Fabrication of Pixelated GaAs Detectors for Ultrafast Hard X-Ray Imaging
Authors: Finnegan Patrick Sean , Wood Michael G , Altamirano Donika , Weiner Phillip , Hains Chris P. , Stephenson Chad A , Klem John F. , Sanchez Marcos , Jordan Matthew , Looker Quinn 
Published: December 2020

Sandia National Laboratories is developing GaAs photodiode arrays compatible with the Ultra-Fast X-ray Imager (UXI) platform for sub-nanosecond hard x-ray diagnostics. These 25 µm pitch pixelated detector arrays are particularly useful in the diagnostics of high energy density (HED) experiments to capture spatial variations as they evolve. Existing UXI sensors consist of a backside illuminated Si detector array bonded to a read-out integrated circuit (ROIC), which offers peak sensitivity to soft (<10keV) x-rays [1]. Substituting the Si detector with materials with higher atomic numbers Z, such as GaAs and CdTe, the sensitivity of the imager can be improved for higher x-ray energies [2]. The integration of these higher Z materials at useful pixel sizes and appropriate thicknesses (10s of µm) is challenging due to less mature processing and integration techniques. Here, we report on the fabrication of small pixel arrays using epitaxially-grown GaAs p-i-n diodes, integrated onto Si fanout packages with sub-nanosecond response times.

We have previously reported on discrete fast, hard x-ray GaAs detectors [3]. This paper focuses on transferring this prior work on single-element detectors to fine-pitch pixelated imaging arrays. Fabrication of the backside-illuminated detector array begins with the separate fabrication of the Si readout and GaAs detector chips, continues with In bump flip-chip bonding, and is then completed by removing the GaAs substrate and making backside contact to bias the detectors. A cross-sectional schematic of the fabricated device is given in Fig. 1.

The Si fanouts were fabricated in the Sandia MESA SiFab facility. Each fanout chip has a series of 1x1 mm2 bonding sites with 25 µm pitch pixels arrays ranging in size from 3x3 to 36x36. Growth and fabrication of the GaAs detectors was carried out in the Sandia MESA MicroFab facility. Molecular beam epitaxy is used to grow a series of wafers with GaAs p-i-n diode structures on commercial n-type GaAs wafers. The epitaxial structure consists of, from the substrate up, 200 nm of undoped GaAs, 400 nm of undoped Al0.4Ga0.6As to serve as an etch stop, 1 µm of n-type GaAs, 4 to 40 µm of undoped GaAs, and 400 nm of p-type GaAs.

The GaAs devices are fabricated using standard III-V semiconductor processing techniques. The bond-side (p-contact) metal contact is made with a Ti/Pt/Au metal stack. Next, the pixels are electrically isolated by wet etching through the p-GaAs and into the undoped GaAs region. A silicon nitride passivation is then deposited by PECVD and patterned to reduce dark current [3]. Finally, underbump metal and In bumps are deposited on both the Si fanout and GaAs detector chips. After In deposition, the chips are singulated and flip-chip bonded. After bonding, the parts are underfilled with a low viscosity epoxy to provide additional mechanical stability. The left chip in Fig. 2(a) shows a bonded pair after flip-chip bonding.

After bonding, the GaAs substrate is removed to allow backside metallization and illumination. The bulk of the substrate is first removed by mechanical lapping to thin the detector chip from approximately 630 µm to 70 µm. Next, a selective wet etch (5:1 citric acid:H2O2) is used to uniformly etch to the stop layer. Finally, a timed wet etch is used to remove the etch stop layer and expose the backside n-contact. These thinning steps are shown in Figs. 2(a) and (b). Backside metal contacts are formed using a low-temperature process to prevent melting the In bumps [4]. After fabrication, devices are packaged onto a custom PCB designed to allow for flexible readout as shown in Fig. 2(d).

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

SAND2020-5273A

REFERENCES

[1] L. Claus, et al., “Design and characterization of an improved, 2 ns, multi-frame imager for the Ultra-Fast X-ray Imager (UXI) program at Sandia National Laboratories,” Proc. SPIE, 10390, 2017.

[2] Q. Looker, et al., “Synchrotron characterization of high-Z, current-mode x-ray detectors,” Rev. Sci. Instrum.
91, 023509, 2020.

[3] Q. Looker, et al., “GaAs x-ray detectors with sub-nanosecond temporal response,” Rev. Sci. Instrum.
90, 113505, 2019.

[4] M. G. Wood, et al., “Formation of Ohmic contacts to n-GaAs at temperatures compatible with indium flip-chip bonding,” Proc. Electrochem. Soc., 881, 2019.

Figure 1