Indium Bump Research Articles

A chip removal facility for indium bump bonded pixel detectors

Yield maximization in multichip hybrid pixel sensors is a crucial issue in large volume productions planned for future High-Energy Physics experiments. Bump bonding process optimization can guarantee statistical single bump failure rates at the acceptable level of 10–100 ppm; nevertheless, systematic effects connected to process repeatability can affect the functionality of a full chip in a module to a much larger extent. Because of this, the reversibility of the bonding procedure has been investigated. A feasibility study on single chip assemblies for the ATLAS experiment has been successfully completed, proving the possibility of reworking. As a result of it, a dedicated facility has been conceptually designed, engineered and commissioned. The characteristics of the facility in terms of motion, temperature and tensile strength control are outlined, together with the main results.

Microwave techniques for high-density electronics interconnect bonding and hybridization

Microwave Bonding Instruments, Inc. (MBI), has been investigating die-to-die (Level-0) bonding of independent devices, or device-substrate hybridization, as part of a system-on-a-package (SOP). MBI applies microwaves to selectively heat the interconnect metal to bond the substrates. MBI bonded a 320/spl times/256 array of indium bump of infrared detectors made of GaAs to the CMOS readout on silicon substrates. This selective heating technique has doubled the bump-to-bump bond strength to 0.2 g/bump and minimized the processing time (less than 15 s) at an overall low-substrate temperature (86/spl deg/C). The thermal mismatch between substrates is reduced in this bonding process. The technique was also applied to higher melting temperature metals. Bonding of gold-tin-to-gold-tin all the way up to gold-to-gold has been demonstrated on parts up to 400/spl times/400 bumps. A SOP for advanced electronics requiring a variety of substrates will benefit from this technology.

Characteristics of Indium Bump for Flip-Chip Bonding Used in Polymeric-Waveguide-Integrated Optical Interconnection Systems

We have flip-chip-bonded vertical-cavity surface-emitting laser (VCSEL) arrays on polymeric-waveguide-integrated optical interconnection systems. Using indium solder bumps, thermal damage to the polymeric waveguide can be avoided. Fracture occurs between the indium solder bumps and the VCSEL chip pad during the die shear test. It is inferred that both the low bonding temperature and the oxide layer formed on the surface of the indium solder prevent the bump from interacting with the chip pad. To reveal the microstructures of the joints between the bump and the chip pad, several specimens are cut into cross sections and polished. Scanning electron microscopy (SEM) with an energy dispersive X-ray (EDX) spectroscopic system is used to examine the microstructures and analyze the element compositions. Also, the optoelectronic characteristics of VCSEL arrays that were flip-chip-bonded under different bonding conditions are compared by current-voltage (I-V) and light-current (L-I) inspection.

Spectral crosstalk by radiative recombination in sequential-mode, dual mid-wavelength infrared band HgCdTe detectors

For small pixel, infrared (IR) focal plane arrays (FPAs), Raytheon Vision Systems’ architecture for integrated, dual-band detectors uses the sequential mode of the n-p+-n configuration. There is a single indium bump per pixel, leaving the p+ layer floating, and the operating polarity of the bias selects the spectral sensitivity by reverse-biasing the active p-n junction. Photogenerated minority carriers in the absorber layer of the forward-biased inactive photodiode are lost through recombination. This paper is the first report of a new optical crosstalk mechanism that occurs in sequential-mode, dual-band detectors. In the long-wavelength mode under out-of-band, short-wavelength illumination, radiative recombination yields emission near the bandgap energy of the short-wavelength absorber layer, resulting in a spurious short-wavelength response that appears as spectral crosstalk. We present experimental and device modeling results on the spectral crosstalk in molecular-beam-epitaxy-grown HgCdTe arrays with the cutoff wavelength of both bands in the 4–5-µm range.

Demonstration of a 256×256 middle-wavelength infrared focal plane array based on InGaAs/InGaP quantum dot infrared photodetectors

We report a demonstration of an infrared focal plane array based on InGaAs/InGaP quantum dot infrared photodetectors. The middle-wavelength infrared quantum-dot infrared photodetector (QDIP) structure was grown via low-pressure metal organic chemical vapor deposition. A detectivity of 3.6×1010 cm Hz1/2/W was achieved at T=95 K and a bias of −1.4 V. The background limited temperature of our QDIP was 140 K with a 45° field of view. A 256×256 detector array was fabricated with dry etching, and hybridized to a Litton readout chip by indium bumps. Thermal imaging was achieved at temperatures up to 120 K. At T=77 K, the noise equivalent temperature difference was measured as 0.509 K with a 300 K background and f/2.3 optics.

Design and fabrication of the detector technology for SCUBA-2

The design and fabrication is described of the prototype IR detector for the SCUBA-2 80×80 pixel array (Submillimetre Common User Bolometer Array), which is to be mounted on the James Clerk Maxwell Telescope in Hawaii. The detector technology is based on silicon micromachining, with transition edge sensors (TES) being used to detect incoming radiation with wavelengths of 450 and 850 μm. Each TES is located on a λ/4 silicon brick suspended on a silicon nitride membrane and supported by a silicon micromachined waffle. Low-temperature indium bump bonding connects each TES to a SQUID multiplexer chip. The paper details the design considerations and the technology used to fabricate the detector wafer.

Improvement of Adhesion and Microwave Transmission Characteristics of Indium Bump by Silver Coating for Low Temperature Flip-Chip Applications

ABSTRACTWe have conducted low-temperature flip-chip bonding for both optical interconnect and microwave applications. Flip-chip bonding of vertical-cavity surface-emitting laser (VCSEL) arrays was performed on a fused silica substrate that provides propagation paths of laser beams and also supports a polymeric waveguide. To avoid thermal damage of the polymeric waveguide during the flip-chip bonding, indium bumps were used and the bonding condition of the flip-chip was determined as a heating temperature of 150 °C and a pressure of 500 gf. Experimentally, a thin silver (Ag) layer coated on the indium bump was very effective to enhance the adhesion strength between the indium bump and the VCSEL chip pads. In addition, the microwave characteristic of coplanar waveguide (CPW) package was slightly improved by the Ag coating.

Optoelectronic and microwave characteristics of silver coated indium bumps for low temperature flip-chip applications

The effect of silver coating on pure indium bumps for low temperature and high speed flip-chip applications is presented with respect to optoelectronic and microwave characteristics. The current-voltage and light power-current characteristics of flip-chip bonded vertical-cavity surface-emitting laser (VCSEL) arrays are improved by coating a thin silver layer of 0.2 µm. The flip-chip assembled coplanar waveguide packages using silver coated bumps show the insertion loss is about 3.3 dB/cm at 20 GHz and the return loss is under 13 dB.

SCUBA-2: A large-format TES array for submillimetre astronomy

SCUBA-2, which replaces the Submillimetre Common User Bolometer Array (SCUBA) (Mon. Not. R. Astron. Soc. 303 (1999) 659) on the James Clerk Maxwell telescope in 2006, will be the first CCD-like array for submillimeter astronomy. Unlike previous detectors which have used discrete bolometers, SCUBA-2 has two DC-coupled, monolithic, filled arrays with a total of ∼10,000 bolometers. It will offer simultaneous imaging of an 8×8 arcmin field of view at wavelengths of 850 and 450μm. SCUBA-2 is expected to have a huge impact on the study of galaxy formation and evolution in the early Universe as well as star and planet formation in our own Galaxy. Mapping the sky to the same S/N up to 1000 times faster than SCUBA, it will also act as a pathfinder for the new submillimetre interferometers such as ALMA. SCUBA-2's absorber-coupled pixels use superconducting transition edge sensors (Ph.D. Thesis, Stanford, 1995) operating at ∼120mK for photon noise limited performance. The monolithic silicon detector arrays are deep-etched by the Bosch process to isolate the pixels on silicon nitride membranes (Nucl. Instr. and Meth. A, these proceedings). Electrical connections are made through indium bump bonds to a backplane that incorporates a SQUID time-domain multiplexer. We describe the key technologies that make SCUBA-2 possible and give an update on the considerable progress in the detector development and instrument design that has taken place over the last 2 years.

Fabrication of the SCUBA-2 detector arrays

We describe the techniques used to fabricate SCUBA-2, the first large-format, filled array of bolometers for sub-millimeter astronomy. With two monolithic arrays of ∼10,000 bolometers, SCUBA-2 is made possible by a unique combination of advanced technologies. The detectors are made from thermally bonded and thinned silicon wafers whose surface is ion-implanted to match the impedance of free space. SCUBA-2s pixels are suspended on a 500nm silicon nitride membrane with low tensile stress. Deep-etch micromachining to 100μm by the Bosch process isolates each pixel thermally. Proximity effect transition edge sensors formed from Mo/Cu bilayers (Nucl. Instr. and Meth. A, these proceedings) are the temperature sensing elements for the bolometers. To read out such a large number of pixels, SCUBA-2 uses a superconducting quantum interference device readout for time domain multiplexing (Nucl. Instr. and Meth. A, these proceedings). The detector wafer is flip-chip bonded to the multiplexer wafer by indium bumps which provide electrical and thermal connections. The technologies that make SCUBA-2 possible have applications for large-format arrays from the submillimeter through the X-ray spectral regions.

Fabrication of indium bumps for hybrid infrared focal plane array applications

Hybrid infrared focal plane arrays (FPAs) have found many applications. In hybrid IR FPAs, FPA and Si read out integrated circuits (ROICs) are bonded together with indium bumps by flip-chip bonding. Taller and higher uniformity indium bumps are always being pursued in FPA fabrication. In this paper, two indium bump fabrication processes based on evaporation and electroplating techniques are developed. Issues related to each fabrication technique are addressed in detail. The evaporation technique is based on a unique positive lithography process. The electroplating method achieves taller indium bumps with a high aspect ratio by a unique “multi-stack” technique. This technique could potentially benefit the fabrication of multi-color FPAs. Finally, a proposed low-cost indium bump fabrication technique, the “bump transfer”, is given as a future technology for hybrid IR FPA fabrication.

Demonstration of 256 x 256 focal plane array based on Al-free GaInAs-InP QWIP

We report the first demonstration of an infrared focal plane array based on aluminum-free GaInAs-InP quantum-well infrared photodetectors (QWIPs). The long wavelength QWIP structure was grown via a low-pressure metal-organic chemical vapor deposition. Corrugated light coupling structure of QWIP was fabricated with a dry etching process. A 256/spl times/256 detector array was also fabricated with dry etching and hybridized to a Litton readout integrated circuit via indium bumps. A unique positive lithography method was developed to perform indium-bump liftoff. The noise equivalent differential temperature (NE/spl Delta/T) of 29 mK was achieved at 70 K with f/2 optics.

MCT heteroepitaxial 4 × 288 FPA

4 × 288 heteroepitaxial mercury–cadmium telluride (MCT) linear arrays for long wavelength infrared (LWIR) applications with 28 × 25 micron diodes and charge coupled devices (CCD) silicon readouts were designed, manufactured and tested. MCT heteroepitaxial layers were grown by MBE technology on (0 1 3) GaAs substrates with CdZnTe buffer layers and had cutoff wavelength λ co≈11.8±0.15 μm at T=78 K. To decrease the surface influence of carrier recombination processes the compositionally dependent layers with increase of Cd content both toward the surface and HgCdTe/CdZnTe boundary interface were grown. Silicon readouts with CCD multiplexers with input direct injection circuits were designed, manufactured and tested. The testing procedure, to qualify readout integrated circuits on wafer level at T=300 K, was worked out. The silicon readouts for 4 × 288 arrays, with skimming and partitioning functions included were manufactured by n-channel MOS technology with buried or surface channel CCD register. The HgCdTe arrays and Si CCD readouts were hybridized by cold welding indium bumps technology. With skimming mode used for 4 × 288 MCT n–p-junctions, the detectivity for several tested 4 × 288 arrays was within D λ ∗≅9×10 10–1.8×10 11 cm Hz 1/2/W for background temperature T b=295 K.

Effects of bulk and surface conductivity on the performance of CdZnTe pixel detectors

We studied the effects of bulk and surface conductivity on the performance of high-resistivity CdZnTe (CZT) pixel detectors with Pt contacts. We emphasize the difference in mechanisms of the bulk and surface conductivity as indicated by their different temperature behaviors. In addition, the existence of a thin (10-100 A) oxide layer on the surface of CZT, formed during the fabrication process, affects both bulk and surface leakage currents. We demonstrate that the measured I-V dependencies of bulk current can be explained by considering the CZT detector as a metal-semiconductor-metal system with two back-to-back Schottky-barrier contacts. The high surface leakage current is apparently due to the presence of a low-resistivity surface layer that has characteristics which differ considerably from those of the bulk material. This surface layer has a profound effect on the charge collection efficiency in detectors with multi-contact geometry; some fraction of the electric field lines originated on the cathode intersects the surface areas between the pixel contacts where the charge produced by an ionizing particle gets trapped. To overcome this effect we place a grid of thin electrodes between the pixel contacts; when the grid is negatively biased, the strong electric field in the gaps between the pixels forces the electrons landing on the surface to move toward the contacts, preventing the charge loss. We have investigated these effects by using CZT pixel detectors indium bump bonded to a custom-built VLSI readout chip.

Readout Techniques for Drift and Low Frequency Noise Rejection in Infrared Arrays

Three different methods are presented to subtract thermal drifts and low-frequency noise from the signal of infrared array. The first is dead pixels with open Indium bumps, the second is reference output as implemented on the Hawaii2 multiplexer, and the third is dark pixels to emulate reference cells having a capacity connected to the gate of the unit cell field-effect transistor (FET). The third method is the most effective and yields a reduction in readout noise from 15.4-9.4 erms. A novel method will be described to extend this readout technique to the Aladdin 1K×1K InSb array.

Characterization of indium and solder bump bonding for pixel detectors

A review of different bump-bonding processes used for pixel detectors is given. A large-scale test on daisy-chained components from two vendors has been carried out at Fermilab to characterize the yield of these processes. The vendors are Advanced Interconnect Technology Ltd. (AIT) of Hong Kong and MCNC in North Carolina, USA. The results from this test are presented and technical challenges encountered are discussed.

Development of a bump bonding interconnect technology for GaAs pixel detectors

Pixel detectors made of semi-insulating GaAs have already been successfully tested for their application as tracking detectors in high energy physics experiments. For the operation of large arrays of pixel sensors, a technology allowing a two dimensional interconnect with a high yield is necessary. For this purpose an indium based bump bonding process has been developed. Especially the generation of the indium bumps on the surface of the GaAs pixel detector was optimized. The development of these processing steps is described. In order to investigate the influence of these steps and of the bonding procedure on the detector performance, I– V-characteristics of bump bonded GaAs pixel detectors were analyzed.

Study of indium bumps for the ATLAS pixel detector

The bump-bonding technology is used to join the front-end read-out chips to the silicon substrate of the ATLAS pixel detector. We review the current status of the technology used by Alenia Marconi Systems and we report on the electrical and mechanical properties and the defect rate of the Indium bumps.

High resolution CdZnTe pixel detectors with VLSI readout

CdZnTe pixel detectors with a custom VLSI readout, are being developed at Caltech/JPL for use in focusing hard X-ray telescopes. We have recently tested several prototype detector assemblies, each consisting of a 2 mm thick CdZnTe pixel detector indium bump bonded to our VLSI readout chip. A complete pulse height analysis chain is located directly below each 680 by 650 /spl mu/m pixel and includes a preamplifier, shaping amplifiers, and a peak stretcher/discriminator. Here we report on the first fully functional operation of these detector/VLSI hybrids. Using an /sup 241/Am source, collimated to illuminate a single pixel, excellent energy resolution of 670 eV FWHM was measured for the 59.5 keV line at -10 C, with electronic noise of only 340 eV FWHM. Illumination with an uncollimated /sup 241/Am source was performed to assess the uniformity of pixel response and to exercise the readout chip's ability to process multiple pixel events arising from X-rays interacting above pixel boundaries. The imaging capability of the detector was demonstrated using a tungsten slit mask.

A novel micromachined vibrating rate-gyroscope with optical sensing and electrostatic actuation

This paper reports the fabrication and preliminary characterization of a novel vibratory rate-sensor. The sensor employs the modulated integrative differential optical sensing (MIDOS) to detect the output mode amplitude. The mechanical part of the sensor is fabricated by bulk micromachining using a double side anisotropic wet etching process. A CMOS chip containing the detecting photodiodes and their readout electronics is fabricated through MOSIS. Electrical and mechanical integration of the two parts is achieved by using the indium bumps technology. The proof mass is aligned with the detecting photodiodes, so that when at rest, equal portions of the two photodiodes are exposed. Several prototypes have been fabricated and tested on a rotating table. Good linearity (<1%) was shown on a −1000–1000°/s range and so far, a minimum detectable rate (MDR) below 1°/s was demonstrated. Future research will focus on lowering the MDR below 0.01°/s.