ARTICLE IN PRESS

Nuclear Instruments and Methods in Physics Research A 520 (2004) 483–486

*Corresp

E-mail a

0168-9002/$

doi:10.1016

Fabrication of the SCUBA-2 detector arrays

Michael D. Audleya,*, William D. Duncana, Wayne S. Hollanda,Anthony Waltonb, William Parkesb, Camelia Dunareb,

Alan Gundlachb, Tom Stevensonb, Kent D. Irwinc, Gene C. Hiltonc, Eric Schulted,Peter A.R. Adee, Carole Tuckere

aUnited Kingdom Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, Scotland, UKbScottish Microelectronics Centre, The University of Edinburgh, The King’s Buildings, Edinburgh EH9 3JF, Scotland, UK

cNational Institute of Standards and Technology, 325 Broadway, Boulder, CO 80305, USAdRaytheon Vision Systems, Goleta, CA, USA

eDepartment of Physics and Astronomy, University of Wales, Cardiff, 5 The Parade, Cardiff CF24 3YB, Wales, UK

Abstract

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 B10; 000 bolometers, SCUBA-2 is made possible by a uniquecombination 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 500 nm

silicon nitride membrane with low tensile stress. Deep-etch micromachining to 100 mm 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.

r 2003 Elsevier B.V. All rights reserved.

PACS: 85.25.Pb; 07.57.Kp; 85.25.Na; 85.40.�e

Keywords: Microfabrication; Bolometer array; Bump bonding; Deep etching

1. Introduction

SCUBA-2, initiated by the United KingdomAstronomy Technology Centre (ATC) at theRoyal Observatory in Edinburgh is the second

onding author.

ddress: mda@roe.ac.uk (M.D. Audley).

- see front matter r 2003 Elsevier B.V. All rights reserve

/j.nima.2003.11.379

generation sub-millimeter detector array for theJames Clark Maxwell Telescope [1,2]. The newinstrument will detect point sources 10 times fasterthan its predecessor [3] with improved imagefidelity and an increase of more than 15 times inthe field of view [4]. The detailed design andfabrication of the detectors is largely shared by theScottish Microelectronics Centre (SMC) and the

d.

ARTICLE IN PRESS

M.D. Audley et al. / Nuclear Instruments and Methods in Physics Research A 520 (2004) 483–486484

National Institute of Standards and Technology(NIST) at Boulder. Production of the SCUBA-2prototype subarrays is now underway.

2. Array design

The focal plane is populated by four subarraysof 40� 32 pixels, each of which is fabricated from3 in: Si wafers. The layout of a subarray is shownin Fig. 1. The detector wafer is thinned to 63 mm or100 mm; depending on the detection wavelength,and masked so that deep etching of the pixel wellleaves a silicon ‘‘brick’’ on a silicon nitridemembrane. The surface of the brick is ion-implanted to 377 O=& to match free space andto dissipate incident radiation as heat. Underneatheach brick is a transition edge sensors (TES)thermometer which also acts as a backshort. Bumpbonds connect each TES to a correspondingsuperconducting quantum interference device(SQUID) which detects the change in conductioncurrent due to the incident radiation and ampli-fies the signal. A second stage of SQUIDsprovides further amplification and multiplexingfor readout [5].

3. Microfabrication technology

SCUBA-2 uses a unique combination of ad-vanced techniques: thermal bonding and thinningof wafers; ion implantation to match detector toimpedance of free space; deep-etch micromachin-ing to 100 mm by the Bosch process to isolate eachpixel; deposition of 500 nm silicon nitride mem-

Fig. 1. Representation of a group of detector pixels within a

40� 32 subarray. The array pitch is 1:135 mm and the

interpixel walls are 50 mm thick.

brane with low tensile stress [6] TES as the detectoron each pixel [7]; multiplexed SQUID readout [5];and flip-chip bonding to the multiplexer (MUX)chip by indium bumps.Each process step was verified individually at

first. As the development has progressed we havecombined steps during optimisation to check thereare no adverse interactions.

4. Processing steps

SCUBA-2s subarrays are fabricated from three3 in: wafers. The ‘‘handle’’ wafer provides mechan-ical support during processing, but is etched awayat the end. The detector wafer forms the sub-millimeter bolometers. The MUX wafer carries theSQUID multiplexer circuitry. The detector andhandle wafers are preselected for flatness to ensurereliability of bump bonding.

4.1. Implantation

The detector wafer is implanted1 to have asurface resistance close to 377 O=& to match theimpedance of free space. We determined thecorrect implant dose by implanting test waferswith different doses and measuring the submilli-meter transmission in a fourier transform spectro-meter (FTS) at Cardiff University. The FTStransmission curves were fitted with a simpletransmission line model to obtain the correct dose.

4.2. Oxidation

A very thin layer of thermal oxide is grown onthe detector and handle wafers to enable them tobe bonded together. The oxide layer on the handlewafer is etched into a ‘‘racetrack’’ pattern which islater used as a mask to isolate the pixels by deepetching.

4.3. Wafer bonding and thinning

The oxide surfaces of the handle and detectorwafers are chemically activated and the wafers are

1 Ion Beam Services, Bathgate UK.

ARTICLE IN PRESS

Fig. 2. Indium bumps 20 mm square used to hybridise the

detector and multiplexer wafers.

Fig. 3. SEM image showing deep-etched silicon quarter-wave

bricks and walls. The lighter colour of the bricks is caused by

charge buildup. The bricks are suspended on silicon nitride

membranes and are thus less well grounded than the walls.

Fig. 4. SEM image showing a 10 mm trench. The etch stopped

on the nitride membrane. The trench is 100 mm deep.

M.D. Audley et al. / Nuclear Instruments and Methods in Physics Research A 520 (2004) 483–486 485

brought together and are then annealed. The yieldfor this step is about 50% due to the presence ofsmall particles on the surfaces, making it the mainyield-limiting step for the entire process. The backof the detector wafer is then ground down to therequired thickness2 ð63 mm for the l ¼ 850 mmarray and 100 mm for the l ¼ 450 mm array) andpolished.

4.4. Nitride deposition

A 0:5 mm layer of nitride is grown on the back ofthe detector wafer. It is silicon rich for low stress.The TES, heaters and bump bond pads are thenpatterned onto this nitride membrane at NIST.

4.5. Hybridisation

The assembly is then bump bonded to the MUXwafer (see Fig. 2). The bumps are superconductingindium and provide the electrical and thermalconnections between the MUX chip and detectors.There are about 80 bumps per pixel. Such a largenumber is needed for heat conduction because thebumps are superconducting. There is also a‘‘forest’’ of bumps around the edges of eachsubarray, giving a total of B160; 000 bumps persubarray.

4.6. Deep etching

Deep-etch silicon micromachining by the Boschprocess is then used to remove the handle waferand etch a trench around each pixel down to the

2Valley Design Corp., Santa Cruz, CA.

nitride layer as shown in Figs. 3 and 4. The maskfor this is the racetrack pattern left behind in theoxide layer from the handle wafer. Each pixel isnow thermally isolated and suspended on a nitridemembrane which forms the bolometer’s thermallink to the heat bath.

References

[1] W.D. Duncan, et al., Proceedings LTD-9, AIP, Vol. 605,

Madison, WI, July 2001, p. 577.

ARTICLE IN PRESS

M.D. Audley et al. / Nuclear Instruments and Methods in Physics Research A 520 (2004) 483–486486

[2] W.D. Duncan, et al., Nucl. Instr. and Meth. A, these

Proceedings.

[3] W.S. Holland, et al., Mon. Not. R. Astron. Soc. 303 (1999)

659.

[4] M.D. Audley, et al., Nucl. Instr. and Meth. A, these

Proceedings.

[5] K.D. Irwin, et al., Nucl. Instr. and Meth. A, these

Proceedings.

[6] G.C. Hilton, et al., Nucl. Instr. and Meth. A, these

Proceedings.

[7] K.D. Irwin, Ph.D. Thesis, Stanford, 1995.