Abstract:
A method for passivating a photodiode so as to reduce dark current, I sdark , due to the exposed semiconductor material on the sidewall of the device. The method includes etching away sidewall surface damage using a succinic acid-hydrogen peroxide based sidewall etch. This is followed by a subsequent hydrochloric acid (HCl)-based surface treatment which completes the surface treatment and reduces the dark current I sdark . Finally, a polymer coating of benzocyclobutene (BCB) is applied after the surface treatment to stabilize the surface and prevent oxidation and contamination which would otherwise raise the dark current were the diodes left with no coating. The BCB is then etched away from the contact pad areas to allow wirebonding and other forms of electrical contact to the diodes. Such method effectively stabilizes the etched surfaces of photodiodes resulting in significantly reduced and stable dark current.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to semiconductor devices, and more particularly to methods for passivating such semiconductor devices.  
         BACKGROUND  
         [0002]    As is known in the art, one type of semiconductor device is a photodiode as shown in FIGS. 1A and 1B. As is also known in the art, such devices are typically used for converting amplitude modulated light signals in fiber optics to electrical currents which represent digital or analog information. For example, data might be modulated on a light source at a rate of 40 billion binary digits/sec (40 Gb) with a “1” being represented by the light being on and a “0” represented by the light being off. Since the photodiode&#39;s current output is proportional to the intensity of illumination on the photodiode, ideally, the current waveform will be a representation of the digital data carried by the light.  
           [0003]    These photodiodes, such as photodiode  1  in FIGS. 1A and 1B convert light, having a wavelength near 1.55 μm, to hole-electron pairs (carriers) which result in an electrical current, Id flow between the anode pad  2  and the cathode pad  3 . Under illumination, hole-electron generation occurs mostly in the absorption layer  18 . In order to efficiently convert 1.55 μm wavelength photons to carriers, the absorption layer  18  must have a bandgap which is less than that of the photon energy. During operation, the photodiode&#39;s anode  2  is biased at a negative voltage with respect to the cathode  3 . This bias voltage, V d , results in an electrical field which separates the holes and electrons. Up to a point, the higher the V d  and the electrical field, the faster the electrons and holes (carriers) move. High carrier velocity is needed for high switching speed implying the need for a higher V d . However, if V d  is too high, the diode can break down resulting in burnout and/or a large component of Id that is unrelated to illumination, i.e. dark current, I dark . In any event, I dark  normally increases with V d . Photodiodes, used here for 1.55 μm wavelength detection, use an In .53 Ga .47 As absorption layer to provide a good compromise between absorption (i.e., relatively high In content) and good breakdown and low dark current characteristics (i.e., relatively low In content).  
           [0004]    Photodiodes are rated according to several figures of merit. These include responsivity, R; dark current, I dark ; and breakdown voltage, V br . In addition, when illumination levels are changing rapidly e.g. the light source is switching on and off at a very high rate, say in a 40 Gb optical data link; the time required for the photodiode current to change with the illumination might be a significant portion of the optical pulsewidth, T w . The ability for the photodiode&#39;s current to keep up with rapidly changing illumination levels is related to the 3 dB bandwidth of the photodiode, f c . When the illuminating light source power amplitude is varied sinusoidally with time at a frequency, f op , the diode current (I d ) also varies sinusoidally in accordance with the illumination. Let P ac  be the variation of the optical power level where P ac =P max −P min  where the optical power varies between a high of P max  to a low of P min . Also let I dac =I dmax −I dmin  where I dac  is the difference in the high (I dmax ) and low (I dmin ) values of I d . Also define the responsivity, R ac , as I dac /P ac . If P ac  is held constant, I dac  will remain nearly constant for low values of f op . However I dac  and R ac  fall as f op  approaches f c . At f op =f c , R ac  is 0.707 times its value at low f op , i.e. R ac  is 3 dB down from its low-frequency value. A high f c  is desirable because this implies that the photodiode current is able to follow the rapid changes in illumination level required for high-speed data and/or analog optical links. High values of R ac  are desirable because this reduces the optical power, P ac  needed to produce diode current, I dac .  
           [0005]    Dark current (I dark ) is the photodiode current measured with no illumination. It is desirable to minimize I dark  because it contributes an error signal and noise to the I d  when the photodiode is used to detect light. A major component of dark current arises when generated holes and electrons are separated by the electric field due to the anode bias and appear at the photodiode terminals as current. I dark  sources can also arise from conductive leakage paths on the diode&#39;s surface.  
           [0006]    I dark  has two physically contributing effects namely surface current, I sdark  and bulk current, I bdark , such that I dark =I bdark +I sdark . I bdark  is the dark current that originates from within the absorption layer  18  and I sdark  is that which originates from the photodiode&#39;s sidewall surface  6 . While a small amount of I bdark  is also generated from the natural hole-electron pair production processes in those portions of the absorption layer that are flawless, crystal defects in the absorption layer  18  and surface  6  act as generation centers which are prolific sources of hole-electron pairs. I sdak  can also arise from surface  6  contamination and leakage. FIGS. 1A and 1B show respectively the cross-sectional and top views of a metamorphic photodiode  1  on a GaAs substrate  12 . However, these photodiodes could also be grown on InP substrates. The current state of semiconductor technology allows the growth of high-quality photodiode material using both lattice-matched and metamorphic substrates. This means that the majority of dark current, especially in small-diameter photodiodes, is due not to bulk effects, but rather the sidewall surface  6  of the photodiode  1 . The sidewall surface  6  contributes most of I dark , especially in small devices and is highly dependent on the fabrication technology used to make the photodiode  1 . For instance, if the photodiodes&#39; anode mesas  4  are formed using dry etching, the sidewall surface  6  often comes out with large numbers of crystal defects (damage) due to its exposure to the energetic gas ions inherent in dry etching processes. This damage results in high I sdark  and I dark . Not only are the high values of dark current undesirable but contaminated and/or damaged sidewall surface  6  can also result in sudden failures due to anode to cathode short circuits which appear under anode bias.  
           [0007]    Alternatively, the photodiodes&#39; anode mesas  4  can also be formed using water-based etchants, i.e. wet etching processes. Wet etching results in an insignificant level of damage to the sidewall surface  6  and might help to reduce I sdark  as a result. Unfortunately, oxidation and contamination of the photodiode surface  6  can also contribute to I sdark . Also, wet etching is generally isotropic, meaning that the etch rate is the same in all directions. The resulting undercut during etching makes it difficult to fabricate photodiodes which have a small L 1  (FIG. 1A) relative to the anode mesa  4  thickness, L t . It is desirable to minimize L 1 /L t , especially for small photodiode diameters (L d ) because the photodiode area represented by L 1  does not contribute to photo-generated current, I dac , but does increase the photodiode&#39;s capacitance and hence reduces the photodiode&#39;s f c  and its ability to follow rapid changes in illumination.  
           [0008]    Therefore, dry etching is often used because it etches vertically with very little undercutting and sideways etching, allowing minimization of L 1 . Therefore, some kind of sidewall surface  6  treatment must be incorporated in the diode process to remove the damaged material, oxidation, and/or surface contamination, so as to obtain acceptable values of dark current, I sdark .  
         SUMMARY  
         [0009]    In accordance with the present invention, a method is provided for passivating a photodiode so as to reduce dark current, I sdark , due to the exposed semiconductor material on the sidewall  6  of the device. The method includes first etching away sidewall surface damage. This is followed by a hydrochloric acid (HCl)-based surface treatment which is effective in reducing dark current. Finally, a polymer coating of benzocyclobutene (BCB) is applied after the surface treatment to stabilize the surface and prevent oxidation and contamination which would otherwise raise the dark current were the diodes left with no coating. The BCB is then etched away from the pad areas to allow wirebonding and other forms of electrical contact to the diodes.  
           [0010]    Such method effectively stabilizes the etched surfaces of photodiodes resulting in significantly reduced dark current. 
       
    
    
     DESCRIPTION OF DRAWINGS  
       [0011]    [0011]FIGS. 1A and 1B are cross-sectional side and top views, respectively, of a photodiode according to the PRIOR ART, the cross-section of FIG. 1A being taken along line  1 A- 1 A of FIG. 1B;  
         [0012]    [0012]FIGS. 2A-2G cross-sectional sketches of a photodiode fabricated in accordance with the invention at various stages in such fabrication;  
         [0013]    [0013]FIG. 3 is a plot of the dark current of several untreated photodiodes as a function of time when biased at −10 V anode to cathode. FIG. 4 is a plot of the dark current of several photodiodes from the same wafer as those in FIG. 3 except here the diode sidewalls were etched with a solution having succinic acid and subsequently treated with hydrochloric acid in accordance with the process of FIGS. 2A-2F;  
         [0014]    [0014]FIG. 5 is a comparison of photodiodes having a sidewall with the succinic acid solution and subsequent hydrochloric (HCl) etch in accordance with the process of FIGS. 2A-2F compared with those having the sidewall etch and HCl treatment followed by a benzocyclobutene (BCB) coating and curing procedure in accordance with the process described in connection with FIG. 2G 
     
    
     DETAILED DESCRIPTION  
       [0015]    Referring now to FIGS. 2A-2G, the process for forming a photodiode semiconductor device is accordance with the invention is shown. Thus, referring to FIG. 2A, diode layers  14 ,  16 ,  18  and  20  are grown on a metamorphic GaAs or InP substrate  12 . Here, layer  16  is an n+In 0.52 Al   048  As backside layer and layer  14  is an InAlGaAs layer. Layer  14  is used to lattice match the layer  16  to the substrate  12  and is included only when GaAs substrates are used. Layer  14  is not used on diodes having InP substrates. Layer  18  is an In 0.53 Ga 0.47 As absorption layer which converts the incident light into hole-electron pairs. Layer  20  is a p+ InGaAs anode contact ohmic layer.  
         [0016]    Next, referring to FIG. 2B, p+ ohmic metal layer  22  is deposited over the layer  20  and is patterned into a ring-shaped structure, such ring having an aperture  21  therein using conventional photolithographic etching techniques. The aperture  21  is free of metal, and allows light to enter through the p+ InGaAs ohmic layer,  20 , into the InGaAs absorption layer. Next, referring to FIG. 2C, a dielectric and anti-reflection coating layer  24 , here silicon nitride, is deposited over the surface of the structure shown in FIG. 2B. It is noted that portions of the deposited layer  24  are disposed on the portion of the layer  20  within the aperture  21 .  
         [0017]    Next, referring to FIG. 2D, the photodiode anode mesa  4  is either dry-etched or wet etched into the structure shown in FIG. 2C, such mesa  4  having portions of p+ ohmic contact layer  20  and underlying portions of the absorption layer  18 . Dry etching results in the outer sidewall surfaces  6  of the anode mesa  4  having large numbers of crystal defects (damage) due to its exposure to the energetic gas ions inherent in dry etching processes. Both wet and dry etched surfaces can accumulate oxidation and contamination products over time. Crystal defects and/or surface oxidation/contamination can result in significant increases in dark current.  
         [0018]    Next, referring to FIG. 2E, the cathode mesa  25  is etched into the structure shown in FIG. 2D. the cathode mesa serves to electrically isolate the photodiode cathodes from each other and future metal structures. The cathode metal  26  is then deposited to provide electrical contact to layer  16  of the photodiode cathode mesa  27 .  
         [0019]    Next, referring to FIG. 2F, a conductive airbridge  28  is fabricated to provide electrical contact to the p+ ohmic metal rings  22  which are in ohmic contact with the anode p+ layer  20 . The substrate  14  may be thinned and via holes are added (if necessary) and metalized.  
         [0020]    Next, the diode structure is treated with the passivation etch in accordance with the invention. More particularly, the first step involves a selective etch of the anode mesa  4  using a mixture of a succinic acid solution which has its pH adjusted to 5.2 (hereafter called SA) and a 30% solution of ammonium hydroxide and water (hereafter called NH 4 OH). The SA is prepared by mixing 200 g of solid succinic acid with 1000 ml deionized water (hereafter called H 2 O). The NH 4 OH is stirred into the SA while monitoring the pH of the solution, until the pH=5.2. Solubility of succinic acid increased as NH 4 OH was added and pH increased. As NH 4 OH is added, an increasing proportion of the succinic acid dissolves, and at pH=5.2, nearly all the succinic acid is dissolved. During preparation of the SA, a waiting period is included after each addition of NH 4 OH to allow the succinic acid solution to reach equilibrium before the pH is measured. When the pH is satisfactory, i.e. within pH=5.1-5.35, then the SA is filtered by drawing it through a filter paper. The resulting filtered succinic acid solution (SA) is ready to be used in creating the sidewall etchant.  
         [0021]    The etchant used to etch portions of the sidewalls  6  of anode mesa  4  is prepared by mixing 6 parts of SA, prepared as described above, with 1 part of a 30% solution of hydrogen peroxide in water (hereafter called H 2 O 2 ). The sidewall etchant should be used within about 1 hr 30 min of mixing the SA and H 2 O 2 . The sidewall etchant described here etches the sidewall  6  of In 0.53 Ga 0.47 As layers  18  and  20  at approximately 600 Å/min while its etch rate of In .52 A .48 As layer  16  is under 10 Å/min. The low etch rate of In 0.52 Al 0.48 As layer  16  reduces the undesired etching of the n+ InAlAs layer  16  that interfaces to the n+ cathode metal layer  26 . Etching of layer  16  would result in an increase of the photodiode&#39;s resistance which would increase the response time of the photodiode to an optical pulse signal. The layer  24  of silicon nitride (SiNx) covers the top of the anode mesa  4  and the p+ ohmic metal layer  22 . The SiNx layer  24  coating prevents the sidewall etchant from undercutting the p+ ohmic metal  22  and also serves as an antireflection coating to improve the efficiency of light transfer from air to the absorption layer  18  of the photodiode.  
         [0022]    Thus, the photodiode is first subjected to an etch, consisting of the volume ratio of 6 parts SA with 1 part H 2 O 2 , for 1 min. The pH of this solution is 5.1-5.3. This etch removes a few hundred Angstroms from the semiconductor surfaces including all damaged semiconductor and contamination products from the anode mesa  4  sidewalls  6 .  
         [0023]    This SA:H 2 O 2  etch is then followed with a 30 sec HCl:H 2 O etch where the HCl:H 2 O solution is prepared by mixing a concentrated HCl solution with H 2 O in the volume ratio of 5 parts concentrated HCl solution with 95 parts H 2 O. The concentrated HCl solution is a 37-38% solution of HCl in water.  
         [0024]    Referring now to FIG. 2G, as part of the passivation, the diode can be optionally coated with a layer  30  of a polymer coating, here benzocyclobutene (BCB) manufactured by Dow chemical Cyclotene 4024-40. The BCB is thinned using a BCB solvent, i.e. T 1100 from Dow Chemical, before application to the photodiode wafer.  
         [0025]    More particularly, immediately following the HCl treatment, the photodiodes are then spin coated with BCB:BCB solvent solution consisting of the volume ratio of 1 part BCB to 5 parts T 1100 BCB solvent. Both chemicals were obtained from Dow Chemical. This BCB solution is spun on the wafer at 4000 RPM for 60 sec. The BCB coating is then cured in a vacuum oven at 250 C for 1 hour. The BCB coating can be removed from the photodiode contact pads ( 2  and  3 ) by patterning with photoresist and etching the BCB. Etching of the BCB is accomplished using a photoresist mask and dry etch. The procedure which describes the patterning of the BCB coating  30  process, according to the invention, follows below:  
         [0026]    1. Spin on Shipley 1827 photoresist at 2800 RPM for 60 sec spin time.  
         [0027]    2. Softbake 115 C on a hotplate.  
         [0028]    3. Expose with pad mask (UV light on pad regions) at 400 nm wavelength and 600 mJ/cm 2 .  
         [0029]    4. Develop photoresist mask using Microposit MF319 1 min 15 sec, rinse in deionized H 2 O, and blow-dry with N 2 .  
         [0030]    5. Etch BCB in a reactive ion etcher using the following gas flows, RF power, chamber pressure, and etch time respectively: 20 SCCM O 2 , 5 SCCM NF 3 , 50 W RF, 100 mT pressure. Etch for 2 min.  
         [0031]    6. Apply an oxygen plasma descum for 10 min, strip photoresist mask in acetone 5 min, followed by a squirt of acetone from the bottle, followed by 5 min dip in isopropanol, an isopropanol squirt from the bottle and N 2  blow-dry.  
         [0032]    Now, the BCB layer  30  is removed from diode pads  2  and  3  to allow external electrical contact to the diode and the photodiode is ready to test.  
         [0033]    In most photodiodes dark current often rises slightly with time as the photodiode is held under bias. Surface defects and contamination often result in high initial dark current and/or a dark current that drifts rapidly upward with time under bias. The passivation technique described by the invention reduces both the dark current and its upward drift with time as illustrated by comparing FIGS. 3 and 4.  
         [0034]    [0034]FIG. 3 is a plot of the dark current curves of several untreated photodiodes as a function of time when biased at room temperature with −10 V from anode to cathode. The photodiodes of FIG. 3 had no surface treatment and were left in air for several months. FIG. 4 is a plot of the dark current of several photodiodes from the same wafer as those in FIG. 3 except here such wafers were tested less than a day after having their sidewalls etched and treated with the SA and HCl solutions as described above in connection with the process of FIGS. 2A-2F.  
         [0035]    [0035]FIG. 5 shows the benefit of having the BCB coating applied after the sidewall SA etch and HCl treatment described in accordance with FIG. 2A-2G. Here, photodiodes were stressed by biasing them at −10 V (normal bias for optical detection is −5 V to −10 V) while holding them at 150 C in air for 65 hours. More particularly, FIG. 5 shows dark current increasing less in photodiodes that have the BCB coating as opposed to those that had only the sidewall SA:H 2 O 2  and HCl:H 2 O etches described above but with no coating.  
         [0036]    Thus, FIG. 4 shows dark current for photodiodes that have had the sidewall SA etch and HCl treatment described in accordance with FIG. 2A-2F. Such diodes show much lower dark currents and lower dark current drift. While the surface etch and treatment reduces dark currents, it provides little long-term protection from sidewall contamination and oxidation—both of which will raise dark current with time and/or stress. Therefore, the surface etch and treatments need to be followed by a procedure which encapsulates the photodiode and seals the sidewalls against oxygen and other contaminates. Such is the purpose of the BCB coating and curing as described in the invention in accordance with FIG. 2G.  
         [0037]    To test the effectiveness of the BCB coating, two sets of photodiodes, from the same wafer, were fabricated with the first set having only the sidewall etch and HCl treatment and the next having the sidewall etch and HCl treatment followed by the BCB deposition as described in the invention. All devices receiving only the sidewall etch and HCl treatment were baked at 200 C for 15 min in a vacuum oven immediately following the HCl treatment. Devices getting the BCB coating were coated right after the HCl treatment and then baked after the BCB coating at 250 C for 1 hour in the vacuum oven as described in the invention.  
         [0038]    Thus, FIG. 5 is a comparison of photodiodes having a sidewall etch and HCl treatment in accordance with the process of FIGS. 2A-2F compared with those having the sidewall etch and HCl treatment followed by a BCB coating and curing procedure in accordance with the process described in connection with FIG. 2G Photodiodes lacking the BCB coating were baked at 200 C in a vacuum oven following the HCl treatment whereas those coated with BCB were coated immediately after drying off the HCl treatment. The BCB was then cured at 250 C in a vacuum oven for an hour. Photodiodes were stressed by holding them at 150 C in air under −10 V anode to cathode bias for 65 hours. The photodiodes were then cooled to room temperature and the dark current was measured at −10 V anode bias after holding the anode at −10 V for 1 min.  
         [0039]    Other embodiments are within the spirit and scope of the appended claims.