of the present invention, light emitting devices are provided that include a first LED string that includes a first LED that has a recipient luminophoric medium that includes a first luminescent material that emits light having a peak wavelength within the green color range and a second LED that has a recipient luminophoric medium that includes a second luminescent material that emits light having a peak wavelength within the yellow color range. These light emitting devices further include a second LED string that includes at least one LED that emits light having a peak wavelength within the red color range. A drive circuit is provided that is configured to provide first drive current to the first LED string and a second drive current that is independent of the first drive current to the second LED string.
In some embodiments, the color point of the light emitted by the first LED may be within the green color range, and the color point of the light emitted by the second LED may be within the yellow color range. In other embodiments, the recipient luminophoric medium of the first LED may further include a third luminescent material that emits light having a peak wavelength within the yellow color range, and the recipient luminophoric medium of the second LED may further include a fourth luminescent material that emits light having a peak wavelength within the green color range.
In some embodiments, these light emitting devices may also include a third LED string that includes at least one unsaturated LED that emits light having a peak wavelength within either the green color range or the yellow color range. In some cases, this third string may include at least one LED that emits light having a peak wavelength within the green color range and at least one LED that emits light having a peak wavelength within the yellow color range.
Certain embodiments of the present invention are directed to packaged semiconductor light emitting devices that include multiple “strings” of light emitting devices such as LEDs. Herein, a “string” of light emitting devices refers to a group of at least one light emitting device, such as an LED, that are driven by a common current source. The common current source may be used to drive multiple strings, which strings may be arranged in series, in parallel, or in other configurations.
At least some of the light emitting devices in the multiple strings have associated recipient luminophoric mediums that include one or more luminescent materials. Moreover, some or all of these multiple strings may be driven by independently controllable current sources. For example, in some embodiments, the packaged semiconductor light emitting device may include two independently controllable strings, which may allow the packaged semiconductor light emitting device to be adjusted to emit light having a desired color. In other embodiments, the packaged semiconductor light emitting device may include three or more independently controllable strings. In some embodiments, the device may be adjusted at the factory to emit light of a desired color, while in other embodiments, end users may be provided the ability to select the color of light emitted by the device from a range of different colors.
In some embodiments, the packaged semiconductor light emitting device may include at least blue, green, yellow and red light sources. For example, a device may have three strings of LEDs, where the first string comprises one or more blue LEDs that each have a recipient luminophoric medium that contains a yellow light emitting phosphor, the second string comprises one or more blue LEDs that each have a recipient luminophoric medium that contains a green light emitting phosphor, and the third string comprises one or more red LEDs or, alternatively, one or more blue LEDs that each have a recipient luminophoric medium that contains a red light emitting phosphor.
As used herein, the term “semiconductor light emitting device” may include LEDs, laser diodes and any other light emitting devices that includes one or more semiconductor layers, regardless of whether or not the light emitting devices are packaged into a lamp, fixture or the like. The semiconductor layers included in these devices may include silicon, silicon carbide, gallium nitride and/or other semiconductor materials, an optional semiconductor or non-semiconductor substrate, and one or more contact layers which may include metal and/or other conductive materials. The expression “light emitting device,” as used herein, is not limited, except that it be a device that is capable of emitting light.
A packaged semiconductor light emitting device is a device that includes at least one semiconductor light emitting device (e.g., an LED or an LED coated with a recipient luminophoric medium) that is enclosed with packaging elements to provide one or more of environmental protection, mechanical protection, light mixing, light focusing or the like, as well as electrical leads, contacts, traces or the like that facilitate electrical connection to an external circuit. Encapsulant material, optionally including luminescent material, may be disposed over the semiconductor light emitting device. Multiple semiconductor light emitting devices may be provided in a single package.
Semiconductor light emitting devices according to embodiments of the invention may include III-V nitride (e.g., gallium nitride) based LEDs fabricated on a silicon carbide, sapphire or gallium nitride substrates such as various devices manufactured and/or sold by Cree, Inc. of Durham, N.C. Such LEDs may (or may not) be configured to operate such that light emission occurs through the substrate in a so-called “flip chip” orientation. These semiconductor light emitting devices may have a cathode contact on one side of the LED, and an anode contact on an opposite side of the LED, or may alternatively have both contacts on the same side of the device. Some embodiments of the present invention may use semiconductor light emitting devices, device packages, fixtures, luminescent materials, power supplies and/or control elements such as described in U.S. Pat. Nos. 7,564,180; 7,456,499; 7,213,940; 7,095,056; 6,958,497; 6,853,010; 6,791,119; 6,600,175, 6,201,262; 6,187,606; 6,120,600; 5,912,477; 5,739,554; 5,631,190; 5,604,135; 5,523,589; 5,416,342; 5,393,993; 5,359,345; 5,338,944; 5,210,051; 5,027,168; 5,027,168; 4,966,862, and/or 4,918,497, and U.S. Patent Application Publication Nos. 2009/0184616; 2009/0080185; 2009/0050908; 2009/0050907; 2008/0308825; 2008/0198112; 2008/0179611, 2008/0173884, 2008/0121921; 2008/0012036; 2007/0253209; 2007/0223219; 2007/0170447; 2007/0158668; 2007/0139923, and/or 2006/0221272. The design and fabrication of semiconductor light emitting devices are well known to those skilled in the art, and hence further description thereof will be omitted.
Visible light may include light having many different wavelengths. The apparent color of visible light to humans can be illustrated with reference to a two-dimensional chromaticity diagram, such as the 1931 CIE Chromaticity Diagram illustrated in
As shown in
Each point in the diagram of
As a heated object becomes incandescent, it first glows reddish, then yellowish, and finally bluish with increasing temperature. This occurs because the wavelength associated with the peak radiation of the black-body radiator becomes progressively shorter with increased temperature, consistent with the Wien Displacement Law. Illuminants that produce light which is on or near the black-body locus 4 can thus be described in terms of their correlated color temperature (CCT). The 1931 CIE Diagram of
The ability of a light source to accurately reproduce color in illuminated objects is typically characterized using the color rendering index (“CRI Ra”). The CRI Ra of a light source is a modified average of the relative measurements of how the color rendition of an illumination system compares to that of a reference black-body radiator when illuminating eight reference colors that are referred to as r1 through r8. Thus, the CRI Ra is a relative measure of the shift in surface color of an object when lit by a particular lamp. The CRI Ra equals 100 if the color coordinates of a set of test colors being illuminated by the illumination system are the same as the coordinates of the same test colors being irradiated by the black-body radiator. Daylight generally has a CRI Ra of nearly 100, incandescent bulbs have a CRI Ra of about 95, fluorescent lighting typically has a CRI Ra of about 70 to 85, while monochromatic light sources have a CRI Ra of essentially zero. Light sources for general illumination applications with a CRI Ra of less than 50 are generally considered very poor and are typically only used in applications where economic issues preclude other alternatives. Light sources with a CRI Ra value between 70 and 80 have application for general illumination where the colors of objects are not important. For some general interior illumination, a CRI Ra value of greater than 80 is acceptable. A light source with color coordinates within 4 MacAdam step ellipses of the black-body locus 4 and a CRI Ra value that exceeds 85 is more suitable for general illumination purposes. Light sources with CRI Ra values of more than 90 provide good color quality.
For backlight, general illumination and various other applications, it is often desirable to provide a lighting source that generates white light having a relatively high CRI Ra, so that objects illuminated by the lighting source may appear to have more natural coloring to the human eye. Accordingly, such lighting sources may typically include an array of semiconductor lighting devices including red, green and blue light emitting devices. When red, green and blue light emitting devices are energized simultaneously, the resulting combined light may appear white, or nearly white, depending on the color points and relative intensities of the red, green and blue sources. However, even light that is a combination of red, green and blue emitters may have a low CRI Ra, particularly if the emitters generate saturated light, because such light may lack contributions from many visible wavelengths.
As noted above, CRI Ra is an average color rendering value for eight specific sample colors that are generally referred to as r1-r8. Additional sample colors r9-r15 are now also often used in evaluating the color rendering properties of a light source. The sample color r9 is the saturated red color, and it is generally known that the ability to reproduce red colors well is key for accurately rendering colors, as the color red is often found mixed into processed colors. Accordingly, all else being equal, lamps with high r9 values tend to produce the most vivid colors. Generally speaking, lamps with r9 values of above 90 are desirable in many settings.
Pursuant to embodiments of the present invention, semiconductor light emitting devices are provided that may be designed to emit warm white light and to have high CRI Ra values including CRI Ra values that can exceed 90. These devices may also exhibit high r9 values (e.g., r9 values that exceed 90), and may have high luminous power output and efficacy.
In some embodiments, the semiconductor light emitting devices may comprise multi-emitter devices that have one or more light emitting devices that emit radiation in three (or more) different color ranges or regions. By way of example, the semiconductor light emitting device may include a first group of one or more LEDs that combine to emit radiation having a first color point on the 1931 CIE Chromaticity Diagram that falls within a first color range or region, a second group of one or more LEDs that combine to emit radiation having a second color point on the 1931 CIE Chromaticity Diagram that falls within a second color range or region, and a third group of one or more LEDs that combine to emit radiation having a third color point on the 1931 CIE Chromaticity Diagram that falls within a third color range or region.
The drive current that is provided to a first of the groups of LEDs may be adjusted to move the color point of the combined light emitted by the first and second groups of LEDs along a line that extends between the first color point and the second color point. The drive current that is provided to a third of the groups of LEDs may likewise be adjusted to move the color point of the combined light emitted by the first, second and third groups of LEDs along a line that extends between the third color point and the color point of the combined light emitted by the first and second groups of LEDs. By adjusting the drive currents in this fashion the color point of the radiation emitted by the packaged semiconductor light emitting device can be adjusted to a desired color point such as, for example, a color point having a desired color temperature along the black-body locus 4 of
In some embodiments, the first group of LEDs may comprise one or more blue-shifted-yellow LEDs (“BSY LED”), and the second group of LEDs may comprise one or more blue-shifted-green LEDs (“BSG LED”). The third group of LEDs may comprise one or more red LEDs (e.g., InAlGaP LEDs or organic LEDs) and/or one or more blue-shifted-red LEDs (“BSR LED”). For purposes of this disclosure, a “red LED” refers to an LED that emits nearly saturated radiation having a peak wavelength between 600 and 720 nm, and a “blue LED” refers to an LED that emits nearly saturated radiation having a peak wavelength between 400 and 490 nm. A “BSY LED” refers to a blue LED and an associated recipient luminophoric medium that together emit light having a color point that falls within a trapezoidal “BSY region” on the 1931 CIE Chromaticity Diagram defined by the following x, y chromaticity coordinates: (0.32, 0.40), (0.36, 0.48), (0.43, 0.45), (0.42, 0.42), (0.36, 0.38), (0.32, 0.40), which is generally within the yellow color range. A “BSG LED” refers to a blue LED and an associated recipient luminophoric medium that together emit light having a color point that falls within a trapezoidal “BSG region” on the 1931 CIE Chromaticity Diagram defined by the following x, y chromaticity coordinates: (0.35, 0.48), (0.26, 0.50), (0.13, 0.26), (0.15, 0.20), (0.26, 0.28), (0.35, 0.48), which is generally within the green color range. A “BSR LED” refers to a blue LED that includes a recipient luminophoric medium that emits light having a dominant wavelength between 600 and 720 nm in response to the light emitted by the blue LED. A BSR LED will typically have two distinct spectral peaks on a plot of light output versus wavelength, namely a first peak at the peak wavelength of the blue LED in the blue color range and a second peak at the peak wavelength of the luminescent materials in the recipient luminophoric medium when excited by the light from the blue LED, which is within the red color range. Typically, the red LEDs and/or BSR LEDs will have a dominant wavelength between 600 and 660 nm, and in most cases between 600 and 640 nm.
As shown in
As further shown in
Typically, a packaged semiconductor light emitting device such as the device 10A of
Unfortunately, a number of factors may make it difficult to produce semiconductor light emitting devices that emit light at or near a desired color point. As one example, the plurality of LEDs that are produced by singulating an LED wafer will rarely exhibit identical characteristics. Instead, the output power, peak wavelength, FWHM width and other characteristics of singulated LEDs from a given wafer will exhibit some degree of variation. Likewise, the thickness of a recipient luminophoric medium that is coated on an LED wafer or on a singulated LED may also vary, as may the concentration and size distribution of the luminescent materials therein. Such variations will result in variations in the spectral power output of the light emitted by the luminescent materials.
The above-discussed variations (and others) can complicate a manufacturers efforts to produce semiconductor light emitting devices having a pre-selected color point. By way of example, if a particular semiconductor light emitting device is designed to use blue LEDs having a peak wavelength of 460 nm in order to achieve a specified color temperature along the black-body locus 4 of
In order to reduce the number of LED wafers that must be grown or purchased, an LED manufacturer can, for example, increase the size of the acceptable range of peak wavelengths by selecting LEDs on opposite sides of the specified peak wavelength. By way of example, if a particular design requires LEDs having a peak wavelength of 460 nm, then use of LEDs having peak wavelengths of 457 nm and 463 nm may together produce light that is relatively close to the light emitted by an LED from the same wafer that has a peak wavelength of 460 nm. Thus, a manufacturer can “blend” multiple LEDs together to produce the equivalent of the desired LED. A manufacturer may use similar “blending” techniques with respect to variations in the output power of LEDs, FWHM width and various other parameters. As the number of parameters is increased, the task of determining combinations of multiple LEDs (and luminescent materials) that will have a combined color point that is close to a desired color point can be a complex undertaking.
Pursuant to embodiments of the present invention, methods of tuning a semiconductor light emitting device are provided that can be used to adjust the light output thereof such that the emitted light is at or near a desired color point. Pursuant to these methods, the current provided to at least two different strings of light emitting devices that are included in the device may be separately adjusted in order to set the color point of the device at or near a desired value. These methods will now be described with respect to
Referring to
The color point of the overall light output of the device 10A will fall on a line 31 in
The device 10A may be designed, for example, to have a color point that falls on the point on the black-body locus 4 that corresponds to a color temperature of 3200K (this color point is labeled as point 27 in
For example, pursuant to some embodiments, the color point of the light emitted by the combination of the first string of BSY LEDs 11 and the second string of BSG LEDs 12 may be moved along line 30 of
Next, the device 10A may be further tuned by adjusting the relative drive current provided to string 13 as compared to the drive currents provided to strings 11 and 12. In particular, the drive current provided to string 13 is increased relative to the drive current supplied to strings 11, 12 so that the light output by device 10A will move from color point 26 to the right along a line 32 that extends between point 23 and point 25 to point 27, thereby providing a device that outputs light having a color temperature of 3200K on the black-body locus 4. Thus, the above example illustrates how the drive current to the LED strings 11, 12, 13 can be tuned so that the device 10A outputs light at or near a desired color point. Such a tuning process may be used to reduce or eliminate deviations from a desired color point that result from, for example manufacturing variations in the output power, peak wavelength, phosphor thicknesses, phosphor conversion ratios and the like.
It will be appreciated in light of the discussion above that if a semiconductor light emitting device that includes independently controllable light sources that emit light at three different color points, then it may be theoretically possible to tune the device to any color point that falls within the triangle defined by the color points of the three light sources. Moreover, by selecting light sources having color points that fall on either side of the black-body locus 4, it may become possible to tune the device to a wide variety of color points along the black-body locus 4.
While the graph of
By way of example,
It will be appreciated that many modifications can be made to the above-described semiconductor light emitting devices according to embodiments of the present invention, and to methods of operating such devices. For example, the device 10B of
It will also be appreciated that all of the strings 11, 12 and 13 need not be independently controllable in order to tune the device in the manner described above, For example,
As yet another example,
It has been discovered that including both BSY and BSG LEDs (or BSYG LEDs) in one or both of the first and second strings may provide semiconductor light emitting devices that exhibit improved efficiency. In particular, LEDs may exhibit different efficiency levels as a function of drive current. If a target color point on the 1931 CIE Chromaticity Diagram has been selected for a particular semiconductor light emitting device, it may be preferable to have the color point of the combined output of the first string of LEDs (i.e., point 21 on
In the embodiment of
As should be clear from the above discussion, embodiments of the present invention provide both a means for adjusting the light output of a packaged semiconductor light emitting device to have a desired color point on the 1931 CIE Chromaticity Diagram while achieving good color rendering properties, but also provide ways of operating at high efficiency levels. These goals may be achieved, for example, by selecting the LEDs to include in at least the first string of LEDs 11D and the second string of LEDs 12D such that the combined output of the first string of LEDs 11D and the second string of LEDs 12D when those strings are operated at a desired drive current level (which is typically a drive current level that provides good efficiency) is approximately on a line on the 1931 CIE Chromaticity Diagram that is defined by the color point for the combined output of the third string of LEDs 13 and a desired color point for the entire light output of the light emitting device. Once the LEDs are selected in this manner, then the process for the tuning the light output of the packaged semiconductor light emitting device that is described above with respect to
In still further embodiments the second string 12D of LEDs that is included in the embodiment of
In embodiments of the present invention that only include the first string of LEDs 11D and the third string of LEDs 13, the BSY LEDs, BSG LEDs and/or BSYG LEDs that are included in the first string of LEDs 11D may be selected so that a color point of the combined light output of the first string of LEDs 11D is on a line on the 1931 CIE Chromaticity Diagram that is defined by a color point of the third string of red LEDs 13 and a point associated with a desired correlated color temperature on the black body locus. The relative drive currents supplied to the first and third strings of LEDs 11D, 13 may then be adjusted to move the color point of the combined light output of both strings to a point on or about the black body locus, which point should be substantially at the desired correlated color temperature. Such designs provide less flexibility for adjusting the overall color point of the light emitting device (as they provide only two degrees of freedom), but may be suitable for many applications, particularly if the LEDs included in one or both of the strings are preselected to have a desired color point.
In the embodiments of the present invention described above, the tuning process started with the adjustment of the relative drive currents that are supplied to the first and second string of LEDs 11, 12. However, it will be appreciated that in other embodiments the tuning process need not start with this particular adjustment. For example, in another embodiment, the relative drive currents supplied to the BSY LED string 11 and the red LED string 13 may be adjusted first (which moves the color point for the overall light output of the device along a line 33 of
It will likewise be appreciated that if more than three strings of LEDs are provided, an additional degree of freedom may be obtained in the tuning process. For example, if a fourth string of BSC LEDs was added to the device 10A of
It will likewise be appreciated that embodiments of the present invention are not limited to semiconductor devices that include BSY, BSG, BSC, BSYG, BSR and/or red LEDs. For example,
In still other embodiments, the light emitting device 10E of
As noted above, in some embodiments, the second string 12E of LEDs may be blue LEDs that each have a luminophoric medium that includes luminescent materials that emit light having a color point that is generally green in color, but the color point is outside the BSG LED region on the 1931 CE Chromaticity Diagram. In these embodiments, the color point may be within at least eight MacAdam ellipses from one or more points that are within the BSG LED region. In other example embodiments, the color point may be within at least five MacAdam ellipses from one or more points that are within the BSG LED region. Similarly, the third string 13E of LEDs may be blue LEDs that each have a luminophoric medium that includes luminescent materials that emit light having a color point that is generally yellow in color, but the color point is outside the BSY LED region on the 1931 CE Chromaticity Diagram. In these embodiments, the color point may be within at least eight MacAdam ellipses from one or more points that are within the BSY LED region. In other example embodiments, the color point may be within at least five MacAdam ellipses from one or more points that are within the BSY LED region. Such LEDs may also be used in the first and/or second LED strings 11D, 12D of the light emitting device 10D of
In some embodiments, the LEDs in the third string 13 of FIGS. 3 and 6-8 may emit light having a dominant wavelength between 600 nm and 635 nm, or even within a range of between 610 nm and 625 nm. Likewise, in some embodiments, the blue LEDs that are used to form the BSY LEDs, BSG LEDs and/or BSYG LEDs of the devices of FIGS. 3 and 6-8 may have peak wavelengths that are between about 430 nm and 480 nm, or even within a range of between 440 nm and 475 nm. In some embodiments, the BSG LEDs may comprise a blue LED that emits radiation having a peak wavelength between 440 and 475 nm and an associated recipient luminophoric medium that together emit light having a color point that falls within the region on the 1931 CIE Chromaticity Diagram defined by the following x, y chromaticity coordinates: (0.35, 0.48), (0.26, 0.50), (0.13, 0.26), (0.15, 0.20), (0.26, 0.28), (0.35, 0.48).
As shown in
A packaged semiconductor light emitting device 40 according to embodiments of the present invention will now be described with reference to
As shown in
Each LED 48 is mounted to a respective die pad 44 that is provided on the top surface of the submount 42. Conductive traces 46 are also provided on the top surface of the submount 42. The die pads 44 and conductive traces 46 can comprise many different materials such as metals (e.g., copper) or other conductive materials, and may be deposited, for example, via plating and patterned using standard photolithographic processes. Seed layers and/or adhesion layers may be provided beneath the die pads 44. The die pads 44 may also include or be plated with reflective layers, barrier layers and/or dielectric layers. The LEDs 48 may be mounted to the die pads 44 using conventional methods such as soldering.
In some embodiments, the LEDs 48 may include one or more BSY LEDs, one or more BSG LEDs and one or more saturated red LEDs. In other embodiments, some or all of the saturated red LEDs may be replaced with BSR LEDs. Moreover, additional LEDs may be added, including, for example, one or more long-wavelength blue LEDs and/or BSC LEDs. LED structures, features, and their fabrication and operation are generally known in the art and only briefly discussed herein.
Each LED 48 may include at least one active layer/region sandwiched between oppositely doped epitaxial layers. The LEDs 48 may be grown as wafers of LEDs, and these wafers may be singulated into individual LED dies to provide the LEDs 48. The underlying growth substrate can optionally be fully or partially removed from each LED 48. Each LED 48 may include additional layers and elements including, for example, nucleation layers, contact layers, current spreading layers, light extraction layers and/or light extraction elements. The oppositely doped layers can comprise multiple layers and sub-layers, as well as super lattice structures and interlayers. The active region can include, for example, single quantum well (SQW), multiple quantum well (MQW), double heterostructure and/or super lattice structures. The active region and doped layers may be fabricated from various material systems, including, for example, Group-III nitride based material systems such as GaN, aluminum gallium nitride (AlGaN), indium gallium nitride (InGaN) and/or aluminum indium gallium nitride (AlInGaN). In some embodiments, the doped layers are GaN and/or AlGaN layers, and the active region is an InGaN layer.
Each LED 48 may include a conductive current spreading structure on its top surface, as well as one or more contacts/bond pads that are accessible at its top surface for wire bonding. The current spreading structure and contacts/bond pads can be made of a conductive material such as Au, Cu, Ni, In, Al, Ag or combinations thereof, conducting oxides and transparent conducting oxides. The current spreading structure may comprise spaced-apart conductive fingers that are arranged to enhance current spreading from the contacts/bond pads into the top surface of its respective LED 48. In operation, an electrical signal is applied to a contact/bond pad through a wire bond, and the electrical signal spreads through the fingers of the current spreading structure into the LED 48.
Some or all of the LEDs 48 may have an associated recipient luminophoric medium that includes one or more luminescent materials. Light emitted by a respective one of the LEDs 48 may pass into its associated recipient luminophoric medium. At least some of that light that passes into the recipient luminophoric medium is absorbed by the luminescent materials contained therein, and the luminescent materials emit light having a different wavelength distribution in response to the absorbed light. The recipient luminophoric medium may fully absorb the light emitted by the LED 48, or may only partially absorb the light emitted by the LED 48 so that a combination of unconverted light from the LED 48 and down-converted light from the luminescent materials is output from the recipient luminophoric medium. The recipient luminophoric medium may be coated directly onto the LED or otherwise disposed to receive some or all of the light emitted by its respective LED 48. It will also be appreciated that a single recipient luminophoric medium may be used to down-convert some or all of the light emitted by multiple of the LEDs 48. By way of example, in some embodiments, each string of LEDs 48 may be included in its own package, and a common recipient luminophoric medium for the LEDs 48 of the string may be coated on a lens of the package or included in an encapsulant material that is disposed between the lens and the LEDs 48.
The above-described recipient luminophoric mediums may include a single type of luminescent material or may include multiple different luminescent materials that absorb some of the light emitted by the LEDs 48 and emit light in a different wavelength range in response thereto. The recipient luminophoric mediums may comprise a single layer or region or multiple layers or regions, which may be directly adjacent to each other or spaced-apart. Suitable methods for applying the recipient luminophoric mediums to the LEDs 48 include the coating methods described in U.S. patent application Ser. Nos. 11/656,759 and 11/899,790, the electrophoretic deposition methods described in U.S. patent application Ser. No. 11/473,089, and/or the spray coating methods described in U.S. patent application Ser. No. 12/717,048. Numerous other methods for applying the recipient luminophoric mediums to the LEDs 48 may also be used.
As noted above, in certain embodiments, the LEDs 48 can include at least one BSY LED, at least one BSG LED, and at least one red light source. The BSY LED(s) may comprise blue LEDs that include a recipient luminophoric medium that has YAG:Ce phosphor particles therein such that the LED and phosphor particles together emit a combination of blue and yellow light. In other embodiments, different yellow light emitting luminescent materials may be used to form the BSY LEDs including, for example, phosphors based on the (Gd,Y)3(Al, Ga)5O12:Ce system, such as Y3Al5O12:Ce (YAG) phosphors; Tb3−xRExO12:Ce (TAG) phosphors where RE=Y, Gd, La, Lu; and/or Sr2-x-yBaxCaySiO4:Eu phosphors. The BSG LED(s) may comprise blue LEDs that have a recipient luminophoric medium that include LuAG:Ce phosphor particles such that the LED and phosphor particles together emit a combination of blue and green light. In other embodiments, different green light emitting luminescent materials may be used including, for example, (Sr,Ca,Ba) (Al,Ga)2S4: Eu2+ phosphors; Ba2(Mg,Zn)Si2O7: Eu2+ phosphors; Gd0.46Sr0.31Al1.23OxF1.38:Eu2+0.06 phosphors; (Ba1-x-ySrxCay)SiO4:Eu phosphors; BaxSiO4:Eu2+ phosphors; Sr6P5BO20:Eu phosphors; MSi2O2N2:Eu2+ phosphors; and/or Zinc Sulfide:Ag phosphors with (Zn,Cd)S:Cu:Al. In some embodiments, the BSG LEDs may employ a recipient luminescent medium that includes a green luminescent material that has a FWHM emission spectrum that falls at least in part into the cyan color range (and in some embodiments, across the entire cyan color range) such as, for example, a LuAG:Ce phosphor that has a peak emission wavelength of between 535 and 545 nm and a FWHM bandwidth of between about 110-115 nm. The at least one red light source may comprise BSG LEDs and/or red LEDs such as, for example, conventional AlInGaP LEDs. Suitable luminescent materials for the BSR LEDs (if used) include Lu2O3:Eu3+ phosphors; (Sr2−xLax)(Ce1−xEux)O4 phosphors; Sr2Ce1−EuxO4 phosphors; Sr2−xEuxCeO4 phosphors; SrTiO3:Pr3+,Ga3+ phosphors; (Ca1−xSrx)SiAlN3:Eu2+ phosphors; and/or Sr2Si5N8:Eu2+ phosphors. It will be understood that many other phosphors can used in combination with desired solid state emitters (e.g., LEDs) to achieve the desired aggregated spectral output.
An optical element or lens 55 may be provided over the LEDs 48 to provide environmental and/or mechanical protection. In some embodiments the lens 55 can be in direct contact with the LEDs 48 and a top surface of the submount 42. In other embodiments, an intervening material or layer may be provided between the LEDs 48 and the top surface of the submount 42. The lens 55 can be molded using different molding techniques such as those described in U.S. patent application Ser. No. 11/982,275. The lens 55 can be many different shapes such as, for example, hemispheric, ellipsoid bullet, flat, hex-shaped, and square, and can be formed of various materials such as silicones, plastics, epoxies or glass. The lens 55 can be textured to improve light extraction. For a generally circular LED array, the diameter of the lens can be approximately the same as or larger than the diameter of the LED array.
The lens 55 may also include features or elements arranged to diffuse or scatter light, including scattering particles or structures. Such particles may include materials such as titanium dioxide, alumina, silicon carbide, gallium nitride, or glass micro spheres, with the particles preferably being dispersed within the lens. Alternatively, or in combination with the scattering particles, air bubbles or an immiscible mixture of polymers having a different index of refraction could be provided within the lens or structured on the lens to promote diffusion of light. Scattering particles or structures may be dispersed homogeneously throughout the lens 55 or may be provided in different concentrations or amounts in different areas in or on a lens. In one embodiment, scattering particles may be provided in layers within the lens, or may be provided in different concentrations in relation to the location of LEDs 48 (e.g., of different colors) within the packaged device 40. In other embodiments, a diffuser layer or film (not shown) may be disposed remotely from the lens 55 at a suitable distance from the lens 55, such as, for example, 1 mm, 5 mm, 10 mm, 20 mm, or greater. The diffuser film may be provided in any suitable shape, which may depend on the configuration of the lens 55. A curved diffuser film may be spaced apart from but conformed in shape to the lens and provided in a hemispherical or dome shape.
The LED package 40 may include an optional protective layer 56 covering the top surface of the submount 42, e.g., in areas not covered by the lens 55. The protective layer 56 provides additional protection to the elements on the top surface to reduce damage and contamination during subsequent processing steps and use. The protective layer 56 may be formed concurrently with the lens 55, and optionally comprise the same material as the lens 55.
As shown in
The current control circuits 14, 15, 16 (see, e.g.,
To promote heat dissipation, the packaged device 40 may include a thermally conductive (e.g., metal) layer 92 on a bottom surface of the submount 42. The conductive layer 92 may cover different portions of the bottom surface of the submount 42; in one embodiment as shown, the metal layer 92 may cover substantially the entire bottom surface. The conductive layer 92 may be in at least partial vertical alignment with the LEDs 48. In one embodiment, the conductive layer is not in electrical communication with elements (e.g., LEDs) disposed on top surface of the submount 42. Heat that may concentrate below individual LEDs 48 will pass into the submount 42 disposed directly below and around each LED 48. The conductive layer 92 can aid heat dissipation by allowing this heat to spread from concentrated areas proximate the LEDs into the larger area of the layer 92 to promote dissipation and/or conductive transfer to an external heat sink (not shown). The conductive layer 92 may include holes 94 providing access to the submount 42, to relieve strain between the submount 42 and the metal layer 92 during fabrication and/or during operation. In certain embodiments, thermally conductive vias or plugs that pass at least partially through the submount 42 and are in thermal contact with the conductive layer 92 may be provided. The conductive vias or plugs promote passage of heat from the submount 42 to the conductive layer 92 to further enhance thermal management.
While
Methods of tuning a multi-emitter semiconductor light emitting device to a desired color point according to embodiments of the present invention will now be further described with respect to the flow chart of
As shown in
In some embodiments, the first string of LEDs may include at least one BSY LED, and the second string of LEDs may include at least one BSG LED. The third string of at least one LED may include at least one red LED and/or at least one BSR LED. The color point on the 1931 CIE Chromaticity Diagram of the combined output of the multi-emitter semiconductor light emitting device may be within three MacAdam ellipses from a selected color point on the black-body locus.
In some embodiments of the present invention, the drive currents supplied to the strings may be set in the fashion described above at the factory in order to tune the device to a particular color point. In some cases, adjustable resistors or resistor networks, digital to analog converters with flash memory, and/or fuse link diodes may then be set to fixed values so that the packaged semiconductor light emitting device will be set to emit light at or near the desired color point.
According to further embodiments of the present invention, semiconductor light emitting devices may be provided which allow an end user to set the color point of the device. For example, in some embodiments, semiconductor light emitting devices may be provided that include at least two different color temperature settings. By way of example, a device might have a first setting at which the drive currents to various strings of light emitting devices that are included in the device are set to provide a first light output having a color temperature of between 4000K and 5000K, which end users may prefer in the daytime, and a second light output having a color temperature of between 2500K and 3500K, which users may prefer at night.
Turning to the particular embodiment depicted in
A wide variety of changes may be made to the device 200 of
According to still further embodiments of the present invention, tunable multi-emitter semiconductor light emitting devices are provided which automatically adjust the drive currents provided to one or more of multiple strings of light emitting devices included therein. By way of example, it is known that when LEDs constructed using different semiconductor material systems (e.g., GaN-based LEDs, InAlGaP-based LEDs and/or organic LEDs) are used in the same light emitting device, the characteristics of the LEDs may vary differently with operating temperature, over time, etc. As such, the color point of the light produced by such devices is not necessarily stable. Pursuant to further embodiments of the present invention, tunable packaged multi-emitter semiconductor light emitting devices are provided with automatically adjusting drive currents that compensate for such variable changes. The automatic adjustment may, for example, be pre-programmed or responsive to sensors.
The device 300 also includes first, second and third current control circuits 314, 315, 316. The first, second and third current control circuits 314, 315, 316 are configured to provide respective drive currents to the first, second and third strings of LEDs 311, 312, 313, and may be used to set the drive currents that are provided to the respective first through third strings of LEDs 311, 312, 313 at levels that are set so the device 300 will emit combined radiation at or near a desired color point.
The device 300 further includes a control system 317 and a sensor 320. The sensor 320 may sense various characteristics such as, for example, the temperature of the device 300. Data regarding the sensed characteristics is provided from the sensor 320 to the control system 317. In response to this data, the control system 317 may automatically cause one or more of the first, second and third current control circuits 314, 315, 316 to adjust the drive currents that are provided to the respective first, second and third strings of LEDs 311, 312, 313. The control system 317 may be programmed to adjust the drive currents that are provided to the respective first, second and third strings of LEDs 311, 312, 313 in a manner that tends to maintain the color point of the light emitted by the device 300 despite changes in various characteristics such as the temperature of the device 300.
In some embodiments, the control system 317 may also be pre-programmed to make adjustments to the drive currents that is not responsive to data from sensor 320. For example, if the emissions of, for example, the LEDs in the third string of LEDs 313 degrades over time more quickly than the emissions of the first and second strings of LEDs 311, 312, then the control system 317 may be pre-programmed to, for example, cause the third current control circuit 316 to slowly increase the drive current that is provided to the third string of LEDs 313 over time (e.g., in discrete steps at certain time points) in order to better maintain the color point of the light emitted by the device 300 over time.
It will be appreciated that the sensor 320 and control system 317 of device 300 of
The light emitting devices according to embodiments of the present invention may exhibit excellent CRI with very high efficiency. Moreover, as noted above, this high performance may be achieved for a wide variety of correlated color temperatures (e.g., 2500K to 6500K).
In particular,
As noted above, pursuant to certain embodiments of the present invention, light emitting devices (“BSG+BSY+R” devices) are provided that include a string of BSY LEDs, a string of BSG LEDs and a string of red LEDs. As shown at curve 404 in
Pursuant to further embodiments of the present invention, it has been discovered that for BSG+BSY+R light emitting devices, the color rendering performance (i.e., CRI Ra and r9 performance) may, at least in some cases, be optimized with little loss in efficiency. By way of example,
As discussed above, in some embodiments of the present invention, the color point of a semiconductor light emitting device may be adjusted to fall closer to a desired color point by adjusting the drive current provided to one or more independently controllable LED strings. It will be appreciated that drive current can be adjusted in a variety of ways. For example, in some embodiments, an absolute drive current level provided to one or more of the LED strings may be adjusted to move the color point. In other embodiments, the drive current provided to one or more LED strings may be turned on and off (e.g., using pulse width modulation) in order to reduce the average drive current that is provided to those LED strings. It will be appreciated that many other techniques may also be used.
Various embodiments of the present invention that are discussed above adjust the drive current supplied to one or more of multiple strings of light emitting devices that have separate color points in order to adjust a color point of the overall light output of the device. It will be appreciated that there are numerous ways to provide strings of light emitting devices that have different color points. For instance, in some of the embodiments discussed above, identical LEDs may be used in each of the multiple strings, while each of the strings use different recipient luminophoric mediums in order to provide multiple strings having different color points. In other embodiments, some strings may use the same underlying LEDs and different recipient luminophoric mediums, while other strings use different LEDs (e.g., a saturated red LED) in order to provide the multiple strings having different color points. In still further embodiments, some strings may use the recipient luminophoric mediums and different underlying LEDs (e.g. a first string uses 450 nm blue LEDs and a BSY recipient luminophoric medium and a second string uses 470 nm blue LEDs and the same BSY recipient luminophoric medium), while other strings use different LEDs and/or different recipient luminophoric mediums in order to provide the multiple strings having different color points.
Many different embodiments have been disclosed herein, in connection with the above description and the drawings. It will be understood that it would be unduly repetitious and obfuscating to literally describe and illustrate every combination and subcombination of these embodiments. Accordingly, the present specification, including the drawings, shall be construed to constitute a complete written description of all combinations and subcombinations of the embodiments described herein, and of the manner and process of making and using them, and shall support claims to any such combination or subcombination.
While embodiments of the present invention have primarily been discussed above with respect to semiconductor light emitting devices that include LEDs, it will be appreciated that according to further embodiments of the present invention, laser diodes and/or other semiconductor lighting devices may be provided that include the luminophoric mediums discussed above.
The present invention has been described above with reference to the accompanying drawings, in which certain embodiments of the invention are shown. However, this invention should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the thickness of layers and regions are exaggerated for clarity. Like numbers refer to like elements throughout. As used herein the term “and/or” includes any and all combinations of one or more of the associated listed items.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that, when used in this specification, the terms “comprises” and/or “including” and derivatives thereof, specify the presence of stated features, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, operations, elements, components, and/or groups thereof.
It will be understood that when an element such as a layer, region or substrate is referred to as being “on” or extending “onto” another element, it can be directly on or extend directly onto the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” or extending “directly onto” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present.
It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, components, regions and/or layers, these elements, components, regions and/or layers should not be limited by these terms. These terms are only used to distinguish one element, component, region or layer from another element, component, region or layer. Thus, a first element, component, region or layer discussed below could be termed a second element, component, region or layer without departing from the teachings of the present invention.
Furthermore, relative terms, such as “lower” or “bottom” and “upper” or “top,” may be used herein to describe one element's relationship to another element as illustrated in the figures. It will be understood that relative terms are intended to encompass different orientations of the device in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as being on the “lower” side of other elements would then be oriented on “upper” sides of the other elements. The exemplary term “lower”, can therefore, encompasses both an orientation of “lower” and “upper,” depending on the particular orientation of the figure.
Embodiments of the invention are described herein with reference to cross-section illustrations that are schematic illustrations of idealized embodiments (and intermediate structures) of the invention. The thickness of layers and regions in the drawings may be exaggerated for clarity. Additionally, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, embodiments of the invention should not be construed as limited to the particular shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing.
In the drawings and specification, there have been disclosed embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.
The present application claims priority under 35 U.S.C. §120 as a continuation-in-part of U.S. patent application Ser. No. 13/039,572, filed Mar. 3, 2011, the entire content of which is incorporated herein by reference as if set forth in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 13039572 | Mar 2011 | US |
| Child | 13546099 | US |
