Patent Application
20140001588
Photodetectors can be used as ambient light sensors (ALSs), e.g., for use as energy saving light sensors for displays, for controlling backlighting in portable devices such as mobile phones and laptop computers, and for various other types of light level measurement and management. For more specific examples, an ALS can be used to reduce overall display-system power consumption and to increase Liquid Crystal Display light source (LCD) lifespan by detecting bright and dim ambient light conditions as a means of controlling display and/or keypad backlighting. Without an ALS, LCD display backlighting control is typically done manually whereby a user will increase the intensity of the LCD as the ambient environment becomes brighter. With the use of an ALS, a user can adjust the LCD brightness to their preference, and as the ambient environment changes, the display brightness adjusts to make the display appear uniform at the same perceived level; this results in battery life being extended, user eye strain being reduced, and LCD lifespan being extended. Similarly, without an ALS, control of the keypad backlight is very much dependent on the user and software. For example, a keypad backlight can be turned on for 10 seconds by a trigger which can be triggered by pressing the keypad, or a timer. With the use of an ALS, keypad backlighting can be turned on only when the ambient environment is dim, which will result in longer battery life. In order to achieve better ambient light sensing, an ALS preferably has a spectral response close to the human eye response and has excellent infrared (IR) noise suppression (also referred to as IR rejection). Such a spectral response is often referred to as a “true human eye response” or a “photopic response”.
A potential problem with using a photodetector (such as a photodiode) as an ALS is that it detects both visible light and non-visible light, such as infrared (IR) light, which starts at about 700 nm. By contrast, the human eye does not detect IR light. Thus, the response of a photodetector can significantly differ from the response of a human eye, especially when the light is produced by an incandescent light, which includes large amounts of IR light. This would provide for significantly less than optimal adjustments if the photodetector were used as an ALS, e.g., for adjusting backlighting, or the like. Accordingly, various techniques have been attempted to provide light sensors (also referred to as optical sensors) that have a spectral response closer to that of the human eye, so that such light sensors can be used, e.g., for appropriately adjusting the backlighting of displays, or the like. Some of these techniques involve covering photodetectors with optical filters.
As can be appreciated from the above discussion, one potential desired response for a photodetector is a photopic response. However, this is just one exemplary response. For example, it may be desired that the response of one photodetector indicate how much red light is detected, the response of another photodetector indicate how much green light is detected, and the response of a further photodetector indicate how much blue light is detector. The responses of these three photodetectors can be combined, e.g., to provide a photopic response. Alternatively, the responses of these three photodetectors can be individually used as feedback to adjust colors in digital images captured using a digital camera and/or a digital video recorder, e.g., so that the captured images/videos more closely resemble what a person operating the camera/video recorder actually viewed. The responses of these three photodetectors can also be used for color adjustment for an LED back light system or an LED projector, for color detection and/or for white balance adjustment. Another potential desired response for a photodetector is detection of IR light and rejection of visible light, e.g., if the photodetector is being used in an IR based proximity and/or motion detector. Regardless of the exact response desired, it would be beneficial if photodetectors having any particular desired response can be fabricated in a manner that provides high accuracy and high yield.
Low cost semiconductor optical sensors are typically silicon photodiodes underneath mono-layer organic color filters. For example, conventional sensor designs often include dyed organic filters (also referred to as organic color filters) that are directly deposited on a passivation layer that covers a photodiode sensor region. The passivation layer is typically located on one or more inter-metal dielectric (IMD) layer(s) that also cover the photodiode sensor region. The dyed organic filters, which absorb specific light frequency ranges, have the advantage of low cost and ease of integration into conventional integrated circuit (IC) fabrication flows. A disadvantage of dyed organic filters is that they allow excessive infrared (IR) energy transmission. In other words, dyed organic filters are not good at absorbing wavelengths greater than 700 nm. However, where there is a desire to provide a photopic response, or to provide responses indicative of specific visible colors (e.g., red, green and/or blue), there is a need to filter out or otherwise reject wavelengths greater than 700 nm. Further, there is often a desire to include in the same package both an ALS and an IR-based proximity sensor. In such cases, there is a need to provide both a photodiode that rejects IR light, and a photodiode that detects IR light, within the same package.
In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific illustrative embodiments. It is to be understood that other embodiments may be utilized and that mechanical and electrical changes may be made. The following detailed description is, therefore, not to be taken in a limiting sense. In the description that follows, like numerals or reference designators will be used to refer to like parts or elements throughout. In addition, the first digit of a reference number identifies the drawing in which the reference number first appears.
Certain embodiments of the present invention, which are described below, relate to monolithic optical sensor devices that include photodetectors (e.g., photodiodes), one or more wafer-level patterned inorganic dielectric optical filter(s), as well as one or more wafer-level patterned organic color filter(s). Certain embodiments of the present invention, which are described below, enable one or more photodetectors to reject IR light, while one or more further photodetectors detect IR light, even though all such photodetectors and filters are fabricated in/on a common semiconductor wafer substrate.
The entire surface of the wafer substrate 104 is covered by one or more inter-metal dielectric (IMD) layer(s), which can include one or more oxide and/or nitride, but is not limited thereto. One or more passivation layer(s) is/are likely above the uppermost IMD layer(s) 106. Passivation layers are typically categorized as either “hard” or “soft”. Hard passivation is typically silicon nitride while soft passivation is typically polyimide which is usually deposited over the hard passivation layer. Alternative passivation materials are also possible. The hard passivation layer(s) may or may not be planarized using CMP (chemical mechanical polishing). It is preferable for the passivation surface to be planar in optical sensor applications, but planar passivation is not a requirement for this invention. The IMD layer(s) and passivation layer(s) are collectively labeled 106.
A wafer-level inorganic dielectric optical filter 108 is patterned to cover PD1, PD2, PD3 and PD4, but not PD5. The patterned wafer-level inorganic dielectric optical filter 108 includes multiple layers of thin inorganic dielectric films. The individual thin film thicknesses typically range from about 10 nm to 300 nm, but are not limited thereto. The total thickness of the optical dielectric filter can range, e.g., from 2 μm to 10 μm, but is not limited thereto. Such inorganic dielectric films can be deposited using conventional semiconductor tooling to have, e.g., a high-low-high-low (HLHL) pattern of alternating refractive indices. Various conventional deposition methods can be employed to pattern the wafer-level inorganic dielectric optical filter 108, such as chemical vapor deposition (CVD), physical vapor deposition (PVD), plasma-enhanced CVD (PECVD), low-pressure CVD (LPCVD), metalorganic CVD (MOCVD), molecular beam epitaxy (MBE), epitaxy, evaporation, sputtering, atomic layer deposition (ALD), in-situ jet vapor deposition (JVD), and the like.
The dielectric materials used to form the wafer-level inorganic dielectric optical filter 108 can include silicon dioxide (SiO2), silicon hydride (SixHy), silicon nitride (SixNy), silicon oxynitride (SixOzNy), tantalum oxide (TaxOy), gallium arsenide (GaAs), gallium nitride (GaN), and the like. Alternating layers in the optical filter may have a constant or varying film thickness throughout the filter stack, in order to achieve the desired optical response. By careful choice of the exact composition, thickness, and number of these layers, it is possible to tailor the reflectivity and transmissivity of the optical filter to produce almost any desired spectral characteristics. For example, the reflectivity can be increased to greater than 99.99%, to produce a high-reflector (HR) coating. The level of reflectivity can also be tuned to any particular value, for instance to produce a mirror that reflects 90% and transmits 10% of the light that falls on it, over some range of wavelengths. Such mirrors have often been used as beam splitters, and as output couplers in lasers. Alternatively, the wafer-level inorganic dielectric optical filter 108 can be designed such that the mirror reflects light only in a narrow band of wavelengths, producing a reflective optical filter.
Generally, layers of high and low refractive index materials are alternated one above the other. This periodic or alternating structure significantly enhances the reflectivity of the surface in the certain wavelength range called band-stop, which width is determined by the ratio of the two used indices only (for quarter-wave system), while the maximum reflectivity is increasing nearly up to 100% with a number of layers in the stack. The thicknesses of the layers are generally quarter-wave (then they yield to the broadest high reflection band in comparison to the non-quarter-wave systems composed from the same materials), designed such that reflected beams constructively interfere with one another to maximize reflection and minimize transmission. Using the above described structures, high reflective coatings can achieve very high (e.g., 99.9%) reflectivity over a broad wavelength range (tens of nanometers in the visible spectrum range), with a lower reflectivity over other wavelength ranges, to thereby achieve a desired spectral response. By manipulating the exact thickness and composition of the layers in the reflective stack, the reflection characteristics can be tuned to a desired spectral response, and may incorporate both high-reflective and anti-reflective wavelength regions. The wafer-level inorganic dielectric optical filter 108 can be designed as a long-pass or short-pass filter, a bandpass or notch filter, or a mirror with a specific reflectivity. In specific embodiments of the present invention, the wafer-level inorganic dielectric optical filter 108 is designed as a short-pass filter that passes visible wavelengths less than 700 nm, and rejects wavelengths (e.g., including IR wavelengths) above 700 nm, in which case the wafer-level inorganic dielectric optical filter 108 can be referred to as an IR-cut filter.
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It is noted that a wafer-level inorganic dielectric optical filter, such as the filter 108, is typically relatively expensive to implement, because the deposition process (e.g., sputtering or evaporation) of alternating dielectric material at very fine geometries (tenth to hundreds of nanometer), with precision control to the layer thickness and material composition, typically takes several hours. Additionally, a wafer-level inorganic dielectric optical filter is typically patterned using a photoresist lift-off in a chemical solvent bath, which is typically costly due to the relatively long residence time (i.e., soak duration) in the photoresist solvent bath, and due to a relatively narrow process margin. Thus, if there is a desire to achieve multiple (e.g., three or more) different photodetector responses using a single monolithic optical sensor device, it would be quite costly to achieve the multiple different responses using multiple separate wafer-level inorganic dielectric optical filters to achieve the multiple responses. This is because it would require multiple deposition and multiple lift-off processes to form multiple different wafer-level inorganic dielectric optical filters on a common semiconductor substrate, which would require a very long cycle time. Specific embodiments of the present invention, described herein, take advantage of the common denominator of the multiple desired response, e.g., IR rejection, in order utilize a common wafer-level inorganic dielectric optical filter in combination with multiple organic color filters for achieving multiple different photodetector responses using a single monolithic optical sensor device.
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Where each of the organic color filters 110 is essentially a dyed photoresist material, each organic color filter layer can be patterned using photolithography in the same manners that photoresist is conventionally patterned. There exist both positive and negative types of photoresists, and depending on the exact materials used, the organic color filters 110 can either behave as a positive type of photoresist, or a negative type of photoresist. When a positive photoresist is exposed to UV light the chemical structure of the photoresist changes so that it becomes more soluble in a developer. The exposed photoresist is then washed away by the developer, leaving windows in the photoresist where the photoresist was exposed to UV light. Accordingly, when using a positive photoresist the photomask includes an exact copy of the pattern which is to remain on the wafer. Negative photoresists behave in the opposite manner. That is, exposure to the UV light causes the negative photoresist to become less soluble in a developer. Therefore, the negative photoresist remains on the surface wherever it was exposed, and the developer removes only the unexposed portions. Accordingly, a photomask used with a negative photoresist includes the inverse (or photographic “negative”) of the pattern to be transferred.
As mentioned above, hundreds of such monolithic optical sensor devices are likely being fabricated on a same wafer. Accordingly, after the various organic color filters 110 are patterned, as explained with reference to
In
In
The high level flow diagram of
More or less than three PD regions can be included. For example, there can also be a fourth PD region in the substrate that is intended to be used to detect IR light for IR based proximity and/or motion detection. As was described above with reference to
The wafer-level inorganic dielectric optical filters (e.g., 108, 708, OF1, OF2, OF3 and OF4) and the wafer-level organic color filters (e.g., 110R, 110G, 110B and 110Bk) described herein are considered “wafer-level” filters because they are formed on a wafer prior to dicing the wafer into a plurality of dies (where each die is, or includes, one of the monolithic optical sensor devices described herein). In certain embodiments of the present invention, a wafer, prior to dicing, can include a semiconductor substrate (e.g., 104), PDs formed in the substrate, IMD and passivation layer(s) (e.g., 106) formed above the substrate, as well as the inorganic dielectric optical filters (e.g., 108, 708, OF1, OF2, OF3 and OF4) and the organic color filters (e.g., 110R, 110G, 110B and 110Bk) formed above the IMD and passivation layer(s). A waver can include alternative configurations, e.g., one example of which was described above with reference to
Monolithic optical sensor devices of embodiments of the present invention can be used in various systems, including, but not limited to, mobile phones, cameras, video recorders, projectors, tablets, personal data assistants, laptop computers, netbooks, other handheld-devices, as well as non-handheld-devices. Such sensor devices can be used to achieve various different responses, which depend on the specific applications in which the sensor devices are to be used. For example, a monolithic optical sensor device according to an embodiment of the present invention can include a PD that indicates how much red light is detected, another PD that indicates how much green light is detected, and another PD that indicates how much blue light is detector. The responses of these three PDs can be combined, e.g., to provide a photopic response. Alternatively, the responses of these three PDs can be individually used as feedback to adjust colors in digital images captured using a digital camera and/or a digital video recorder, e.g., so that the captured images/videos more closely resemble what a person operating the camera/video recorder actually viewed. The responses of these three PDs can also be used for color adjustments for a television, an LED back light system or an LED projector, or for color detection and/or for white balance adjustment. The responses of these three PDs can be combined for use as an ALS, or the response of one of the PDs can be used alone as an ALS. The responses of these three PDs can also be used to decrease shutter speeds in a digital camera. For example, a CPU of a digital camera can use the responses of the three PDs to perform temperature calculations, instead of requiring that the CPU perform the color temperature calculations based on signals from a pixel array behind a camera lens.
Referring to the system 1000 of
In accordance with an embodiment, one or more PD regions can be covered by a light blocking material (e.g., a metal layer) that does not let any light through. The PD region(s) that are covered by the light blocking material will produce a current, known as a dark current or a leakage current, that varies with changes in temperature and variations in processing conditions. Similarly, a small portion of the current generated by the other PD region(s) (not covered by a light blocking material) will be due to a dark current, while the remaining portion of the current is primarily indicative of detected light (the wavelengths of which are dependent upon the filter(s) above the PD region(s)). By covered a PD region by the light blocking material, the dark current generated by PD region covered by the light blocking material can be subtracted from a current(s) generated by the other PD region(s), to remove the affects of the dark current.
Alternatively, or additionally, one or more of the PD region(s), which is not covered by any filter, and thus can be referred to as a naked PD region, can be used detect both ambient visible light and ambient IR light. Assume other PD region(s) are covered by one or more filters designed to filter out ambient visible light while passing ambient IR light, and thus, produce a current indicative of ambient IR light. By subtracting the current indicative of ambient IR light from the current generated by the naked optical sensor device(s), a current indicative of ambient visible light can be produced. Other variations are also possible, depending upon the filter design and the desired optical response.
Optical sensor devices produced in accordance with embodiments of the present invention should provide a better performance to cost ratio compared to sensors including either a color organic filter or an inorganic optical dielectric filter alone. Embodiments of the present invention also allow an IR-based proximity and/or motion sensor to be fabricated on a same wafer alongside an ambient light sensor (ALS) and/or one or more sensors configured to detect light of specific colors, such as, but not limited to, red, green and blue (RGB). In other words, a monolithic semiconductor device can include a plurality of light sensors each of which has a different response intended for a different purpose. Alternatively, or additionally, responses of two or more light sensors within the same monolithic device can be combined to provide a desired response, such as a photopic response.
In certain embodiments, other circuitry, such as, amplifier circuitry that is used to amplify photocurrents produced by PD regions and/or driver circuitry that can be used to selectively drive the a light source (for use in proximity and/or motion sensor applications) can be fabricated into the same semiconductor substrate that includes the PD regions that are selectively covered by one or more filters, as described above.
Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement, which is calculated to achieve the same purpose, may be substituted for the specific embodiments shown. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.
