The Cosmology Large Angular Scale Surveyor (CLASS) is a ground-based instrument that will measure the polarization of the cosmic microqave background to search for gravitational waves form a posited epoch of inflation early in the universe's history. This measurement will require integration of superconducting transition-edge sensors with microwave waveguide inputs with good conrol of systematic errors, such as unwanted coupling to stray signals at frequencies outside of a precisely defined microwave band. To address these needs we will present work on the fabrication of silicon quarter-wave backshorts for the CLASS 40GHz focal plane. The 40GHz backshort consists of three degeneratively doped silicon wafers. Two spacer wafers are micromachined with through wafer vins to provide a 2.0mm long square waveguide. The third wafer acts as the backshort cap. The three wafers are bonded at the wafer level by Au-Au thermal compression bonding then aligned and flip chip bonded to the CLASS detector at the chip level. The micromachining techniques used have been optimized to create high aspect ratio waveguides, silicon pillars, and relief trenches with the goal of providing improved out of band signal rejection. We will discuss the fabrication of integrated CLASS superconducting detectors with silicon quarter wave backshorts and present current measurement results

Bennett, Charles L.

Chuss, David T.

Crowe, Erik J.

Denis, Kevin L.

Eimer, Joseph

Lourie, Nathan

Marriage, Tobias

Moseley, Samuel H.

Rostem, Karwan

Stevenson, Thomas R.

Towner, Deborah

U-yen, Kongpop

Wollack, Edward J.

NASA Technical Reports Server

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Fabrication of Silicon Backshorts with Improved 
Out-of-Band Rejection for Waveguide-Coupled 
Superconducting Detectors 
Erik J. Crowe'·3 , Charles. L. Bennetr, David. T. Chuss', Kevin. L. Denis', Member, IEEE, Joseph Eime~, Nathan 
Lourie'·5, Tobias Marriage2, Samuel H. Moseley', Karwan Rostem1•4, Thomas R. Stevel1lionl, Deborah Towner',), 
Kongpop U-yen' , Edward J. Wollackl 
AbstrtIcI- The Cosmol"", Large Allgular Soale Surveyor. 
(CLASS) is a ground-based instrument that will measare the 
polarization of the cosmic mierowave background to searcb for 
gravitational waves from a p.sited epocll of i.fIatio. early in tbe 
unive ... ', history. Thb measuremeot will require integrstio. of 
supercondading tran.itioD-e~le seD son with m.icrowave 
.. avOlulde inputs "ith good control of syatematic or"n, lucb a. 
un ... nted coupling to stray si ... als at frequencies outside of a 
pr .. bely defined mlcro .. ave band. To address these needs we 
will pr .... t work on the fabrication of lilicon quarter-wave 
backallom for the CLASS 40GHz (ooal plano. The 40GHz 
backahort conliots of three doconoratively doped silicon .. afers. 
Two spacer waf en are mlcrom.cllilled with throliJh w_ror. vias 
to provide a 2.04mm long square .. _VOlulde. The third warer 
acts as the backahort cap. Tbe three waren .re hoaded .t the 
wafer level by Au-Au thermal compre •• loD bonding tIIen "laDed 
and Dip chip bonded to tbe <:JASS detector at tbe cbip leveL 
The mlerom_chinlng tecbniques u,ed have beeD optimized to 
cre.te high a.pect ratio waveguides, silicon pillar., and relief 
trORebes with the goal of providing improved out of band signal 
rejection. We will di.c .... the fabrication of Integrated CLASS 
. supercondactiilg detectors with silicon quarter wave backabom 
and prete.t c.rreat mea,u.remeat results. 
Index T...........s.pereonductioll mi.rostrip, tr ••• ition edge 
,ensors, deep reactive ion etching, .. afer bondinK. ..m.e 
roughness 
I. iNTRODUCTION 
Gravitational waves produced during inflation create a unique polarization pattern on the cosmic microwave 
background (CMB). This signature offers an important 
tool with which to investigate the high-energy physics of the 
inflationary epoCh of the early Universe. The discovery of this 
signature will provide the !irst direct evidence for inflation and 
will rule out most competing explanations for the initial 
conditions of the Universe. Characterization of this signal 
offers a way to explore the !irst 10.32 seconds of the Universe. 
The polarized signal from inflation is anticipaled to be 10" of 
the 2.725 K isotropic cMB. Thus for an instrument to be 
Manuscript """ived Ootobel9; 2012. This worl< wos",uppom:d in part by 
the NASA _<II Opportunities in Spaoc and Earth Science·APRA 2008 
E. J. Crowe ;, with NASA Goddard Spaoc Flight Center. and MEl 
Technologies. Greenbelt, MD 20771 USA (3010604-7177; fax: 301-286-1672; 
o-mail: crik.j.crowc@n .... gov. 
successful fu measuring this signal it must have (I) the 
requisile sensitivity, (2) excellent control over systematic 
errors in measurement, and (3) multiple spectral bands' for 
foreground removal. NASA Goddard Space Flight Center in 
collaboration with 10hns Hopkins University are developing 
the Cosmology Large Angular Scale Surveyor (CLASS), a 
ground based Ielescope designed to search for the polarized 
divergence free uB-mode" signal in the CMB. 
The CLASS instrument takes an innovative approach to 
address the scientific requirements for measuring the B-mode 
signal. CLASS will operale at three spectral bands (40, 90 and 
150GHz) for foreground removal. Excellent beam control 
enabled by feedhoms is combined with the sensitivity 
provided by transition-edge,sensing (TES) bolometers. The 
sensor integration is enabled via the use of a broad-band 
planar orthomode transducer (OMT) that . symmetrically 
couples the independent polarizations in the waveguides into 
separate microstrip lines. The microstrip line for each 
polarization terminales in a resistor that is thermally coupled 
to a bolomeler. 'Ibis archilecture provides the very pOwerful 
advantage of incorporating the band-defining filters into the 
microstrip circuitry. 
A critical enabling component for the CLASS instrument is 
the detector Iechnology. The CLASS 400Hz detector 
archilecture was described in [I]. The full detector assembly 
is made up of three parts, a detector chip which consists of 
planar superconducting microwave antenna and filters along 
with transition edge sensors which is flip chip bonded to a 
micromachined silicon quarter wave backshort .. This structure 
is bonded to a silicon interli!.ce plate that mates the detector 
assembly to the focal plane. In 'a previous paper(REF) we 
described the fabrication pIocess for the 400Hz CLASS 
detectors. These fabrication processes are reviewed in section 
II. In section ill we describe the fabrication of the quarter 
wave backshort. Section IV reviews recent measurements of 
deviCe perfonnance. 
II. FABRICATION 
A. Detector chip fabrication 
Fabrication of the CLASS 400Hz detectors has been 
described previously in [I]. Here we give a brief overview of 
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the process. Fabrication begins with a silicon· on insulator 
wafer with a 5um thick device layer and 375um thick bandle 
wafer. The device layer will serve as the microstrip. dielectric 
as well as the thennal link between the leg isolated silicon 
membranes and the thennal bath, After patterning and etching 
a niobium ilround plane the wafer is honded to a 
degeneratively doped silicon wafer with a polymer adhesive. 
Subsequently the handle wafer is . removed from the wafer 
stack by a plasma dry etching step. · This naturally stops on the 
SOl buried oxide which is removed in buffered HF solution. 
At this point the wafer resembles a modified SOl wafer where 
the buried silicon oxide is replaced by the polymer adhesive, 
the handle wafer is degeneratively doped silicon and the 
silicon device layer has a buried niobium layer. Subsequent 
processing follows standard device lilbrication since the 
polymer adhesive, when properly cured is inert in standard 
micromaching chemicals. Additionally, after the silicon oxide 
is removed the silicon layer is clean, unrOugbned and provides 
an excellent surface for formation of the transition edge 
sensors. 
The next step is deposition and characterization of the 
transition edge sensors (TES). We use MoIAu TES consisting 
of 500A ofMo and 2700A of gold. The deposition is done in 
a two stage process where the fiist stage is a sputter deposition 
of a MoAu seed layer and the second stage is an ebeam 
deposition of gold to the ihickness required to suppress the Mo 
transition temperatore which is typically 900mK to our target 
of ISOmK. . The sputtered gold in the seed layer is thick 
enough to prevent subsequent molybdenum oxidation while 
not too thick to suppress bilayer Tc significantly. Our seed 
layers have typical Tc of 700mK. The benefit of this 
approach is the availability of a batch process where a number 
of product Wafers along with test wafer seed films can be 
processed at the same time and have similar transmission 
coupling between the molybdenum and gold. Also, the 
molybdenum transition temperatures have minimal variation 
for films deposited in the same vacuum pump down. Gold 
electron beam depositions are done subsequently after an in-
situ surfilce cleaning on the witness wafers to bracket the 
bilayer transition as a function of gold thickness prior to 
committing our product wafers. Electron beam deposited gold 
gives a higher residual resistivity ratio than room temperatore 
sputtered gold and this can help suppress bilayer Tc for 
minimaladditioual gold (2). We anneal our MoAu bilayers at 
150C after deposition to stabiliZe them against subsequent 
high temperatore processes (3). 
The TES gold is pattemed and ion mill etched stopping on 
the molybdenum. Next the niobium microstrip is deposited by 
sputter deposition. The niobium is then reactive ion etched in 
a fluorine based plasma in two steps. In the fiist step for the 
majority of the microstrip geometry the plasma chemistry is 
adjusted for a vertical niobium sidewall slope profile. This is 
to minimize additioual microwave loss due to sloped 
sidewalls. A second lithography step patterns the niobium in 
areas where it must make contact to other metal layers and this 
requires a sloped sidewall for good step coverage. The plasma 
chemistry is adjusted by adding oxygen to the chamber. The 
niobium etching is done in a bench top reactive ion etching 
tool using fluorine and oxygen plasma chemiStry. We have 
found that the etching process roughens the silicon under the 
niobium upon completion of the. etch. We have done a 
number of experiments to determine the extent of silicon 
roughening and ways of minimizing it. 
Detector Fabrication Process 
. --.... 
$Icon on InIUIatDr (SOl) WIIw: Depod Nt! 
paund,,1nd PD¥-'I¥rfor ... bonding 
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Iherrn8lIIOIdon pftlCllling on lie davie. • erda 
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indium tunp bof'Id NIlIIrfti!y fabricated .!Icon 
..-oer .nd .nicon ~ chip _ waI •• lilleon 
photonlc chou dIIp. 
Following the second niobium step the gold broadhand load 
is deposited and patterned by liftoff. The broadband load is 
used · as a tennination in the magic-T and cross-over 
components. It is also .used as a contact between the niobium 
microstrip and the PdAu termination resistor. These 
connections require the niobium microstrip to bave a sloped 
sidewall for adequate step coverage.. The PdAu deposition is 
optimized for 50hm/square resistance. The final metal 
deposition step is ebeam deposition of Pd which is used to 
provide the necessary heat capacity on the TES membrane for 
required response time and to maintain the TES membrane at 
an isothermal temperature. Following the Pd deposition and 
liftoff the silicon legs and the TES membrane are defined and 
etched. This is done in an SF6 plasma using a capacitive 
coupled RIE. 
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A typical profile of the silicon legs and the TES membrane 
is shown in figure 2. 
below. The silicon is etched isotropically with 2um of lateral 
etching for5um of vertical etching. The leg cut mask 
continues through the buried niobium ground plane and stops 
on the polymer bonding material. Next, the wafer is 
temporarily .bondedto a pyrex handle wafer and the backside 
is patterned for deep reactive ion etching. The DRIE defines 
the detector chip extent as well as the membranes for the 
antenna, crossover and ·maglc.T. Next the polymer layer is 
removed in an 02 plasma stopping on the niobium ground 
plane and the silicon. . The wafer is then soaked in solvent to 
remove the pyrex handle wafer and the individual chips are 
released. An image of the final detector chip is shown in 
figure 
III. SILI(;ON BACKSHORT F ABRlCATION 
For the 40GHz CLASS detector the backshort must be 
placed 2.04mm from the OMT. We elected to integrate the 
backshort into silicon using micromaching techniques. For 
CLASS the silicon backshorl assembly not only provides a 
backshort but also provides mitigation against high frequency 
leakage from the . focal plane that can be picked up in .the TES 
or . in unshielded circuit components. An additional 
component called the silicon interface chip is bonded to the 
detector chip. This component is used to shield the backside 
of the TES and OMT from high frequency leakage as well as 
to extend the ground plane o{the detector chip improving the 
efficiency of photonic choke which is machined into the focal 
plane. In this section we wili discuss the fabrication of the 
silicon backshort as well as the silicon interface chip. 
A. Silicon backshort 
The silicon backshort for the 40GHz detector is made up of 
3 silicon wafers. I. A 0.87mm thick interface wafer, 2. A 
1.l6Omm thick spacer wafer, and 3. A 0.325um thick cap 
wafer. All of the wafers are degeneratively doped silicon 
with room temperature resistance between I to 3mohm-cm 
and RRR of 1.7. Each of the 3 wafers are processed 
separately then subsequently bonded at the wafer level with 
gold thermocompression bonding. . 
We split up the backshort into a stack of three wafers to 
improve the sidewall profile of the waveguide which is etched 
hy deep reactive ion et£hing using the well known Bosch 
process. The Bosch process consists of alternating a plasma 
passivation step with an etching step. By adjusting the ratio 
betWeen the etch time and the passivation time the etch rate 
and sidewall slope can be controlled. For very thick wafers 
however, it is difficult to maintaiD. a high aspect slope without 
the buildup of silicon grass which fonDs due to unetched 
.passivation. The gritss can ultimately limit silicon etching. To 
sitnplify the requirements we split the through wafer etching 
into two steps where we etch halfway through the wafer from 
either side. This requires a front to back alignment and results 
in an holirglass waveguide profile with a 2·3 degree slope off 
of 9Odeg. A representative profile of the \.l6mm thick 
spacer is shown in the . 
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The O,87mm thick wafer· is processed similarly to the 
1.16mm wafer where the through wafer DRIE is etched from 
both sides. Since this wafer forms the interface to the deteetar 
chip there is an additional sOwn deep relief etch completed on 
the surface of the backshort wafer which mates to the detector 
chip to accommodate the Nb microstrip. The backshort wafer 
contacts the detector wafer around the OMT and around the 
TES membranes with only a sinall ISOurn wide mousehole to 
accommodate the niobium wiring, The sizes of the cavities 
around the TES are designed such )hat they are too smail to 
support hish frequency out of band modes that could couple 
into the TES. An SEM image of the relief etch is shown in 
figure 4. We additionally micro-machine silicon bumps at the 
contact interface. When the bonding epoxy is applied to the 
silicon bumps, capillary action forces it to flow between the 
bumps only in the contact area .while maintaiiring it only in 
areas where bonding should occur. 
Once the three backshort wafers are complete they are 
cleaned and coated with O.Sum of gold by electron beam 
evaporation. The gold coats both the sidewalls of the 
waveguide as well as the surface of the silicon. The gold 
coated wafets are bonded in a Karl Suss SB6e substrate 
bonder at 360C and 400 pressure following standard Au-Au. 
thermo-compression bonding procedures [7]. . The bonding 
sequence consists of first bonding the cap wafer to the 
1.16mm spacer wafer, then bonding the cap/spacer slack to the 
backshort interface wafer. The surface which interfuces with. 
the detector chip is then coated with niobium to reduce 
microwave loss. The final step in the process is to dice the 
2mm thick stack into individual backshorts. An image of the 
S. 
B. Silicon Inteiface Chip 
The last component in the detector assembly is a silicon 
interface chip which mates the detector chip with the CLASS 
focal plane. We elected to incorporate the interface chip in 
order to reduce the required size of the detector chips which 
are the most complicated and time consuming fabrication 
process. This allowed us to increase the number of chips per 
four inch wafer from 4 to 12. The interfuce chip increases the 
effective size of the detector chip ground plane thus improving 
the efficiency of a photonic rooke machined into the focal 
plane. It also caps off the back of the · cavities under the TES 
membrane, magic-T and microstrip crossovers reducing out of 
band leakage. It is etched such that it can be clamped against 
metal posts on the focal plane. The interface chip is 0.32mm 
thick low resistance silicon that is coated by aluminum to 
reduce microwave loss. Assembly of the backshort, detector 
chip and interfuce chip is done by flip chip bonding. First the 
backshort is bonded with epoxy to the detector chip and next 
the interface chip is bonded to the back of the detector chip. 
The photo in figure 6 shows the detector chip bonded to the 
interface chip. 
REFERENCES 
[I] K. L. Denis, N. T. Cao, D. T. Chuss, J. Eimer, J. R. Hinderks, W.-T. 
Hsieh, S. H. Moseley, T. R. Stevenson, D. J. Talley, K. U.-yen,aDd E. J. 
Wollack, \Fabrication of an antenna-ooupled bolometer for cosmic 
miclowave baokground polarimetry," AlP Conference Proceedings 
1185(1), pp. 371 {374, 2009. . 
, [2] K. Rootcm, C.LBenneU, D.T. Chuss, N. Costm, E. Crowe, K.LDeni3, 
J.R. Eimer, N. Lourie, T. Essinger-Hilem .... T.A. Marriage, S.H. 
Moseley, T.R. Stevenson, D.W. Towner, G. Vocllm." E.J. Wollack, L. 
Zcns. ':Deteetor Architcct= of the Cosmology Large AngUlar Scale 
Surveyor", SPIE Astrooomieal Telesoopes aDd Instrumentation 
[3] J. M. _is, G.C. HiltOll, K.D. Irwin, DA WoOllman, "Calculation 
ofTe in a Normal-Superconductor Bilayer using the Microscopic Based 
U,adel Theory", Nuclear Instruments and Methods in· Ph)'llics Research 
A 444 (2000) 23-27 
[4] M. Parra-Bordenas, L Fcrrumdcz-Mart,ncz, L.Fabniga, A. Carnon, O. 
Gil, R. Gonzalez-Arrabal, J. Sese, J. L. Costa-Kramer, B. Warot-
Fonrosc, V. Serin, F. Briones, "Thermal Stability ofMoAu Bilayers for 
11lS Applicatioos", Supm;ond, Sci. Technol, 25, (2012) 095001 
[51 M Ghargh~ and S. Sivoththaman,"Formation of Nanoscale Columnar 
Structmes in Silicon by a Maskless Reactive Ion Etching Process", J. 
Vac. Sci. Techno!.,A24(3),May/June2006 . 
[6] G. C. Schwartz and P.M Schaible, "Reactive ion _ing of .iIi"",,·, J. 
Ves;. Sci. Teelmo!., 16(2) MarJApr. 1979 
[7] M. Go12a, B. Saint-Cricq, M. Dotoit, P.H. Jouneau, "Natural Masking 
for Producing sub lOOnm Silicon Nanowlres .. ,. Microekctronic 
Engineering, 27, (1995) pp 129-132 
[8] C. H. Ts .... M.· A. Scbmid~ S.M. Spearing, "Chanu:tcrizatiOD of low 
temperature, wafer level gold-gold thcrmocompression bonds", Proc. 
MateriaisScieoce ofMicroElectroMechanical Systems (MEMS) Devices 
11, 1999, ppI71-176. 


Fabrication of Silicon Backshorts with Improved Out-of-Band Rejection for Waveguide-Coupled Superconducting Detectors

Fabrication of Silicon Backshorts with Improved Out-of-Band Rejection for Waveguide-Coupled Superconducting Detectors

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