Rapid prototyping of single-photon-sensitive silicon avalanche photodiodes

A chip-to-chip integration process for rapid prototyping of silicon avalanche photodiode (APD) arrays has been developed. This process has several advantages over wafer-level 3D integration, including: (1) reduced cost per development cycle since a dedicated full-wafer read-out integrated circuit (ROIC) fabrication is not needed, (2) compatibility with ROICs made in previous fabrication runs, and (3) accelerated schedule. The process provides several advantages over previous processes for chip-to-chip integration, including: (1) shorter processing time as the chips can be diced, bump-bonded, and then thinned at the chip-level faster than in a wafer-level back-illumination process, and (2) the CMOS substrate provides mechanical support for the APD device, allowing integration of fast microlenses directly on the APD back surface. This approach yields APDs with low dark count rates (DCRs) and higher radiation tolerance for harsh environments and can be extended to large arrays of APDs.

BACKGROUND

Geiger-mode avalanche photodiodes (GmAPDs) can detect single photons and time stamp their arrivals with sub-nanosecond precision. Such arrays are useful in lidar applications as well as high-speed optical communication. Silicon APDs have low defect densities, leading to low dark count rates with minimal cooling. Silicon also offers superior material uniformity leading to high-yielding low-cost detector arrays. Silicon provides exceptional detection performance in the visible and NIR wavebands (400-900 nm).

Typically, silicon APD arrays are fabricated on one wafer and integrated onto respective readout integrated circuits (ROICs) fabricated on a separate complementary metal-oxide-silicon (CMOS) wafer. This provides freedom to optimize the detector and CMOS fabrication processes separately. It also provides the freedom to mate different detector designs to existing ROICs for detection optimization in different wavebands or, conversely, to mate existing detectors to different ROICs for different functionality.

FIG. 1Aillustrates wafer-to-wafer process for integrating APDs with ROICs to make a silicon GmAPD array chip. The ROICs are fabricated in a CMOS wafer110by a CMOS foundry run that can cost $250,000 to $10,000,000. In the meantime, the APDs are fabricated in a detector wafer120, in a process that takes up to 24 months. The detector wafer120is bonded to the CMOS detector wafer110in a wafer bonding step101that yields a bonded detector/CMOS readout wafer130that can be diced into individual APD/ROIC chips. Although this process creates great defect-free single-photon imager, it is too expensive and time-consuming to use for fabricating a handful of parts.

Three-dimensional (3D) APD/ROIC integration processes resulting in high pixel yield and spatial uniformity have been demonstrated using the Ziptronix Direct Bond Interconnect® (DBI) process. After 3D integration, these APD arrays need to be backside-illuminated, requiring uniform thinning to a target thickness that is a small fraction of the initial APD wafer thickness. These 3D integration and backside illumination processes are usually done on the wafer scale, requiring a costly dedicated full-wafer CMOS foundry run and careful co-design of the APD device layout and reticle stepping plan. For an advanced CMOS technology node, ROIC fabrication alone can cost millions of dollars.

Chip-to-chip integration, therefore, is favorable for prototyping hybridized APD arrays. For silicon, however, the APD substrate is opaque and needs to be removed for the APDs to detect light. This leads to challenges as the active device (APD) layer is typically less than 10 μm in thickness for APDs designed for visible wavelengths. Uniformly removing the bulk of the silicon substrate and leaving a uniform 10 μm thick silicon APD detector is challenging for either bulk grinding or chemical etching over a non-planar topology, such as that of an APD layer bump-bonded to a ROIC. In the past, a method for producing mechanically stable chips on quartz wafer has been used for bump-bondable silicon APDs. However, this complex process involves two wafer bonding steps and yields a low number of devices. Moreover, the imager is illuminated through a quartz support layer, limiting the speed (f-number) of microlenses that can be used to increase the detector fill factor.

FIG. 1Billustrates a bump-bonding process for making silicon APD chips. In the first step151, a silicon handle wafer150is oxide bonded to a device wafer152. In the second step153, the device wafer152is thinned to yield a device layer154on the silicon handle wafer150. The APDs are formed in the device layer154in step155, then the device layer154is bonded to a quartz handle wafer160in step157for mechanical stability. The silicon handle wafer is removed in step159, opening contacts in the device layer154for bump bonding to a CMOS ROIC162in step161. This process yields high-quality devices but is time consuming because it involves both backside processing in step155and frontside processing in step159. It also includes the addition of the quartz handle wafer160in step157, which hinders integration of other components with the APD array. The quartz handle wafer160can be thinned mechanically, but too much thinning can compromise the APDs.

SUMMARY

This disclosure relates to an approach for rapid die-level assembly of fully depleted, backside-illuminated silicon imaging arrays to CMOS readout integrated circuits (ROICs). This approach comprises 1) fabrication of a custom silicon-on-insulator (SOI) wafer engineered with a built-in backside contact and passivation layer and 2) removal of the handle wafer after the silicon imaging array has been bump-bonded to the ROICs. The specialized SOI wafer facilitates uniform silicon imager substrate removal by selective etching at the die level after bump bonding. The integration process has several advantages over wafer-level 3D integration, including: 1) reduced cost per development cycle since a dedicated full-wafer ROIC fabrication is not needed, 2) compatibility with ROICs in chip-format from previous fabrication runs, and 3) accelerated schedule.

This approach can be applied to Geiger-mode APD (GmAPD) arrays, charge-coupled devices (CCDs), and active pixel sensors (CMOS imagers). These imager architectures have a depleted region of silicon in the bulk of the imager where the photodetection occurs. The back surface is passivated to establish this depleted region without excessive back surface dark current. Electrical performance of APD arrays made using the inventive technique(s) show 100% pixel connectivity and excellent yield before and after substrate removal. The approach is beneficial for development of new detector arrays for various sensitive light sensing applications.

This approach for fabricating an imaging device can be implemented as follows. A thermal oxide layer is deposited on a handle wafer. A backside passivation layer is formed on a device wafer. The backside passivation layer on the device wafer is bonded to the thermal oxide layer on the handle wafer to form an engineered substrate having the thermal oxide layer and the backside passivation layer between a handle layer and a device layer. An imaging array is formed in the device layer of the engineered substrate, then the engineered substrate is diced into an imaging array chip comprising the imaging array. A read-out integrated circuit is bonded to the imaging array chip, then at least a portion of the handle layer is removed from the imaging array chip.

The handle wafer can be a silicon handle wafer, the thermal oxide layer can comprise silicon dioxide, and the device wafer can be a silicon device wafer. The device layer can have a resistivity of at least 20 Ω-cm.

Depositing the thermal oxide layer on the handle wafer can include growing the thermal oxide layer to a thickness of about 20 nm to about 200 nm, e.g., so that the thermal oxide layer can be used as a visible-range anti-reflection coating in the completed device. The thermal oxide layer may be a first thermal oxide layer, in which case forming the backside passivation layer on the device wafer comprises depositing a second thermal oxide layer on the device wafer and implanting a p+dopant into the second thermal oxide layer to form the passivation layer.

Forming the imaging array in the device layer may include forming an avalanche photodiode, a charge-coupled device, and/or an active pixel sensor.

After bonding the backside passivation layer to the thermal oxide layer and before forming the imaging array, the backside passivation layer and the thermal oxide layer can be thermally annealed. After bonding the backside passivation layer to the thermal oxide layer and before dicing the engineered substrate into the imaging array chip, the device layer can be thinned to a thickness of about 10 μm.

Before bonding the read-out integrated circuit to the imaging array chip, the read-out integrated circuit and/or the imaging array chip can be tested, e.g., to ensure that they are fully functional and compatible before being bonded together.

Bonding the read-out integrated circuit to the imaging array chip can comprise forming at least one bump bond between the read-out integrated circuit and the imaging array chip. In this case, epoxy can be disposed between the imaging array chip and the read-out integrated circuit to secure the imaging array chip to the read-out integrated circuit.

Removing the handle layer portion may include chemically etching the handle layer to the thermal oxide layer. The handle layer can be mechanically thinned before being chemically etched to the thermal oxide layer. Optionally, a microlens array can be disposed directly on a surface of the imaging array chip opposite from the read-out integrated circuit.

The fabrication approach disclosed here can yield an imaging device comprising a read-out integrated circuit, an imaging array bump-bonded to the read-out integrated circuit, a passivation layer disposed on the imaging array, and a thermal oxide layer disposed on the passivation layer. The passivation layer can be thermally bonded to the thermal oxide layer. The imaging device may also include a microlens array, bonded directly to the thermal oxide layer, to direct incident light to the imaging array.

Fabricating an imaging device can also be implemented by depositing a first silicon dioxide layer having a thickness of about 60 nm to about 100 nm on a silicon handle wafer having a resistivity of at least about 20 Ω-cm (e.g., 160 Ω-cm) and by depositing a second silicon dioxide layer having a thickness of about 5 nm on a silicon device wafer having a resistivity of about 20 Ω-cm (e.g., 160 Ω-cm). BF2or another suitable dopant is implanted into the second silicon dioxide layer to transform the second silicon dioxide layer into a passivation layer. Then the first silicon dioxide layer is thermally bonded to the passivation layer to form an engineered substrate having the first silicon oxide layer and the passivation layer between a silicon handle layer formed of the silicon handle wafer and a silicon device layer formed of the silicon device wafer. The silicon device layer is thinned to a thickness of about 10 μm, then an avalanche photodiode (APD) array is formed in the silicon device layer. The engineered substrate is diced into an APD chip comprising the APD array. This APD chip is bump-bonded to a read-out integrated circuit before the silicon handle layer is removed from the APD chip. If desired, the APD chip can be tested before being bump-bonded to the read-out integrated circuit. Optionally, a microlens array to the APD chip after removing the silicon handle layer from the APD chip.

BRIEF DESCRIPTIONS OF THE DRAWINGS

FIG. 1Aillustrates a wafer-to-wafer integration process for making avalanche photodiode (APD) arrays bonded to CMOS readout integrated circuits (ROICs).

FIG. 1Billustrates a bump-bonding process for making APD arrays bonded to CMOS ROICs on side and quartz handles on the other side.

FIG. 2illustrates a chip-level bump-bonding process for making an APD chip or other imaging array chip bonded to a CMOS ROIC chip.

FIG. 3Aillustrates a process for making an engineered substrate. The 200 mm diameter device wafer with appropriate resistivity is implanted with BF2, after 5 nm thermal oxide is grown as the screening oxide. This implant layer is buried in the oxide-bonded interface and serves as the backside passivation of APDs, CCDs, or CMOS imagers. The handle wafer has 100 nm thermal oxide grown to serve as the etch stop layer. After these processes, the two wafers are bonded together, and the device layer is thinned to 10 μm via wet etching.

FIG. 3Bshows a photograph of a finished engineered substrate. The engineered substrate is a specialized silicon-on-insulator (SOI) wafer which includes a 10 μm-thick 160 ohm-cm p-type silicon device layer with buried p+ implant for backside passivation on 100 nm thermal oxide bonding layer, which serves as the etch stop for the chip-level thinning process.

FIG. 3Cis a photograph of a wafer with finished frontside-illuminated APDs on engineered substrates. The size of each reticle is about 20×22 mm. Devices are fabricated on 200-mm wafers.

FIG. 4Ashows a cross section of an engineered substrate.

FIG. 4Bshows a cross section of an APD array formed in the device layer of an engineered substrate.

FIG. 5Bshows a 3D laser scan of the indium bumps ofFIG. 5A.

FIG. 6Aillustrates a chip-level thinning process. First, the silicon APD layer is bump-bonded to a read-out integrated circuit (ROIC) and under-filled with epoxy. The epoxy provides mechanical support after the device layer is thinned down to 10 μm. The APDs built on an engineered substrate have an oxide etch stop layer 10 μm away from the bonding interface. Then the bulk of the carrier substrate is removed through a mechanical process using the dicing saw. Finally, the remaining carrier substrate is removed using the chemical etching process stopping on the buried etch stop layer. The chemical etch provides excellent selectivity to oxide.

FIG. 6Bshows a cross section of an APD chip bonded to a ROIC with a microlens array layer over the APD array.

FIG. 7Ashows two 32×32 hybridized APD/ROIC pairs are mounted on precision dicing tape in preparation for mechanical thinning down to 100 μm thick device layer.

FIG. 7Bshows a microscope image of a 32×32 APD ROIC after mechanical thinning. The vertical striations are due to dicing saw blade. The ROIC frame area is protected by polymer coating with blue dye added to aid visual inspection of the coating.

FIGS. 8A and 8Bshow spatial dark count rate (DCR) maps of the 32×32 APD array ofFIG. 7Bbefore mechanical thinning and after mechanical thinning, respectively. The DCR map is largely unchanged.

FIG. 9is a die image of a finished 32×32 APD ROIC after chemical thinning to 10 μm thick device layer. The surface of the APD device layer is black as 100 nm-thick oxide etch stop film also serves as an anti-reflection coating.

FIGS. 10A and 10Bshow spatial DCR maps of the 32×32 APD array ofFIG. 9before chemical thinning and after chemical thinning, respectively. The DCR is slightly higher after chemical thinning, possibly because of light leakage in the measurement setup.

FIG. 11is a plot of reverse bias I-V characteristics of silicon APDs fabricated on two engineered substrates (ES1, ES2) and two 20-μm thick, epitaxial 160-Ω-cm silicon on 0.02-Ω-cm p-type substrates (Epi1, Epi2).

DETAILED DESCRIPTION

Presently, silicon sensors are fabricated in a two- or three-stage process. Single-tier sensors like CCD or CMOS imagers are made in a two-stage process: (1) front-illumination, where device structures and metal interconnect networks are built on the front surface of the wafer, and (2) back-illumination, where the wafer is flipped over and the back surface is processed to optimize light collection. For hybrid sensors like GmAPD imagers (imagers composed of separate detector and ROIC tiers), an additional integration stage occurs between front- and back-illumination where the detector and ROIC tiers are mated. The process of front-illumination, hybridization, and back-illumination is quite long (e.g., 14-18 months).

Here, we introduce a much faster approach to making silicon imaging sensors. In our approach, a fully depleted silicon imager, such as an APD array, CCD, or active pixel array, is built on a specialized SOI wafer with a built-in backside passivation layer. The finished imaging array wafer is diced, bump-bonded to a ROIC chip, underfilled with epoxy, and the handle wafer is removed by selective etching at the die level. This approach has several benefits over other imaging array hybridization processes. First, the schedule to produce the final array is dramatically accelerated, as it does not include a time-consuming backside-illumination process after the frontside-illumination process. Second, the finished imaging arrays are electrically isolated from the handle wafer and any defects that might be introduced during the backside thinning, so the dark count rate (DCR) and current-voltage (I-V) characteristics measured just after the imaging array front-illumination fabrication are similar to those measured on the finished device. Hence, accurate DCR statistics can be studied early in the process development, e.g., before completing the backside-illumination process. Third, this approach uses the ROIC and epoxy underfill as the mechanical support for the device (e.g., APD) layer and does not need a transparent quartz layer above the silicon for mechanical support. The elimination of the transparent quartz layer allows placement of low-f-number microlenses directly on the surface of the silicon. Finally, this chip-level process enables efficient use of ROICs and imaging array dies by ensuring that known-good imager arrays are hybridized to known-good ROICs.

FIG. 2illustrates our process for rapidly prototyping single-photon-sensitive silicon APDs or other silicon imaging arrays. In this process, ROICs are fabricated in a CMOS wafer210in a (shared) foundry run201. The CMOS wafer210is diced into individual ROIC chips212, which can tested and characterized211. In the meantime, a detector wafer220, also called an engineered substrate, is created separately. This detector wafer220can be a silicon wafer for making silicon imaging arrays or a germanium wafer for making germanium imaging arrays. This detector wafer220is used as starting material for fabrication of the silicon imaging array. The finished arrays are characterized and then processed with indium bumps230in preparation for dicing and bump bonding to the ROIC chips212. The detector wafer220is then diced into individual silicon imaging array chips222, which are bump bonded to the ROIC chips212in step231. Finally, the devices undergo chip-level thinning to remove the SOI handle wafer, using the SOI buried oxide layer as an etch stop to form completely integrated imaging array/ROIC devices240. These steps are described in greater detail below.

The process for rapidly prototyping imaging arrays inFIG. 2has several advantages over the bump-bonding fabrication process shown inFIG. 1B. First, it is faster, in part because it does not involve both frontside and backside processing. Second, the imaging arrays are electrically complete before bump bonding, so they can be tested before they are mated to the ROICs. For example, each imaging array chip can be mated to a ROIC with the best-matched electrical characteristics. This increases the yield of the finished devices. Third, the imaging array is not mated to a quartz handle wafer, so it can be mated directly to a fast microlens array that increases the radiometric collection efficiency. Fourth, if its thickness is chosen properly, the thermal oxide layer can act as anti-reflection coating on the finished device.

Engineered Substrates

The imaging arrays layer is typically 10 μm thick. This is thick enough to absorb most of the incident light at visible and near infrared wavelengths, while thin enough not to introduce excessive timing jitter or require high operational voltages. This thickness is not sufficient to provide mechanical support for an entire wafer, so imaging arrays are typically fabricated on a SEMI standard thickness wafer, which is 725 μm thick. After processing the frontside-illuminated imaging arrays, the majority of this silicon (or germanium, for a germanium imaging array) is removed as it blocks the light from entering the active region of a backside-illuminated device. Removing all but 10 μm of silicon over the array involves precision removal with better than 1% non-uniformity. Such controlled removal of silicon can be done using an etch stop layer. Silicon dioxide is an excellent etch stop layer with various chemical etchants available for highly selective etching of silicon over oxide. The silicon dioxide layer can also act as an anti-reflection layer on the finished imaging arrays if its thickness is chosen based on the desired detection wavelength.

Commercial SOI wafers are available from many sources. However, these wafers typically do not have the backside passivation, the desired resistivity (160 Ω-cm), or the desired thickness. Hence, we fabricate our SOI wafers. These specially fabricated SOI wafers are sometimes called engineered substrates and can be made ahead of time and processed as desired to form APD chips.

FIG. 3Aillustrates a process for making an engineered substrate. We grow a layer312of thermal oxide, such as silicon dioxide, on a handle wafer310(step301) made of silicon. The thickness of this thermal oxide layer312may be selected so that the thermal oxide layer312acts as an anti-reflection coating on the finished imaging array. For visible wavelengths, this thickness is about 20 nm thick to about 200 nm thick.

In step303, we grow a thin (e.g., 5 nm thick) thermal oxide layer on a device wafer320. The device wafer has a resistivity of at least 20 Ω-cm (e.g., 160 Ω-cm). After thermal oxidation, the device wafer is implanted with a p+doping, such as 1.5×1014cm−2BF2dopant324at 5 keV (step303), if the body of the semiconductor is p-type. (Likewise, n doping will work if the body of the semiconductor is n-type.) This turns the thermal oxide layer into a backside passivation layer322; for APDs, this backside passivation layer322reduces the surface contribution to DCR and collects the hole current generated by the avalanche process in the operating APDs. The implantation is done after oxidation to reduce diffusion of the boron-doped backside passivation.

After implantation, the two wafers are oxide bonded face-to-face (step305) and annealed at 900° C. for 30 minutes in a nitrogen ambient atmosphere (step307), which both activates the BF2implant and provides high bond strength. The bonded wafers form the engineered substrate220, with the device wafer becoming a device layer and the handle wafer becoming a handle layer and the bonded passivation and thermal oxide layers sandwiched between the device and handle layers.

After the bonding anneal, the device layer320is thinned down to 10 μm thickness via chemical wet etching using HF/nitric/acetic acid mixture. The engineered substrate220may be rotated during this wet etch to achieve good etch uniformity. The device layer's thickness is monitored while thinning. After thinning, the device layer's thickness is measured using white-light interferometry. Thickness uniformity of ±0.2 μm is typically achieved for a 10 μm thick device layer and removal non-uniformity better than 0.3% is achieved over the planar wafer. Edges of the unbonded device layer320are removed with a dicing saw to prevent unbonded silicon from creating particles and defects during the subsequent processing of the APDs.

FIG. 3Bshows a photograph of a finished engineered substrate. The engineered substrate is a specialized silicon-on-insulator (SOI) wafer which includes a 10 μm-thick 160 Ω-cm p-type silicon device layer with buried p+ implant for backside passivation on 100 nm thermal oxide bonding layer, which serves as the etch stop for the chip-level thinning process.

FIG. 3Cis a photograph of a wafer with finished frontside-illuminated APDs on engineered substrates. The APDs are patterned photolithographically in a process that involves masking and illuminating the device layer. In the example shown inFIG. 3C, the APDs were fabricated on a 200-mm wafer with reticles that were about 20 mm×22 mm. After this frontside-illumination (photolithographic fabrication of the APDs), the APDs326are electrically complete, allowing for accurate characterization of the APDs' dark count rates (DCRs). The thermal oxide layer312isolates the APDs326from the handle layer310, and the back passivation is already in place. This allows rapid design optimization for lowering the DCRs.

FIGS. 4A and 4Bshow cross sections of the engineered substrate220before and after fabrication of the APDs, respectively. Before fabrication of the APDs, the device layer310is about 10 μm thick and is on the p+ passivation layer322, which is on a 100 nm thick thermal oxide (e.g., silicon dioxide) layer312, which in turn is on the 725 μm thick handle layer310. The APDs326are formed in the device layer310, with electrical contacts328on the APDs326. The thermal oxide layer312insulates the APDs326from the handle layer310: current flows between contacts328via the APDs326and the passivation layer322as illustrated inFIG. 4B.

Hybridization and Chip-Level Processing

After the wafers are fabricated, under-bump metallization (UBM), which prevents a highly resistive junction from forming between aluminum metallization and indium, is patterned via lift-off.FIG. 5Ashows 8-μm-high indium bumps patterned over the UBM.FIG. 5Bshows a 3D scan of the indium bumps. The bump bonding can be done in a Smart Equipment Technology FC150 automated die/flip chip bonder. Here, the 32×32 100 μm-pitch array is bonded to an asynchronous ROIC that has been developed for optical communication. After bump bonding, the assembled chip is tested for functionality. For 32×32 100 μm-pitch arrays, 100% pixel connectivity is typically achieved.

FIG. 6Ashows the fabrication process from hybridized chip to final thinning. At this point, the APD chip222is bonded to the ROIC chip212with bump bonds230underfilled with epoxy602. Next, most of the handle layer310, which is approximately 725 μm thick, is removed through mechanical sawing (step601). The chip is mounted on a precision-thickness dicing tape, as shown inFIG. 7A. The optical focus provides the height of the ROIC212and the height of the detector array (APDs)326. Using a precision stage, the dicing saw is brought 100 μm above the ROIC212to remove the bulk of the handle layer310. Excellent device yield is achieved through this process. Because the APDs326are separated from the handle layer310by the thermal oxide layer312, the mechanical thinning does not degrade the quality of the APD material.

The remaining silicon on the handle layer310is removed through chemical etching (step603), with the thermal oxide layer312acting as an etch stop. The chemical etching is a dry, vacuum-based process that uses XeF2to selectively and isotropically remove silicon through a reaction that yields xenon and SiF4. The XeF2sublimates from solid crystals to form the vapor phase etchant and provides excellent selectivity (e.g., about 1000:1) over oxide for silicon etching. If desired, the thermal oxide layer312can be etched after the silicon handle layer310has been removed, e.g., by etching with HF2, and replaced with a multi-layer anti-reflective coating to increase the imager's sensitivity over a broader wavelength range.

FIG. 6Bshows an APD/ROIC chip640with an integrated microlens array642. The microlens array642is disposed directly on the APD array326, which is about 10 microns thick, bonded to the ROIC212with bump bonds230and epoxy602. As the microlens array642can be right next to the APD array326, the focal lengths of the microlenses can be arbitrarily small (e.g., 100 μm) as long as the microlenses are align effectively with respect to the APDs.

Experimentally Demonstrated Performance of Fabricated APD Arrays

APD fabrication was performed on engineered substrates like those described above as well as on epitaxial silicon on p+ substrates, the latter of which serve as control wafers. The epitaxial layers of the control wafers have a resistivity of 160 Ω-cm, which is the same as the resistivity of the engineered substrates. The thickness of the epitaxial silicon was approximately 20 μm.

FIGS. 7-10illustrate fabrication of the APDs on the engineered substrate and the DCRs of the APDs during the fabrication process.FIG. 7Ashows two 32×32 hybridized APD/ROIC pairs are mounted on precision dicing tape in preparation for mechanical thinning down to 100 μm thick device layer. The pads on the ROICs were coated with protective polymer, then attached to the precision dicing tape. Once the APD/ROIC pairs were mounted securely, a dicing saw was used to remove the bulk of the substrate as explained above with respect toFIG. 6A.FIG. 7Bshows a microscope image of a 32×32 APD ROIC after mechanical thinning. The vertical striations on the detector level are due to the dicing saw blade. The ROIC frame area is protected by polymer coating with blue dye added to aid visual inspection of the coating.FIGS. 8A and 8Bshow spatial dark count rate (DCR) maps of the 32×32 APD array ofFIG. 7Bbefore and after mechanical thinning, respectively. The DCR map is largely unchanged with a slight increase in the median dark count rate.

FIG. 9is a die image of a finished 32×32 APD/ROIC after chemical thinning to 10 μm thick device layer. The surface of the APD device layer is black as 100 nm-thick oxide etch stop film also serves as an anti-reflection coating.FIGS. 10A and 10Bshow spatial maps of the DCRs for the 32×32 APD array ofFIG. 9before and after chemical thinning, respectively, at 36.6 V. The median DCR increased slightly after chemical thinning, from 13 kHz to 18 kHz. Without being bound by any particular theory, it is possible that the defect density within the silicon APD array increased during the chemical thinning process due to film stress exerted on the 10 μm-thick silicon layer by the epoxy and bumps.

FIG. 11is a plot of reverse bias I-V characteristics of silicon APDs fabricated on two engineered substrates (ES1, ES2) and two 20-μm thick, epitaxial 160-Ω-cm silicon on 0.02-Ω-cm p-type substrates (Epi1, Epi2) for control. The breakdown voltages of the four devices were within 0.2 V of each other and around 30 V. The DCRs were also compared at 5 V over-bias using passive quenching setup using a resistor. The average dark count rate was 14 kHz on the engineered substrate compared to 0.5 kHz on the control wafer for a device with 30 μm cathode diameter and 25 μm multiplier diameter. The higher DCR in the engineered substrate may be due to thermally generated current near the buried silicon dioxide interface, which was expected to have a higher defect density than the epitaxial silicon/p+ silicon interface in the control wafer. In such a case, the higher DCR can be reduced by the improvement of passivation of the backside interface, e.g., by increasing peak doping levels in this back surface field.

The mask set included a couple variations of the 32×32 array of APDs. One of the design variations was the number of contacts to the cathode region. The contacts were made by etching vias in the silicon dioxide and forming a Ti/TiN liner and tungsten plug. The Ti/TiN liner directly in contact with the n+ cathode layer may have been a source of dark current. A benefit of the engineered substrates is that electrical connections to the APDs are complete at the end of the front-illumination process, and hence valid conclusions can be drawn at the time of completion of the front-illumination with regard to the dark count rates. Reducing the number of contacts per pixel from nine to two reduces the DCR by a factor of two or more as shown in TABLE 1. This result suggests that the number of contacts should be reduced on these APD designs. Also, higher doping levels in the cathode layer may suppress the electrons generated at the contact surface and lower their contribution to the total DCR.

TABLE 1Dark count rate measurements at room temperature for the two designvariations with different numbers of contacts to the pixel.32 × 32 Design VersionMedian DCR (kHz) @ 36.6 VV1 (3 × 3 contacts per pixel)31V2 (2 × 1 contacts per pixel)13
Conclusion