An avalanche photodiode (APD) has a first semiconductor substrate having a first doping type. A first semiconductor layer is on top of the first semiconductor substrate. The first semiconductor layer is doped with the first doping type. A second epitaxial layer is on top of the first semiconductor layer. The second epitaxial layer is in-situ doped with the first doping type at a concentration higher than a concentration of the first doping type in the first semiconductor layer. A third epitaxial layer is on top of the second epitaxial layer. The third epitaxial layer is in-situ doped with a second doping type. The doping of the third epitaxial region forms a first p-n junction with the doping of the second epitaxial layer, wherein a carrier multiplication region includes the first p-n junction, and wherein the third epitaxial layer forms an absorption region for photons. A first implanted region is within the third epitaxial layer. The implanted region is doped with the second doping type.

BACKGROUND

The present disclosure relates generally to avalanche photo diodes (APD), and more specifically to APDs used in arrayed devices.

2. Description of Related Art

An APD is a semiconductor photodetector that turns light into an electrical signal. A basic APD has two main regions: an absorption region and a multiplication region. Depending on the APD design, the absorption region may be on one side only of the multiplication region or on both sides. Photons that are absorbed in the absorption region generate an electron-hole pair. The electron (if absorption region is p-type) drifts or is carried by a low-level electric field to the multiplication region. The strong electric field in the multiplication region accelerates the electron to a point where the electron has enough energy to generate more electron-hole pairs through impact ionization within the multiplication region. Depending on the semiconductor and APD design, electrons, holes, or both that reach the multiplication region may be accelerated to the point that they may create additional electron-holes pairs. This process may continue indefinitely or until all carriers are swept out of the multiplication region without generation of additional carriers. Through this process of impact ionization, one electron-hole pair may generate hundreds or thousands more (or significantly more) electron-holes pairs. As the electrons and holes reach the terminals, an electrical current is generated, which may be detected and measured.

The number of electron-holes pairs generated from a single absorbed photon (i.e., the APD gain) varies based on the design and operating point of the APD. There are two main modes of operation for APDs: analog mode and Geiger mode.

In analog mode, the gain of the APD is a function of the APD structure and the reverse bias applied to the APD. Typically, the higher the reverse bias, the higher the gain. For example, an APD may exhibit a gain of 20 at 125V and a gain of 300 at 185V. Gains of up to 1000 or more are possible depending on the materials, the manufacturing process, and the design of the APD. Note that in analog mode, the electrical field in the multiplication region is not strong enough to create electron-hole pairs indefinitely. Eventually all of the carriers will be swept out of the device and the current will drop to zero until another photon generates an electron that reaches the multiplication region.

In Geiger mode, the APD is reverse biased higher than in analog mode and above the APD breakdown voltage. The reverse bias creates a very strong electric field that may result in a gain of 105or 106or the impact ionization process described above may even “latch” the APD and become self-sustaining. If the process becomes self-sustaining, then as long as the electric field is maintained, electron-hole pairs will continue to be generated and a current will continue to flow through the APD.

In addition to the operational mode of the APD, the reverse bias applied to the APD may also determine the probability of a generated electron or hole creating a detectable signal through an impact ionization process. In some APDs, the reverse bias may be set to reduce the probability of one carrier (e.g, a hole) triggering the APD as compared to the probability of the other carrier (e.g., an electron) triggering the APD.

If a photon generates a self-sustaining impact ionization process, either by a generated electron or hole, an APD may use a quench to reset itself. In particular, the APD may use a passive or active quench.

A passive quench may be implemented with a high value resistor connected in series between the cathode or the anode of the APD and the voltage source supplying the reverse bias to the APD. Once a photon generates an electron that triggers a self-sustaining impact ionization process in the multiplication region, a current starts to flow through the APD. The current will cause a significant voltage drop through the high value resistor. The voltage drop across the high value resistor will reduce the electric field in the multiplication region, which will reduce the chance that electrons and holes in the multiplication region will create additional electrons-hole pairs. Once the electric-field drops low enough, the impact ionization process will terminate and the APD will reset because the high value resistor will no longer have a voltage drop across it.

An active quench uses a quench circuit to detect a latched APD. Once the quench circuit detects that the APD has latched, the circuit may disconnect the APD from the voltage source or reduce the reverse bias being applied to the APD. Either of these actions will reduce the electric field in the multiplication region. Once the electric field drops low enough, the self-sustaining impact ionization process will terminate and the APD will stop conducting. The quench circuit may then restore the reverse bias to the APD to reset the APD for the next photon.

Other than the gain, there are several other important parameters that describe the performance of an APD. For example, quantum efficiency is the probability that a photon will generate an electron-hole pair in the absorption region and the electron or hole will reach the multiplication region and initiate an impact ionization process that does not terminate prematurely. Dark counting rate is the rate at which non-photon generated carriers initiate the impact ionization process. It is impossible to differentiate these signals from those generated by photons.

In addition to using APDs as individual discrete devices, multiple APDs may be used in integrated arrays. Arrayed APDs may be useful in, for example, imaging applications. A silicon photomultiplier (SiPM) is an example of a device using an array of APDs. In arrays of APDs, each APD may be known as a pixel of the array.

In addition to the performance parameters of the individual APDs, other performance parameters may be relevant to arrayed APDs. For example, cross-talk is the probability that an impact ionization process in one APD will trigger an impact ionization process in a neighboring APD.

Additional descriptions of APDs may be found in U.S. Pat. No. 7,759,623 and U.S. patent application Ser. No. 11/725,661, filed Mar. 20, 2007, published as US Patent Publication No. 2008/0012087 assigned to the assignee of the present invention, both of which are incorporated herein by reference in their entirety for all purposes.

BRIEF SUMMARY

An exemplary embodiment of an avalanche photodiode (APD) has a first semiconductor substrate having a first doping type. A first semiconductor layer is on top of the first semiconductor substrate. The first semiconductor layer is doped with the first doping type. A second epitaxial layer is on top of the first semiconductor layer. The second epitaxial layer is in-situ doped with the first doping type at a concentration higher than a concentration of the first doping type in the first semiconductor layer. A third epitaxial layer is on top of the second epitaxial layer. The third epitaxial layer is in-situ doped with a second doping type. The doping of the third epitaxial region forms a first p-n junction with the doping of the second epitaxial layer, wherein a carrier multiplication region includes the first p-n junction, and wherein the third epitaxial layer forms an absorption region for photons. A first implanted region is within the third epitaxial layer. The implanted region is doped with the second doping type.

DETAILED DESCRIPTION

The discussion below refers to exemplary embodiments of APDs made of silicon and having a p-type absorption region (e.g., doped with boron) designed for detecting photons with a wavelength of 400-500 nm. However, those skilled in the art will recognize that the concepts discussed below may apply equally to other semiconductors (e.g., Ge or InGaAs), other doping schemes (e.g., using an n-type absorption region), and to detecting other wavelengths.

FIGS. 1A-1Edepict a first embodiment of an APD at various stages during an exemplary process for fabricating an APD. WhileFIGS. 1A-1Edepict a single APD device on a substrate, it should be understood that during manufacturing many APDs are manufactured together on a single substrate or wafer. Additionally, the discussion below does not cover every step necessary to fabricate an APD. Those skilled in the art will recognize other necessary steps to fabricate an APD.

The first step of the exemplary process is providing the starting material.FIG. 1Adepicts wafer100that forms the substrate for the first embodiment. Wafer100may be about 450 μm thick and have n-type doping that results in a resistivity of about 50 mΩ·cm or less. Alternatively, wafer100may be about 200 to 500 μm thick and have a doping concentration of about 1019cm−3to 1020cm−3. Other wafer thicknesses, resistivities, and doping types may also be used.

FIG. 1Bdepicts three expitaxial layers,102,104, and106grown on top of wafer100. The thicknesses, doping, and material of layers102,104, and106(as well as the starting material) may be varied to exclude or include certain wavelengths of photons. In the current embodiment, the APD is designed to use silicon to detect photons with a wavelength of 400-500 nm based on the electrons generated when those photons are absorbed in the silicon. The absorption depth in silicon for these types of photons is less than 1 μm. Additionally, the current embodiment is designed to minimize the APD's response to generated holes. Therefore, the first exemplary embodiment has an absorption region within 1 μm of the surface of the APD in p-type semiconductor. However, in other embodiments designed to detect other wavelengths of photons, the location of the absorption region may be tuned by varying the thickness of layers102,104,106or the doping of these layers and the starting material. For example, the absorption region may be moved to below the p-n junction between layers104and106so that other photon wavelengths may be detected. The absorption region may be moved by reversing the doping types of wafer100and layers102,104, and106.

Layer102spreads the depletion region formed by the p-n junction between layers106and104. By spreading the depletion region, the capacitance of the first embodiment is reduced and the speed at which it operates may be increased. Layer102may have the same doping type (n-type) as the starting material and have a resistivity of 50-500 Ω·cm or a doping concentration of about 1012cm−3to 1015cm−3and a thickness of 10-50 μm. Layer102may be in-situ doped during its epitaxial growth.

Layer104forms part of a multiplication region. Layer104may have the same doping type (n-type) as the starting material and layer102and have a resistivity of 2-20 Ω·cm or a doping concentration of about 1014cm−3to 1016cm−3and a grown thickness of about 3-15 μm. Layer106forms an absorption region and part of the multiplication region. Layer106may have the opposite doping type (p-type) as the starting material, layer102, and layer104and may have a resistivity of 2-20 Ω·cm or a doping concentration of about 1014cm−3to 1016cm−3and a grown thickness of 3-15 μm. Layers104and106may also be in-situ doped during the epitaxial growth. While growing layer106, the doping from layer104may diffuse into layer106so that instead of a p-n junction forming at the physical boundary between layers104and106, a p-n junction is formed within layer106. The junction depth of this p-n junction may be, for example, 1-10 μm below the surface.

The absorption region may be moved to or include the other side of the p-n junction formed by layers104and106. This may tune the APD for specific applications. For example, in the current embodiment, if photon detection is to be based on generated holes in addition to generated electrons, holes generated on the other side of the p-n junction may also trigger the APD if the reverse bias applied to the APD is high enough and the electric field is shaped to allow holes to trigger the APD. Alternatively, the doping of layers102,104, and106, and wafer100may be reversed to allow electrons generated below the p-n junction to trigger the APD. This may be useful, for example, to detect other photon wavelengths in silicon.

Proper shaping of the electrical field in the absorption region and multiplication region may also be used to tune the performance of the APD. For example, a proper shape of the electrical field in the absorption region or multiplication region may reduce the probability of the APD being triggered by holes generated in the absorption region or on the other side of the p-n junction, even when the reverse bias applied to the APD is high. This may be useful, for example, when increasing the reverse bias of the APD to increase the probability of a generated electron triggering the APD while maintaining a low probability that a generated hole will trigger the APD.

Layers102,104,106may all be grown at once in one processing step (i.e., wafer100is not removed from the epitaxy chamber until all three layers are grown). Alternatively, the layers may be grown at different times in different epitaxy chambers. For an example of a process that has additional processing steps between the growths of layers104and106, see the discussion of a second embodiment with respect toFIG. 2below.

FIG. 1Cdepicts wafer100after channel stop regions108have been implanted. These regions may prevent leakage paths. Regions108may be the same type of doping as layer104(n-type).

FIG. 1Ddepicts wafer100after one or more implants has formed region110in layer106. This region helps shape the electric field in the multiplication region. Region110may be of the same type of doping (p-type) as layer106. Region110may also be formed so as to ensure the multiplication region does not extend to the exposed surface of layer106. The peak doping in region110near the exposed surface of layer106may be about 1020cm−3.

Optional regions112at either end of region110may further shape the electric field so as to prevent the multiplication region from expanding laterally. If region112is included, it may be formed by the same implants that forms110or by different implants. For example, the depth of region112may be varied independently of the depth of region110if the surface of layer106above region112has a different screen oxide thickness as compared to the surface of layer106above region110.

FIG. 1Edepicts the first embodiment122of an APD after forming anode electrodes118, cathode electrode120, insulating layer114, and anti-reflective coating116on wafer100. Anode electrodes118electrically contact regions110and112, and cathode electrode120electrically contacts wafer100. The first embodiment122includes absorption region126(dotted box) and multiplication region124(dotted box). The first embodiment122may be from 15-100 μm wide. Again, the first embodiment is designed to detect electrons generated by photons with wavelengths of 400-500 nm in from absorption region126.

The operation of a device formed as described above is as follows. When a large (e.g., >100V) reverse bias is applied between cathode electrode120and anode electrodes118, a strong electrical field will be formed in the multiplication region around the p-n junction formed by layers104and106. If a photon is absorbed into absorption region126, it may generate an electron-hole pair. The hole may be swept to anode electrode118by a weak electric field. The electron may drift or be swept by the weak electric field to multiplication region124where the strong electric field will accelerate it. If the electron gains enough energy, it may generate additional electron-hole pairs through impact ionization in multiplication region124. These carriers will be separated by the strong electric field and may trigger additional electron-hole pairs through impact ionization before being collected by the electrodes and producing a current. If the reverse bias is large enough and the device is properly designed, this process may be self-sustaining. In some cases, the reverse bias will be high enough to increase the chance that generated electrons will trigger the APD but will keep the probability that a hole will trigger the APD to a minimum.

FIG. 1Fdepicts APD128, which is a variation of the first embodiment fabricated with a similar process as discussed above with respect toFIGS. 1A-1E. APD128is similar to the first embodiment122except in APD128, region130further shapes the electric field in the multiplication region. Region130may be formed by first implanting into layer104prior to the growth of layer106. If the implant is shallow enough, the implant will diffuse into layer106and further into layer104as layer106is grown.

Region120may be of the same doping type (n-type) as the doping of layer104. In APD128, because region120has shaped the electric field, the multiplication region may be concentrated at the junction of region110and region120.

FIGS. 2A-2Ddepict several early stages in the fabrication of a second embodiment of an APD according to another exemplary process. The fifth embodiment is similar to the first embodiment except that expitaxial layer102(FIG. 1B) of the first embodiment is formed by bonding two wafers together in the fifth embodiment.

In particular,FIG. 2Adepicts two wafers200and100. Wafer100may be the same wafer100as described with respectFIG. 1Aand the first embodiment. Wafer200may be a wafer of the same doping type as wafer100(n-type) and have a resistivity of about 50-500 Ω·cm. Wafer200has a top exposed surface202and a bottom exposed surface204. Wafer100has a top exposed surface206.

FIG. 2Bdepicts bonded wafer208, which is made of wafers200and100after a wafer bonding process has bonded surface206of wafer100to surface204of wafer200. The wafer bonding process may, for example, use a non-hydrogenated intermediate layer on one of the wafers along with heat and an applied voltage to create a permanent bond between the two wafers. A further explanation of a wafer bonding process is available in U.S. Pat. No. 7,192,841, which is incorporated herein by reference in its entirety for all purposes.

FIG. 2Cdepicts bonded wafer208after wafer200has been thinned to a thickness of 10-70 μm through thinning techniques, such as chemical-mechanical polishing (CMP), etch backs, or a combination of both CMP and etch-backs. Top exposed surface202(FIG. 2B) has been removed in the thinning process. By thinning wafer200, layer210is formed, which is an analogous layer to layer102(FIG. 1B) of the first embodiment.

FIG. 2Ddepicts bonded wafer208after expitaxial layers104and106have been grown on layer210. Layers104and106may be the same layers as described with respect toFIG. 1Babove. After the stage depicted inFIG. 2D, the processing of bonded wafer208may continue as explained above with respect toFIG. 1C-1Eor2.

FIGS. 2E and 2Fdepict optional variations that may be applied to the bonded wafer process described with respect toFIGS. 2A and 2B. In particular,FIG. 2Edepicts insulating layer212that may be on the surface of wafer200or wafer100prior to bonding the wafers together. The addition of insulating layer212may improve cross-talk performance of arrays of APDs using the fifth embodiment. Arrays of APDs are discussed below with respect toFIGS. 6 and 7. Insulating layer212may be, for example, silicon dioxide.FIG. 2Fdepicts bonded wafer208with insulating layer212in addition to implant region214formed in wafer200. Implant region214may allow for backside contact to individual APDs when arrays of the fifth embodiments are used. Arrays of APDs are discussed below with respect toFIGS. 6 and 7. The formation of implant region214is discussed below with respect toFIGS. 3A-3Fand implant region306. By using implant region214, the backside contact schemes described below with respect toFIGS. 3F and 4may be used as an alternative to the contact scheme described above with respect toFIG. 1Eor below with respect toFIG. 5.

FIGS. 3A-3Fdepict various stages in the fabrication of a third embodiment of an APD according to another exemplary process. This exemplary process begins by providing wafer300ofFIG. 3Aas the starting material. Wafer300may form the majority of the absorption region in the third embodiment. Wafer300may be about 450 μm thick and have n-type doping that results in a resistivity of about 10-500 Ω·cm or a doping concentration of about 1013cm−3to 1016cm−3. Wafer300also has a frontside302and a backside304. Because the multiplication region and absorption region will be formed in wafer300, a high quality wafer (e.g., float-zone silicon wafer) may be desirable.

FIG. 3Bdepicts wafer300after implant region306has been implanted on backside304. Implant region306will be of the same doping type (n-type) but a higher concentration as compared to wafer300. For example, implant region306may have a peak doping of 1016cm−3to 1019cm−3.

FIG. 3Ddepicts wafer300after it has been thinned to about 85 μm thick, although wafer300may be thinned to a thickness of about 10-90 μm. By first bonding wafer300to handle wafer308, the difficulties of processing thinned, brittle wafers may be avoided. Wafer300may be thinned by conventional thinning techniques such as grinding, chemical mechanical polishing (CMP), etch back, or a combination of these techniques. Additional processing may be required to return front surface311to a high quality silicon surface. For example, a thermal oxidation of front surface311may consume any silicon damaged from the thinning process.

FIG. 3Edepicts wafer300after several additional process steps. Region312is implanted with doping of the same type (n-type) as wafer300. Region312helps shape the electric field in the multiplication region that is at the p-n junction formed by regions312and314. Region314is implanted with the opposite type of doping (p-type) as wafer300. Region314is similar to region110as discussed above with respect toFIG. 1C. Region314forms a top region of the p-n junction formed by regions312and314. The top region is adjacent to the surface of wafer300. Channel stop regions316may be similar to channel stop regions108as described above with respect toFIG. 1C. Anode electrodes318, anti-reflective layer320, and insulating layer322may be similar to anode electrodes118, anti-reflective layer114, and insulating layer116ofFIG. 1E.

As an alternative to the structure as shown inFIG. 3E, the third embodiment could be processed by processing wafer300shown inFIG. 3Das described above with respect toFIGS. 1B-1Eexcept the cathode electrode120(FIG. 1E) would be omitted unless oxide layer310is not present between wafer300and handle wafer308.

Note that features added inFIG. 3Eare roughly aligned with implant region306so that the p-n junction formed between region312and314overlaps implant region306. Regardless of the how the multiplication region and other features are formed in and on wafer300, there must be some method for aligning these features to implant region306. For example, alignment may be accomplished by adding backside alignment marks to handle wafer308before thinning wafer300. Other alignment schemes may also be used.

FIG. 3Fdepicts the third embodiment324of an APD. The cathode of the p-n junction may be contacted through via324that is formed through handle wafer308so that it is contacting implant region306. A deep reactive-ion etching (DRIE) process may be used to etch the hole for via324. The hole may then be lined with oxide326before filling the hole with a metal plug to form a via.

FIG. 4depicts the third embodiment that adds an optional backside via400. Via400may be filled with a high resistance material to act as a passive quench or with a low resistivity material to allow active quenching.

FIG. 5depicts a fourth embodiment500that adds a backside contact to an APD having similar structure as the first embodiment except that instead of having a frontside anode electrode, the anode electrode may make contact to the anode of the p-n junction through via502. The cathode electrode may then be formed on the backside of handle wafer308. The backside via of the fourth embodiment may also be used with other embodiments of an APD. For example, using the backside via as depicted inFIG. 5may be particularly useful with the third embodiment if the insulating layer between the bonded wafers is not being used.

The third embodiment of an APD may be particularly useful in arrays of APDs. Typically, in APD arrays, it is desirable to detect when each APD adsorbs a photon independently from the other APDs. This requires that each APD in an array have at least one detection line that allows external circuits to determine when that APD has absorbed a photon. If the array has a large number of APDs, these lines may occupy a significant portion of the surface area of the array. By moving these lines to the backside of the array, the array may be smaller or may have more area available for the APDs to collect photons. The third embodiment as depicted inFIG. 3For4or the fourth embodiment as depicted inFIG. 5is particularly suited for an array with backside lines because in contrast to the first and second embodiments ofFIGS. 1E and 2with common backside contacts, each APD will have its own individual backside contact.

FIG. 6depicts the top view of 5×5 array600of APDs with a structure similar to the third embodiment except the anode electrode, insulating layer, and anti-reflective layer have been omitted from the surface for clarity of illustration. Each APD is defined by region114that helps define the multiplication region. The top down view also shows region602which is the region with a doping level defined by the doping of wafer100(FIG. 1A). Channel stop region116is also shown.FIG. 7is a cross-section of 5×5 array600through cut line604(FIG. 6). Arrays of up 100×100 or larger are also possible. Each APD has a discrete p-n junction700,702, and704that vertically overlaps a discrete buried implant region706,708, and710respectively.