Patent Application
20120313201
Photodetectors can be used for various different types of applications, including, but not limited to, for ambient light sensor (ALS) applications, for proximity sensor applications, and for use in long range sensing applications. Such applications typically require high performance optical filters. For example, for ALS applications, an optical filter is typically used to modify the spectral response of a photodetector so that the photodetector and filter achieve a spectral response that is very similar to that of a typical human eye. Such a response can be referred to as a “true human eye” response.
Typically, organic based optical filters can not be used to provide such a true human eye response, because of their poor performance in the infrared range. Rather, non-organic filters, such as filters made of dielectric mirrors, are generally preferred because they provide better performance. Such dielectric mirrors, which are made from stacks of various dielectric films, are conventionally expensive to implement. This is because they are typically deposited during post processing of wafers (i.e., after wafers are completed by a foundry). For example, since foundries typically don't have the expertise and equipment to manufacture such optical filters, specialty vendors often use customized equipment to add such optical filters to wafers or dies.
Understanding that the drawings depict only exemplary embodiments and are not therefore to be considered limiting in scope, the exemplary embodiments will be described with additional specificity and detail through the use of the accompanying drawings, in which:
In accordance with common practice, the various described features are not drawn to scale but are drawn to emphasize specific features relevant to the exemplary embodiments.
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.
Optical sensor devices including front-end-of-line (FEOL) formed optical filters, and fabrication methods for such optical sensor devices, are provided. In the optical sensor devices, the optical filters are formed prior to metallization of the devices. In specific embodiments, the optical filter is composed of a layered stack of dielectric materials that are compatible with high-temperature processing, standard deposition equipment, and standard patterning equipment (definition and etch). The optical filter can be coplanar or non-planar, relative to an active surface of the device. Metal structures such as grids and columns can be patterned over the optical filter.
The optical sensor devices can be employed as various sensors such as ambient light sensors and proximity sensors, or in long range sensing applications that require high performance optical filters.
The optical filter dielectric materials 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.
Various conventional deposition methods can be employed in fabricating the optical filters, such as chemical vapor deposition (CVD), plasma-enhanced CVD (PECVD), low-pressure CVD (LPCVD), metalorganic CVD (MOCVD), molecular beam epitaxy (MBE), epitaxy, evaporation, sputtering, physical vapor deposition (PVD), atomic layer deposition (ALD), in-situ jet vapor deposition (JVD), and the like.
In certain embodiments, the optical filter is composed of dielectric mirrors, which are formed from layered stacks of various dielectric films. More specifically, the dielectric mirrors can be formed of alternating dielectric layers with different optical properties. For example, an oxide and nitride (e.g., SiO2 and Si3N4) can be formed in alternating layers to produce the dielectric mirrors.
The optical filter can be formed directly on top of a photodetector region (also referred to as an optical sensor region) that includes a photo-sensor such as a PN junction or PIN junction photo-diode. The optical filter is formed prior to (and thus, under) metallization on semiconductor wafers. The optical filter can have a surface substantially coplanar with the surface of active devices, which can be referred to as the active device surface. In addition, various structures can be formed on top of the optical filter to shield the edges of the filter, as well as direct and/or block light. In addition, multiple sensors can be formed on the same die with coplanar optical filters formed under the metallization.
In accordance with an embodiment, the semiconductor substrate 104 is a P+ anode substrate, and the semiconductor substrate 106 is a P-type epitaxial layer or a P-type substrate that is low doped for low capacitance. In accordance with an embodiment, the low doping concentration of carriers in the P-substrate 106 is less than 1×1015 atoms/cm3, and preferably about 1×1013 atoms/cm3. Additionally, the thickness of the substrate 106 is preferably between about 5 microns and 30 microns, and preferably about 20 microns. By contrast, the doping concentration of carriers in the P+ substrate 104 is between about 1×1017 atoms/cm3 and about 1×1019 atoms/cm3, and preferably about 1×1018 atoms/cm3. In such an embodiment, the carriers would preferably be fully depleted at a charge of about 1 Volt. In this embodiment, the layer 108 is a cathode layer, such as an N+ diffusion/implant layer, that is formed in the second substrate 106. An N-well region 110 in the second substrate 106 connects the cathode layer 108 to one or more metal surface connectors 112. For the remainder of the description, it will be assumed that the layers 104, 106 and 108 have the polarities described above. However, it is also within the scope of the present invention for the polarities to be reversed, i.e., for the semiconductor substrate 104 to be an N+ cathode substrate, the semiconductor substrate 106 to be an N-type epitaxial layer or a N-type substrate that is low doped for low capacitance, the layer 108 to be a P+ anode layer, and the wells 110 to be P-wells.
The substrate 106 includes a trench 114 that extends downward from a top surface 117 of the substrate 106. In
The optical filter 120 is formed within the trench 114, and in accordance with an embodiment, a top surface 127 of the optical filter 120 is coplanar with the top surface 117 of the substrate 106.
In accordance with an embodiment, the optical filter 120 comprises a stack of alternating dielectric layers 122 and 124, which are formed over the cathode layer 108 such that the cathode layer 108 is adjacent to the bottom of the filter stack. The optical filter 120 is formed to have a vertical sidewall 126. In one embodiment, optical filter 120 can include alternating layers of an oxide and a nitride (e.g., SiO2 and Si3N4) at preselected thicknesses, which form dielectric mirrors.
In
A top surface of the optical sensor device 100 can optionally have a passivation layer, which is etched to expose the optical filter 120. In addition, an optional etch stop layer can be formed in the layers of the optical filter 120.
The pair of optical sensor devices 400a and 400b can provide the optical sensor array 600 with stereo sensing capabilities. In addition the spacing between the optical sensor devices 400a and 400b can be varied to change stereoscopic sensitivity. The pattern of the gratings 430a and 430b can also be optimized as needed such that each pattern can be the same or different for the sensor devices 400a and 400b. For example, a top view of the opaque grating of one of the sensor devices 400a and 400b can resemble the grating pattern 510 in
The sensor array 700 also includes a micro-lens 740a, 740b over each respective grating 430a, 430b. The micro-lenses 740a, 740b focus light over each of the sensor devices 400a, 400b. The micro-lenses 740a, 740b can be formed on top of a top passivation surface 744a, 744b that is on top of the grating 430a, 430b. In alternative embodiments, a micro-lens can be used on only one of the optical sensor devices, or on additional optical sensor devices when more than two optical sensor devices are implemented in the sensor array 700.
Where multiple optical sensor devices (e.g., 400a and 400b) are included in a same die to form a monolithic sensor array, e.g., as in
The grating 830 has a “venetian blind” configuration that provides for an angled incidence of light directed to photodetector region 810. The grating 830 also provides a peripheral shield for blocking light from the periphery of optical filter 120. The optical sensor device 800 can be used, for example, to detect light having a specific incidence angle (or range of angles).
The various metal gratings discussed above can be optimized to “filter” light based on a desired incidence angle. In addition, the metal gratings can be configured as “collimators” to align and channel the light to the surface of the photodetector region. The present FEOL formed optical filters can be covered with BEOL formed metal stacks to achieve a desired grating pattern, with no modifications to the process flow of a conventional device fabrication process.
The BEOL dielectric layers, e.g., passivation, inter-metal-dielectrics (IMD), inter-level-dielectrics (ILD), and the like, can be formed over the optical filter 120, after the optical filter 120 is formed in the trench 114. The BEOL dielectric layers, or portions thereof, can thereafter be removed to expose at least a portion of the optical filter 120. For example, a mask and etch approach can be used, such as a dry etch to etch-stop layer formed to protect the top surface 127 of the optical filter 120. The metal stack can also be used as a boundary for the etch as edges of a mask can overlap the metal shield at the edges.
Various methods can be employed in fabricating the optical sensor devices discussed above on a wafer. Such fabrication methods are described with reference to the drawings as follows.
In one fabrication method, a planar optical sensor device 100 is formed having an optical filter with edge shielding, as was shown in
Referring to
The cathode layer 108 is then formed in the substrate 106, such as by an N+ cathode implant at the bottom 118 of the trench 114, as shown in
As depicted in
At this point, active devices (if any are to be added to the die) are fabricated for the optical sensor device (or for a separate device on the same die) by conventional methods. This can include completion of any wells or diffusions, active area definition (STI or LOCOS), gate oxide and gate electrode patterning, and source/drain diffusions. The active area definition can be achieved using shallow trench isolation (STI) or local oxidation of silicon (LOCOS), but is not limited thereto. The active devices can be, for example, complementary metal-oxide-semiconductor (CMOS) devices, but are not limited thereto. The CMOS devices and/or other devices are fabricated outside of the photodetector area. One or more interlayer dielectrics (ILDs) 940 and other dielectrics are formed as a result of the device formation and isolation, as shown in
After completing the active devices, an ILD 942 is deposited, and a plurality of contacts 112 are formed such as tungsten plugs, which are coupled to deep wells regions 110. A metallization process is then performed by depositing a metal, patterning the metal, and etching the metal to form a metal layer 332. Inter-metal-dielectric layers 948 are then deposited, as shown in
In certain embodiments, the passivation layer 950 and the various dielectric layers 942 and 948 are left intact. In such embodiments, the type and thickness of the passivation layer 950, and types, number and thicknesses of dielectric layers 942 and 948 should be taken into account when designing the optical filter 120 within the trench 114, because these additional layers may affect the optical response of the final optical filter device. For the purpose of this disclosure, the BEOL dielectric layers (e.g., 942 and 948) and the passivation layer 950 are not considered part of the optical filter 120. However, if the BEOL dielectric layers (e.g., 942 and 948) and the passivation layer 950 provide optical filtering, they can be considered part of a BEOL optical filter that is located above the FEOL optical filter 120.
In another fabrication method, the steps described above with respect to
As illustrated in
In an alternative fabrication method, the steps described above with respect to
In another method, various dummy patterns can be formed over an optical filter area to prevent dishing within the exposed area, particularly if the optical sensor is large.
In a further method, the steps described above with respect to
As mentioned above, the dielectric materials used to form the optical filter 120 (or 220) 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 optical filter 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 coating can be designed as a long-pass or short-pass filter, a bandpass or notch filter, or a mirror with a specific reflectivity.
In accordance with specific embodiments of the present invention, an optical filter is used to shape the spectral response of the underlying photo detector region to obtain a true human eye spectral response, i.e., a response that is similar to that of a typical human eye response. Alternative spectral responses are possible, and within the scope of the present invention.
In the above described embodiments, the optical filter formed in the trench was generally described as including dielectric mirrors formed of alternating dielectric layers with different optical properties. A benefit of forming a filter using such dielectric layers is that they can withstand front-end-of-line (FEOL) semiconductor fabrication processes including thermal processes at temperatures of up to at least 1,200 degrees Celsius. Additionally, such dielectric layers can be used to produce high performance optical filters. However, the optical sensor devices of embodiments of the present invention can include alternative types of filters, so long as the filters can be formed as part of a FEOL fabrication processes, e.g., so long as such alternative filters can withstand thermal processes at temperatures of up to at least 1,200 degrees Celsius. For example, semiconductor optical filters can be formed as part of a FEOL fabrication process. Such semiconductor optical filters can include alternating semiconductor layers with different bandgaps. Exemplary semiconductor layers that can be used to form a semiconductor optical filter include, but are not limited to, Gallium nitride (GaN), Aluminum gallium nitride (AlGaN), Indium phosphide (InP) and Gallium arsenide (GaAs).
The high level flow diagram of
After the trench is formed, at step 1304 a photodetector region is formed under the trench, e.g., as was described above with reference to
At step 1306, an optical filter (e.g., 120 or 220) is formed in the trench and over at least a portion of the photodetector region. For example, as was described above with reference to
At step 1308, one or more metal structures that extend above a top surface of the optical filter are formed. Step 1308 can include forming at least one metal connector (e.g., 112) beyond a periphery of the top surface of the optical filter. Additionally, step 1308 can include forming at least one metallization layer (e.g., 128) over at least a portion of the top surface (e.g., 127) of the optical filter (e.g., 120). As was described above with reference to
In accordance with various embodiments, steps 1302, 1304 and 1306 are performed as part of a FEOL semiconductor fabrication process, and step 1308 is performed as part of a BEOL semiconductor fabrication process.
Referring to the system 1400 of
In accordance with an embodiment, one or more of the optical sensor devices (e.g., device 100, 200, 300, 400 or 800, or array 600 or 700) that include a filter (e.g., 120 or 220) described herein can be included in a same die and/or a same system along with one or more further optical sensor devices that is/are covered by a light blocking material (e.g., a metal layer) that does not let any light through. The optical sensors devices 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 optical sensor devices (including a filter 120 or 220) described herein 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)). By forming optical sensors device(s) that are covered by the light blocking material adjacent to the optical sensor device(s) (including a filter 120 or 220) described herein, the dark current generated by optical sensors device(s) covered by the light blocking material can be subtracted from a current generated by the optical sensor device(s) (including a filter 120 or 220) described herein, to remove the affects of the dark current.
Alternatively, or additionally, one or more naked optical sensor devices (that do not include a filter) can be included in a same die and/or a same system along with one or more of the optical sensor devices (including a filter 120 or 220) described herein. The naked optical sensor device(s) will detect both ambient visible light and ambient IR light. Assume the filter(s) 120 or 220 of the optical sensor device(s) described herein are 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.
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.
This application claims priority under 35 U.S.C. 119(e) to U.S. Provisional Patent Application No. 61/496,336, entitled FRONT-END OPTICAL FILTER DEVICES AND FABRICATION METHODS, filed Jun. 13, 2011, and U.S. Provisional Patent Application No. 61/534,314, entitled OPTICAL SENSOR DEVICES INCLUDING FRONT-END-OF-LINE (FEOL) OPTICAL FILTERS AND METHODS FOR FABRICATING OPTICAL SENSOR DEVICES, filed Sep. 23, 2011, both of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61496336 | Jun 2011 | US | |
| 61534314 | Sep 2011 | US |
