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Fabrication of resonator-quantum well infrared photodetector test devices

[+] Author Affiliations
Jason Sun

US Army Research Laboratory, Adelphi, Maryland 20783

Kwong-Kit Choi

US Army Research Laboratory, Adelphi, Maryland 20783

Kimberley A. Olver

US Army Research Laboratory, Adelphi, Maryland 20783

J. Micro/Nanolith. MEMS MOEMS. 13(1), 013004 (Jan 13, 2014). doi:10.1117/1.JMM.13.1.013004
History: Received August 26, 2013; Revised November 21, 2013; Accepted December 18, 2013
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Abstract.  An optimized detector fabrication process is developed for resonator-quantum well infrared photodetectors (R-QWIPs). The R-QWIPs are the next generation of QWIP detectors that use resonances to increase the quantum efficiency (QE). To achieve the expected performance, the detector geometry must be produced in precise specification. In particular, the height of the diffractive elements and the thickness of the active resonator must be uniformly and accurately realized to within 0.05-μm accuracy. To achieve this specification, an optimized inductively coupled plasma etching process with a nearly infinite etching selectivity for the GaAs over the AlxGa1xAs etch-stop layer was developed. Using this etching technique, we have fabricated a number of R-QWIP test detectors with the required dimensions. Their QE spectra were tested to be in close agreement with the QE predictions.

Figures in this Article

Quantum well infrared photodetector (QWIP) focal plane arrays (FPAs) hold many advantages over other detector FPAs, such as inexpensive and large area substrates, simple material design and growth, effective self-passivation, absence of 1/f noise, and high FPA resolution, sensitivity, uniformity, and operability.1 Unfortunately, a typical quantum efficiency (QE) of 5% observed in GaAs/AlGaAs QWIPs necessitates the detectors to operate at a long integration time of 5 ms or longer, which prevents their applications in high speed imaging.1,2 Recently, we have established a three-dimensional finite element electromagnetic (EM) model to calculate QE quantitatively.35 This theoretical tool allows us to design new optical coupling structures to achieve a larger QE. One of these detector structures is known as the resonator-QWIP or R-QWIP.

An R-QWIP consists of a properly sized detector pixel volume. On the top of the pixel, there is an array of diffractive elements (DEs) that are made of the same GaAs contact material and are covered with an ohmic metal layer and a gold layer, which are shown in Fig. 1. On the bottom of the pixel, there is a common ground contact layer, and the substrate underneath is completely removed. The light is incident from the bottom and is scattered by the DEs back to the detector volume. Aided by the EM model, we adjusted the dimensions of the DEs and the active volume such that the scattered light circulates inside the detector resonantly, with which the internal optical intensity can be greatly increased. The higher intensity leads to larger QEs and faster detection speeds.

Graphic Jump LocationF1 :

The figure shows the designed resonator-quantum well infrared photodetector (R-QWIP) structure.

To achieve the expected resonances, the substrate has to be removed to prevent the escape of unabsorbed light from the detector. After scattered by the DEs, the light usually travels at an angle larger than the critical angle for total internal reflection at the bottom semiconductor/air interface. The light reflected back from the bottom layer will thus be able to contribute to constructive interference with other scattered light. Without the substrate, all the unabsorbed light will then be tightly confined to one pixel until it is eventually absorbed. To ensure proper phase relationship for constructive interference, the active material thickness and the height of the DEs have to be fabricated to within 0.05 μm of the EM design specification in order to yield a QE that stays within 10% of the designed value.

To achieve this accuracy, we have optimized two inductively coupled plasma (ICP) etching processes to fabricate the test devices. In an ICP system, there are two independent plasma power sources that provide almost independent control of ion density and ion energy. Ion density is controlled by the ICP source power alone. Increasing ICP source power increases ion density. Meanwhile, ion energy is affected by both ICP source power and RF chuck power. Increasing ICP power decreases induced dc bias. On the contrary, increasing RF chuck power raises dc bias on the chuck. Since an ICP system provides one more process parameter than a reactive ion etching system for plasma control, it is more flexible to use ICP to optimize different etching processes, such as selective (etching GaAs over AlxGa1-xAs stop etching layer) versus nonselective etching, or isotropic versus vertical etching.69

In addition to the etching profile, the electrical damage to the detectors produced by the active gases in the plasma is also an important consideration. There are many physical and chemical processes involved in the plasma etching. A general conclusion attributes the main plasma damage to the high energy ions via physical bombardment and the reactive ions via chemical reactions.6,10,11 The use of a low RF chuck power in ICP will reduce the ion energy; and the use of a chlorine-based gas will eliminate the harmful chemical reactions present in the hydrogen-based gas.

Previously, we had optimized a selective etching process that could yield a near infinite selectivity and a high GaAs etching rate.1215 The etching surface was perfectly smooth and mirror-like after processing. In addition to its simplicity, the process is also highly reproducible and shows no damage to the detector material. In this work, we applied the same processes to fabricate the new detector structures. Each R-QWIP detector is 22×22μm2 in size. To increase the signal-to-noise ratio, we grouped the detectors into arrays of 40×40 elements. The detectors in each array were connected in parallel using a fanout circuit. The pixel pitch is 25 μm.

Our test device fabrication process requires five masks. We used the first mask to create an array of rings as the DEs to diffract normal incident light into nearly parallel propagation. A standard photolithography process was utilized to pattern 4-in. wafers. The DEs were formed by using our optimized selective ICP etching process to reach down to the 15-Å top stop etching layer.15 The optimized etching parameters were: BCl3=20sccm, SF6=10sccm, Ar=10sccm; pressure=0.5mTorr; RF power=0W; ICPpower=200W; and substrate temperature=25°C. The selective etching process was conducted in a Unaxis VLR 700 etch system. The tool uses a 2-MHz RF inductively coupled coil to generate a high density plasma. Ion energy at the wafer surface is independently controlled by a 13.56-MHz RF bias applied to the cathode. Wafer temperature is maintained through the use of a fluid cooled cathode in conjunction with electrostatic clamping and helium backside cooling. A typical schematic of an ICP etching system can be found in Fig. 1 of 12. Since the selective etching process has an almost infinite selectivity (>50001 for Al0.4Ga0.6As), 15- thick stop etching layer is sufficient to define the DE height. The stop etching layer is also important for etching uniformity across the wafer as the etching rate is higher near the wafer edge. Figure 2 is a scanning electron microscope (SEM) image of the DEs. The etching is uniform, and the etching surface is clean and smooth.

Graphic Jump LocationF2 :

A SEM image taken after first inductively coupled plasma etching.

The second masking step defines the ground contact area located outside the pixel area. Our nonselective ICP etching recipe was used to reach the common ground contact layer. The optimized etching parameters were: BCl3=50sccm, Ar=10sccm, pressure=5mTorr, RF Power=80W, ICP Power=800W, and substrate temperature=25°C. In this step, a finite RF power was necessary to create a vertical sidewall. We nevertheless minimized the power to avoid the possible plasma damage. The RF power needed for the creation of the plasma also induces a DC voltage on the chuck, which accelerates the ions toward the etching material. By using a low RF power and a high ICP power, the DC voltage on the chuck is only 120 V. Therefore, the physical impact of these ions onto the detector material can be reduced. We used the third mask to define the lift-off areas for the deposition of Pd/Ge/Au/Pd/Au metal, and it was followed by a furnace annealing at 350°C for 25 min. After this process, the DEs were covered with an ohmic metal layer and a gold layer. The gold layer is used to reflect the incident light and DEs are used to convert the polarization of the incident light from horizontal to vertical through diffraction. Figure 3 shows one corner of the test device after metallization. The outside stripe is ground contact area and the inside square is the 40×40 detector pixel area.

Graphic Jump LocationF3 :

A microscope picture taken after metallization.

We used the fourth mask to define the pixels. We opened the pixel areas while other areas were covered with photoresist. In the pixel areas, the metal squares were used as etching masks and nonselective ICP etching was utilized to create individual pixels. Figure 4 shows microscope pictures taken after the pixel ICP etching. The fifth mask defines the lift-off areas of indium bumps. We utilized a 10-μm-thick negative photoresist and deposited 6-μm-tall indium bumps on the wafer using a thermal evaporator. The wafer was diced into test devices. The candidates were bump bonded to the high resistivity silicon circuit board and were filled with low viscosity epoxy.

Graphic Jump LocationF4 :

Two microscope pictures taken after defining the detector pixels fabricated on two different material wafers (a) shows R-QWIP pixels with 0.4-μm tall diffractive elements (DEs) and 1.0-μm thick active material and (b) shows R-QWIP pixels with 0.45-μm tall DEs and 1.2-μm thick active material. Both bottom contact layers are 0.7-μm thick.

Thinned QWIP FPAs offers several advantages over unthinned FPAs. First, the thermal mass of the FPAs is reduced to lessen the detector cooling time. The thinned layer also makes it easier to adapt to the thermal expansion mismatch between GaAs and the silicon readout circuit. Optical crosstalk among pixels is suppressed by better optical confinement. Besides these general benefits, substrate removal is specifically required for R-QWIP. The thinned R-QWIP FPAs enhance the resonant effects, and the QE can increase by a factor of 3 to 4 according to EM modeling.

The substrate removal process includes two steps, which are mechanical lapping and selective ICP etching. The substrates of the test device were first mechanically lapped, using 3-μm Calcined aluminum oxide lapping medium, to within 50 μm. The edges of the test devices were then hand painted with a surface coating. After baking the test devices in an oven for 1 h at 95°C, the test devices were introduced into the Unaxis VLR 700 Etch System for about one hour of etching. Figure 5 shows two pictures, one was taken after lapping but before ICP etching, and another was taken after etching. As seen in these pictures, the surface of the die is uniform, smooth and mirror-like after etching. The etching surface is close enough to the pixels such that the pixels on the other side can be seen under microscope.

Graphic Jump LocationF5 :

The substrate surface (a) before and (b) after selective etching for substrate removal.

Although we optimized the DE design, due to the developmental nature of our detector processing, we obtained different DE patterns as shown in Fig. 4. In addition, the thicknesses of the DE layer and the quantum well active layer are also different. As a result, different QEs were obtained for the two detectors. We measured the conversion efficiency (CE) spectrum for each R-QWIP and its photoconductive gain g at the voltage where CE saturates. The values of QE (=CE/g) can then be calculated. Figure 6 shows the experimental results for the two detectors. In general, the experimental results agree the predictions satisfyingly. For the detector in Figs. 4(a) and 6(a), the calculated peak is at 9 μm with QE=38.3%. It is slightly larger than the observed QE=34.6% at 8.7 μm, which is deduced from CE=27.4% and g=0.79. For the detector in Figs. 4(b) and 6(b), the calculated peak is 67.3% at 8.6 μm, and the observed value is 70.8% at 8.8 μm, and CE=62.3% and g=0.88. Comparing with the thinned detectors, the detectors with thicker (50μm) substrates have significant lower QE as seen in Fig. 6. On the other hand, these measured thick substrate QEs are larger than that predicted by theory. We attribute the higher QE to the optical crosstalk among pixels, which is expected to be present in thick substrates. The test results of the detectors indicate that no plasma damage to the detector material was observed in either wafer fabrication or substrate thinning.

Graphic Jump LocationF6 :

The figure shows the observed and predicted quantum efficiency (QE) for two detectors. The experimental QE of the detector with thick substrate was scaled by a factor to fit the theoretical prediction.

To verify our high QE R-QWIP designs, optimized selective and nonselective ICP etching processes were developed and applied to fabricate 40×40 small format test detector arrays. Our selective ICP etching process has an optimized BCl3/SF6/Ar composition and shows a nearly infinite etching selectivity for the GaAs over the AlxGa1-xAs etch-stop layer. We used it to create the DEs in the R-QWIP structures and remove the substrate completely. Meanwhile, the nonselective ICP etching process was used to perform straight sidewall, damage-free ground contact etching and pixel mesa etching. The test results of the R-QWIPs agree with EM designs satisfyingly. The highest QE observed is 70.8% with thin detector material and without an anti-reflection coating. The QE could be even higher with further material and structural optimizations. The fabrication of large format 1K×1K FPAs with different pixel sizes (25 and 18 μm) is underway.

Bois  P. et al., “QWIP status and future trends at Thales,” Proc. SPIE. 8268, , 82682M  (2012). 0277-786X CrossRef
Eker  S. U., Arslan  Y., Besikci  C., “High speed QWIP FPAs on InP substrates,” Inf. Phys. Tech.. 54, (3 ), 209 –214 (2011). 1350-4495 CrossRef
Choi  K. K. et al., “Electromagnetic modeling of quantum well infrared photodetectors,” IEEE J. Quantum Electron.. 48, (3 ), 384 –393 (2012). 0018-9197 CrossRef
Choi  K. K., “Electromagnetic modeling of edge coupled quantum well infrared photodetectors,” J. Appl. Phys.. 111, (12 ), 124507  (2012). 0021-8979 CrossRef
Choi  K. K. et al., “Electromagnetic modeling and design of quantum well infrared photodetectors,” IEEE J. Sel. Top. Quantum Electron.. 19, (5 ), 3800310  (2013). 1077-260X CrossRef
Sun  J., Choi  K. K., Lee  U., “Fabrication of pyramidal corrugated quantum well infrared photodetector focal plane arrays by inductively coupled plasma etching with BCl3/Ar,” J. Micro/Nanolithogr. MEMS MOEMS. 11, (4 ), 043003 , (2012). 1932-5150 CrossRef
Mitchell  A. et al., “Real-time, in situ monitoring of GaAs and AlGaAs photoluminescence during plasma processing,” Appl. Phys. Lett.. 56, (9 ), 821 –823 (1990). 0003-6951 CrossRef
Knoedler  C. M., Osterling  L., Shtikman  H., “Reactive ion etching damage to GaAs Layers with etch stops,” J. Vac. Sci. Technol. B. 6, (5 ), 1573  (1988). 0734-211X CrossRef
Pearton  S. J. et al., “Reactive ion etching of GaAs with CCl2F2:O2: etch rates, surface chemistry, and residual damage,” J. Appl. Phys.. 65, (3 ), 1281 –1292 (1989). 0021-8979 CrossRef
Hayes  T. R. et al., “Damage to InP and InGaAsP surfaces resulting from CH4/H2 reactive ion etching,” J. Appl. Phys.. 68, (2 ), 785 –792 (1990). 0021-8979 CrossRef
Etrillard  J. et al., “Anisotropic etching of InP with low sidewall and surface induced damage in inductively coupled plasma etching using SiCl4,” J. Vac. Sci. Technol. A. 15, (3 ), 626 –632 (1997). 0734-2101 CrossRef
Golka  S. et al., “Time-multiplexed, inductively coupled plasma process with separate SiCl4 and O2 steps for etching of GaAs with high selectivity,” J. Vac. Sci. Technol. B.. 27, (5 ), 2270 2279 (2009). 0734-211X CrossRef
Yang  B. et al., “Research on ICP etching technology of InGaAs based on orthogonal experimental design,” Proc. SPIE. 8419, , 84192H  (2012). 0277-786X CrossRef
Lee  J. W. et al., “Advanced selective dry etching of GaAs /AlGaAs in high density inductively coupled plasmas,” J. Vac. Sci. Technol. A. 18, (4 ), 1220 –1224 (2000). 0734-2101 CrossRef
Sun  J. et al., “Advanced substrate thinning process for GaAs-based devices,” J. Micro/Nanolith. MEMS MOEMS. 10, (2 ), 023004  (2011). 1932-5134 CrossRef

Jason Sun is a physicist at the US Army Research Lab, Adelphi, Maryland. He has experiences in a wide range of optoelectronic and RF device physics and fabrication. He has many years of experience in research on conventional and high-Tc superconductors and expertise in high critical temperature (Tc) superconductor epitaxial thin film growth and characteristic tests and analysis. His current interest is QWIP FPA research and fabrication.

Kwong-Kit Choi received his PhD in physics from Yale University in 1984. He is currently a senior research scientist for physical sciences at the Army Research Lab. His interest spans from basic physics to focal plane array demonstration.

Kimberley A. Olver received her BA in chemistry/biochemistry from Goucher College, Towson, Maryland, in 1981. She is currently part of the II-VI Devices Team within the Sensors and Electron Devices Directorate at the Army Research Laboratory in Adelphi, Maryland. Prior to working for the Army Research Laboratory, she was a senior engineer with the Advanced Infrared Technology Department of Martin Marietta Laboratories (now Lockheed Martin Corp.) in Baltimore, Maryland.

© The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

Citation

Jason Sun ; Kwong-Kit Choi and Kimberley A. Olver
"Fabrication of resonator-quantum well infrared photodetector test devices", J. Micro/Nanolith. MEMS MOEMS. 13(1), 013004 (Jan 13, 2014). ; http://dx.doi.org/10.1117/1.JMM.13.1.013004


Figures

Graphic Jump LocationF1 :

The figure shows the designed resonator-quantum well infrared photodetector (R-QWIP) structure.

Graphic Jump LocationF2 :

A SEM image taken after first inductively coupled plasma etching.

Graphic Jump LocationF3 :

A microscope picture taken after metallization.

Graphic Jump LocationF4 :

Two microscope pictures taken after defining the detector pixels fabricated on two different material wafers (a) shows R-QWIP pixels with 0.4-μm tall diffractive elements (DEs) and 1.0-μm thick active material and (b) shows R-QWIP pixels with 0.45-μm tall DEs and 1.2-μm thick active material. Both bottom contact layers are 0.7-μm thick.

Graphic Jump LocationF5 :

The substrate surface (a) before and (b) after selective etching for substrate removal.

Graphic Jump LocationF6 :

The figure shows the observed and predicted quantum efficiency (QE) for two detectors. The experimental QE of the detector with thick substrate was scaled by a factor to fit the theoretical prediction.

Tables

References

Bois  P. et al., “QWIP status and future trends at Thales,” Proc. SPIE. 8268, , 82682M  (2012). 0277-786X CrossRef
Eker  S. U., Arslan  Y., Besikci  C., “High speed QWIP FPAs on InP substrates,” Inf. Phys. Tech.. 54, (3 ), 209 –214 (2011). 1350-4495 CrossRef
Choi  K. K. et al., “Electromagnetic modeling of quantum well infrared photodetectors,” IEEE J. Quantum Electron.. 48, (3 ), 384 –393 (2012). 0018-9197 CrossRef
Choi  K. K., “Electromagnetic modeling of edge coupled quantum well infrared photodetectors,” J. Appl. Phys.. 111, (12 ), 124507  (2012). 0021-8979 CrossRef
Choi  K. K. et al., “Electromagnetic modeling and design of quantum well infrared photodetectors,” IEEE J. Sel. Top. Quantum Electron.. 19, (5 ), 3800310  (2013). 1077-260X CrossRef
Sun  J., Choi  K. K., Lee  U., “Fabrication of pyramidal corrugated quantum well infrared photodetector focal plane arrays by inductively coupled plasma etching with BCl3/Ar,” J. Micro/Nanolithogr. MEMS MOEMS. 11, (4 ), 043003 , (2012). 1932-5150 CrossRef
Mitchell  A. et al., “Real-time, in situ monitoring of GaAs and AlGaAs photoluminescence during plasma processing,” Appl. Phys. Lett.. 56, (9 ), 821 –823 (1990). 0003-6951 CrossRef
Knoedler  C. M., Osterling  L., Shtikman  H., “Reactive ion etching damage to GaAs Layers with etch stops,” J. Vac. Sci. Technol. B. 6, (5 ), 1573  (1988). 0734-211X CrossRef
Pearton  S. J. et al., “Reactive ion etching of GaAs with CCl2F2:O2: etch rates, surface chemistry, and residual damage,” J. Appl. Phys.. 65, (3 ), 1281 –1292 (1989). 0021-8979 CrossRef
Hayes  T. R. et al., “Damage to InP and InGaAsP surfaces resulting from CH4/H2 reactive ion etching,” J. Appl. Phys.. 68, (2 ), 785 –792 (1990). 0021-8979 CrossRef
Etrillard  J. et al., “Anisotropic etching of InP with low sidewall and surface induced damage in inductively coupled plasma etching using SiCl4,” J. Vac. Sci. Technol. A. 15, (3 ), 626 –632 (1997). 0734-2101 CrossRef
Golka  S. et al., “Time-multiplexed, inductively coupled plasma process with separate SiCl4 and O2 steps for etching of GaAs with high selectivity,” J. Vac. Sci. Technol. B.. 27, (5 ), 2270 2279 (2009). 0734-211X CrossRef
Yang  B. et al., “Research on ICP etching technology of InGaAs based on orthogonal experimental design,” Proc. SPIE. 8419, , 84192H  (2012). 0277-786X CrossRef
Lee  J. W. et al., “Advanced selective dry etching of GaAs /AlGaAs in high density inductively coupled plasmas,” J. Vac. Sci. Technol. A. 18, (4 ), 1220 –1224 (2000). 0734-2101 CrossRef
Sun  J. et al., “Advanced substrate thinning process for GaAs-based devices,” J. Micro/Nanolith. MEMS MOEMS. 10, (2 ), 023004  (2011). 1932-5134 CrossRef

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