Extended backside-illuminated InGaAs on GaAs IR detectors

Transcription

1 Extended backside-illuminated InGaAs on GaAs IR detectors Joachim John a, Lars Zimmermann a, Patrick Merken a, Gustaaf Borghs a, Chris Van Hoof a Stefan Nemeth b, a Interuniversity MicroElectronics Center (IMEC); Leuven, Belgium b XenICs; Leuven, Belgium ABSTRACT Diode structures of short wavelength infrared (SWIR) InGaAs material were grown epitaxially on 3 GaAs substrates by molecular beam epitaxy. Despite the large lattice mismatch of 6% between In 0.8 Ga 0.2 As and GaAs the diode performance allows applications in spectroscopy and imaging. Photovoltaic diode characterization measures like R 0 A product and quantum efficiency were extracted from I-V curves. The layers are processed with standard photolithography and micro-structuring tools and finally flip-chip bonded on a silicon read out integrated circuit (ROIC). Linear arrays of 256 and 512 pixel with 25 µm pitch were fabricated as well as focal plane arrays (FPA) of 256 x 320 pixel with 30 µm pitch. Functionality is proven by using the assemblies in systems for spectroscopy and beam profiling up to 2.5 µm wavelength. Keywords: Extended InGaAs, SWIR, Linear array, focal plane array, flip-chip bonding, back-side illumination INTRODUCTION Extended SWIR InGaAs with cut-off wavelength up to 2.5 µm finds its application in spectroscopy and imaging. For the spectroscopy of plastics e.g. the characteristic finger print absorption lines are in the area between 1.7 and 2.5 µm and could not be seen by standard telecom photodiodes like In 0.53 Ga 0.47 As on InP 1,2,3. On the other hand the development of light sources like Lasers and LEDs for wavelengths exceeding 1.7 µm requires detectors working near room temperature for beam profiling and far field measurements. InGaAs with In content larger than 53% is a difficult material to grow on InP 4,5. GaAs substrates provide an alternative that is low cost, mechanically reliable and available up to 6 size. We present a full FPA process on 3 GaAs from the layer growth until the flip-chip bonding on the ROIC. Up to our knowledge this is the first time that a FPA of extended InGaAs material on GaAs substrate is published. LAYER GROWTH In 0.8 Ga 0.2 As material is grown on 3 GaAs (100) substrates with MBE. To overcome the lattice mismatch of 6% and for transparency reasons a buffer layer of the wide band-gap material InAlAs is grown on the GaAs substrates. This buffer layer is followed by an p-n diode structure of InGaAs. The Indium ratio varies, depending on the desired wavelength, between 75-80%. A highly doped n+ top layer is grown for contact reasons. In fig.1 a selection of InGaAs layers are shown with different Indium content and the corresponding measured cut-off wavelength. As a guideline the theoretical band-gap curve is given (solid line). Infrared Technology and Applications XXVIII, Bjørn F. Andresen, Gabor F. Fulop, Marija Strojnik, Editors, Proceedings of SPIE Vol (2003) 2003 SPIE 0277/786X/03/$

2 cut-off wavelength at 250K [ µm] X524 X525 E gap calculation X519 X504 X513 X528 X XRD Indium content [%] Fig.1: Cut-off wavelength versus Indium content of grown In x Ga 1-x As layers. 86 PROCESSING The 3 inch wafer is processed with standard photolithography and microstructuring techniques. The mesa etch is performed with a citric acid solution, contacts are made with TiW/Au metallization. Indium bumps for the forthcoming flip chip bonding are deposit by an electro-plating process. To define the plating an under bump metallization is deposited upon the diodes. Fig. 2 shows a schematic cross-section of the process after flip chip bonding with the ROIC. GaAs SUBSTRATE Insulator UBM SiO InGaAs Au Indium Au UBM Al UBM SiO Indium CMOS ROIC Fig. 2 : Schematic cross section of an assembly, III-V infrared sensor flip chipped on CMOS ROIC. 454 Proc. of SPIE Vol. 4820

3 EXPERIMENTS The MBE growth conditions are in situ controlled by reflection high-energy electron reflection (RHEED). The layer quality, indium contend and remaining strain is determined by high resolution x-ray diffration (HRXRD) measurements. For characterization the samples were mounted and wire bonded in a DIL 24 package modified for backside illumination. Spectral response is measured in a cryogenic measurement set-up using a commercial monochromator and a 1270K blackbody source 6. Cut-off wavelengths are defined at 10% of the measured peak signal 7. Resistance area products are extracted from I-V curves by measuring the dark current at 10mV bias voltage. In a preliminary test a 2D array connected to a ROIC is built in a test board. A pixel yield could be measured as well as a first application as beam profiler could be performed. RESULTS AND DISCUSSION The full width at half maximum (FWHM) of x-ray rocking curves are taken as a measure for layer quality. FWHM between 500 and 600 arcsec are measured for In 0.8 Ga 0.2 As on GaAs substrates. X-ray data show that the layers are relaxed after growth. Peak spectral response is measured to be 0.6 A/W without anti-reflection coating (ARC). The detectivity is determined to be D * =5x10 10 Fig. 3 shows a spectral response curve at 250 K. The cut-off wavelength of InAlAs in that case is 1.3 µm and the cut-off wavelength of InGaAs is 2.3 µm. The measurement is performed without anti-reflection coating (ARC). Response [A/W] T = 250K Wavelength [micrometer] Fig.3 : Spectral Response versus wavelength of an InGaAs diode without anti-reflection coating at 250K. The resistance area product at 0 V bias voltage (R 0 A) is a common measure for photovoltaic diodes. The dependency of the the R 0 A on the temperature gives information about the leakage mechanisms in the diode. In our case R 0 A products versus temperature curves are plotted to compare the influence of the InAlAs buffer layer on the leakage mechanisms. Fig.4 shows 4 diodes grown on InAlAs buffer and in comparison one diode grown on a Proc. of SPIE Vol

4 InGaAs buffer. The performance of the diodes grown with a InAlAs buffer is superior compared to the depicted diode grown on InGaAs buffer in the generation-recombination current regime of the curve between 300 K and 200 K, respectively. This is the working temperature range of the device R 0 A [Ωcm 2 ] # 1 transparent buffer # 2 " # 3 " # 4 " opaque buffer / Temperature [1 / K] Fig.4 : R 0 A product versus inverse temperature of different diodes grown with transparent InAlAs buffer and the best perfomance on InGaAs buffer (opaque buffer). Perimeter effects are parasitic leakage currents occuring at the surface or the sidewalls of a mesa etched structures on not passivated devices. To visualize perimeter effects the inverse resistance area product is plotted versus the ratio of perimeter and area. We measured leakage currents on diodes sizing from 200 µm down to 20 µm at 300 K and 250 K, respectively and did not find any correlation between the leakage current and the diode size as shown in fig. 5. 1/R 0 A [1/ Ωcm 2 ] 80x K 250K Perimeter / Area [1 /cm] Fig.5: Inverse R 0 A product versus perimeter/area ratio at 295K (top of the graph) and 250K (bottom of the graph). 456 Proc. of SPIE Vol. 4820

5 A full processed 3 GaAs wafer with 31 arrays of 256 x 320 pixel with a pitch of 30 µm is shown in fig. 6. On each pixel an indium solder bump of 13 µm diameter is formed. Further a FPA flip chip bonded on a CMOS ROIC wirebonded on a PLCC package is shown. The system of SWIR FPA and CMOS ROIC is mounted on a PCB for signal processing. Fig. 6 : Full processed 3 GaAs wafer including indium solder bumps for the connection to the ROIC. A magnification of the Indium bumps is shown, each bump has a diameter of 13 µm. On the right side the flip-chipped IR sensors is seen on a ROIC in a PLCC package. On the left side the testing board is shown. To test the functionality of the IR FPA it was illuminated by a 1.9 µm LED. Fig.7 shows in the left picture a photo taken from the LED with a standard CCD digital camera. The following photos are taken from this LED with the IR FPA for different injection currents. The light distribution is clearly visible and possible parasitic effects could be detected. Fig. 7 : left : Photo of an 1.9 µm LED center : IR picture of the same LED working with an injection current of 20mA right : IR picture of the same LED working with an injection current of 2mA Beam profiling is made of a telecom LED s operating at 1.5 µm as shown in figure 8. In figure 8a the whole beam is plotted. In figure 8b a cross section through the beam in x and y direction is given, respectively. Proc. of SPIE Vol

6 Fig. 8a : Beam distribution of a 1.5 µm telecom LED measured with an IR-FPA with 2.3 µm cut-off wavelength. Current [na] 10 5 Current [na] X - Direction [µm] Y - Direction [µm] Fig. 8b : Cross-section through the beam of a 1.5 µm telecom LED in x and y-direction respectively. CONCLUSION InGaAs SWIR material with Indium contents between 75 and 83% is grown epitaxially on 3 GaAs. Despite the lattice mismatch of ca. 6% the layers showing sufficient quality to perform as SWIR sensors. Linear arrays (256 and 512 pixel with 25 µm pitch) as well as FPAs with 320x256 pixel with 30 µm pitch were processed. Both kind of devices were integrated via flip chip bonding on a CMOS ROIC. Connected on printed circuit boards (PCB) these systems were tested in applications as beam profiling and imaging. ACKNOWLEDGEMENTS We thank Tom Torfs and Gerlinde Ruttens for setup and run the testsytem. We thank Alex de Kerckhove and Michel Baguet for their system work and packaging. 458 Proc. of SPIE Vol. 4820

7 REFERENCES 1. O.K. Kim, B.V. Dutt, R.J. McCoy, J.R. Zuber, A low dark-current planar InGaAs p-i-n photodiode employing multimode waveguide structure IEEE Photonics Technology Letters, Vol.3, No.9, pp , K. Kato, S. Hata, A. Kozen, J.I. Yoshida, K. Kawano, A high-efficiency 40 GHz InGaAs multimode waveguide photodetector, J. Quant. Electr. 28, p 2728, Handbook of near-infrared physics, practical spectroscopy series, Vol.13, edited by Donald A. Burns, Emil W. Ciurczak, M d Hondt, I Moerman, P Demeester, Dark current optimisation for MOVPE grown 2.5um wavelength InGaAs photodetectors, Electronics Letters, 34 (9), , M d'hondt, I Moerman, P Demeester, "Dark current optimisation of 2.5um wavelength, 2% mismatched InGaAs photodetectors on InP," Proceedings 10th International Conference on Indium Phosphide and Related Materials (IPRM'98), IEEE, , J. John, L. Zimmermann, Stefan Nemeth, Thierry Colin, Patrick Merken, Staf Borghs and Chris Van Hoof, Extended InGaAs on GaAs Detectors for SWIR Linear Sensors, Proceedings of SPIE, Infrared Technology and Application XXVII, Aerosense, Orlando, Hamamatsu photonics K.K., Solid state devision, Product information, Infrared detectors catalogue, June Proc. of SPIE Vol