The objective of this project was to demonstrate the fabrication of extremely tight tolerance collimating grids using a high-Z material, specifically tungsten. The approach taken was to fabricate grids by a replication method involving the coating of a silicon grid substrate with tungsten by chemical vapor deposition (CVD). A negative of the desired grid structure was fabricated in silicon using highly wafering techniques developed for the semiconductor industry and capable of producing the required tolerances. Using diamond wafering blades, a network of accurately spaced slots was machined into a single-crystal silicon surface. These slots were then filled with tungsten by CVD, via the hydrogen reduction of tungsten hexafluoride. Following tungsten deposition, the silicon negative was etched away to leave the tungsten collimating grid structure. The project was divided into five tasks: (1) identify materials of construction for the replica and final collimating grid structures; (2) identify and implement a micromachining technique for manufacturing the negative collimator replicas (performed by NASA/JPL); (3) develop a CVD technique and processing parameters suitable for the complete tungsten densification of the collimator replicas; (4) develop a chemical etching technique for the removal of the collimator replicas after the tungsten deposition process; and (5) fabricate and deliver tungsten collimating grid specimens

Arrieta, Victor M.

Laferla, Raffaele

Tuffias, Robert H.

English

NASA Technical Reports Server

NASA-CR-197688
ULTRAMET 12173 MONTAGUE STREET • PACOIMA, CALIFORNIA 91331
TELEPHONE (B18) 899-0236
TELEX {910) 496-3490
FAX (81B) Bg0-1946
HIGH ENERGY COLLIMATING FINE GRIDS
/_/vAL---
VN- c,qJ
Final Technical Report
February 1995
Contract NAS5-32530
NASA Goddard Space Flight Center
Greenbelt, MD 20771
(NASA-CR-197688) HIGH ENERGY
COLLINATING FINE GRIDS Final
Technical Report, Hay 1993 - Nar.
1994 (Ultramet Co.) IZ p
N95-2_191
Unclas
G3/93 004_780
https://ntrs.nasa.gov/search.jsp?R=19950017771 2020-06-16T08:34:56+00:00Z
INTRODUCTION AND BACKGROUND
This is the final technical report submitted by Ultramet, 12173 Montague Street, Pacoima, CA
91331 to NASA Goddard Space Flight Center, Greenbelt, MD 20771 under contract NAS5-
32530. The period of this contract was from 18 May 1993 to 20 March 1994. The Ultramet
project manager was Dr. Robert H. Tuffias, supported by Victor M. Arrieta as project engineer
and Dr. Raffaele La Ferla as senior scientist. The NASA Goddard COTR was Charles Katz.
Astronomical observation and study rely on the accurate determination of radiation levels, in
order to both observe distant objects and accurately quantify exposure levels. Several upcoming
missions will deploy X-ray and gamma-ray imaging systems that image specific wavelengths (or,
alternatively, energy levels) of high-energy electromagnetic radiation. To provide accurate
imaging systems, collimating grids are required that eliminate off-axis radiation and provide
imaging of an extremely small arc angle. A series of collimating grids is envisioned, with each
grid pair (grid spacing) designed for the imaging of specific energy levels. Similar to optical,
infrared, and radio telescopes, this capability provides imaging and mapping of specific
wavelengths of high-energy radiation. Unfortunately, the fabrication techniques investigated
previously (mainly electrodischarge machining of slots) have not been able to meet the
demanding tolerances (typically 0.0001" or less) required to meet grid performance goals.
Because of the high aspect ratios required in the collimating grid slots and the extreme tolerances
that must be met, it was not expected that alternate machining techniques (e.g. abrasive grinding,
chemical, photochemical, laser, abrasive, water jet) would be able to successfully fabricate the
fine grids required.
The objective of this project was to demonstrate the fabrication of extremely tight tolerance
collimating grids using a high-Z material, specifically tungsten. The approach taken was to
fabricate grids by a replication method involving the coating of a silicon grid substrate with
tungsten by chemical vapor deposition (CVD). A negative of the desired grid structure was
fabricated in silicon using highly advanced wafering techniques developed for the semiconductor
industry and capable of producing the required tolerances. Using diamond wafering blades, a
network of accurately spaced slots was machined into a single-crystal silicon surface. These slots
were then filled with tungsten by CVD, via the hydrogen reduction of tungsten hexafluoride.
Following tungsten deposition, the silicon negative was etched away to leave the tungsten
collimating grid structure.
The primary difficulty in this method lies in the ability to completely fill the high aspect ratio
slots (approximately 50:1 for 0. l-ram pitch grids) with tungsten. This was accomplished through
a directional growth thermal gradient processing technique developed at Ultramet for densification
of metal matrix composites. This deposition technique utilizes a thermal gradient across the slot,
such that deposit growth is initiated at the bottom of the slot and proceeds to grow vertically to
fill the slot. The machined silicon negative grids were supplied by the Jet Propulsion Laboratory
(NASA/JPL).
The project was divided into five tasks:
Task 1=: Identify materials of construction for the replica and final collimating grid structures:
J
Task 2;
Task 3:
Task 4:
Task 5:
Identify and implement a micromachining technique for manufacturing the negative
collimator replicas (performed by NASA/JPL).
Develop a CVD technique and processing parameters suitable for the complete
tungsten densification of the collimator replicas.
Develop a chemical etching technique for the removal of the collimator replicas after
the tungsten deposition process.
i
Fabricate and deliver tungsten collimating grid specimens.
EXPERIMENTAL APPROACH AND RESULTS
A preliminary search for information regarding substrate materials and fabrication techniques for
the negative replica indicated that the micromachining of single-crystal silicon ,,,,'as the best
choice, as technology from the semiconductor industry could be easily applied. Tungsten was
chosen as the grid material due to its high Z number and the advanced state of its deposition
technology.
Micromachining of slits in the single-crystal silicon substrate was performed at NASA/JPL. A
small circular saw with diamond wafering blades was used to produce slits with a width of 45
microns and an aspect ratio of >20:1. Figure 1 shows a schematic of the micromachined negative
replica.
For the deposition of tungsten inside the machined slits, via the hydrogen reduction of tungsten
hexafluoride (WF6), the silicon replica substrate was placed between the two halves of a graphite
holder, sitting on the bottom half and with a 0.005" clearance below the top half, and positioned
exactly one inch from each end of the holder. This setup, shown in Figure 2, would force the
reactant gas flow through the slits of the replica. The replica and the holders were placed inside
a 5" diameter quartz chamber, which was pumped down to 5 torr and leak checked (to prevent
unwanted foreign elements from entering into the system), then resistively heated to 400-500°C
in an argon atmosphere at 200 cc/min. When the substrate reached 400°C, the tungsten and
hydrogen flows were introduced at rates of 50 and 400 cc/min respectively. The direction of the
gas flows was alternated every five minutes in order to obtain uniformity in the deposit. Due to
the extremely low deposition rate (0.72 ktm/hr), the total deposition time was 55 hours, during
which the pressure, substrate temperature, and gas flows were kept constant. To prevent bonding
between the part and holders, the system was disassembled, cleaned, and reassembled after every
six hours of run time. A schematic of the overall CVD apparatus for tungsten deposition is shown
in Figure 3.
Stable metallic tungsten coatings were deposited inside the machined slits, as shown in Figure
4, using the hydrogen reduction of WF 6. This is the "standard" tungsten hexafluoride process,
which has long been used to deposit tungsten by CVD. However, due to the size of the slits, this
reaction had to be slowed down by lowering the deposition temperature by 30% and increasing
the W:H 2 ratio from 1:4 to 1:8. These changes reduced the chemical reduction of the WF6 by the
silicon substrate(i.e. etchingof the substrateby the WF6),shown in Figure 5, and yielded an
extraordinarilyuniform coatingdistribution asshownin Figure 6.
Incompletefilling of the slits, shownin Figure 7, wasdueto the fact that the tungstendeposit
formedby "growing" from the sidewalls of theslits. Whenthe growth from eachsidewall met,
the inner void was sealedoff. However, this should have very little effect, if any, on the
performanceof the collimator grid, sincethe void is only 2-4 _m wide.
In additionto a silanechloridegasetchingmethoddevelopedby JPL,Ultrametdevelopedanew
andlesscomplicatedtechniqueusinga70%potassiumhydroxide/30%sodiumhydroxidesolution
at a constanttemperatureof 40°C. The etchingof the replicaby the hydroxide methodyielded
a clean,silicon-freetungstencollimator grid structureasshownin Figure4. This is now awell-
establishedtechniquefor future use.
CONCLUSIONS AND RECOMMENDATIONS
Silicon and tungsten were chosen as the replica and collimator materials respectively, due
mainly to the small difference between their thermal expansion coefficients.
The micromachining method proved to be inadequate for producing "fine" collimators, but
was very efficient for "coarse" collimators with a grid pitch of 0.155 mm or greater.
Tungsten was successfully deposited within the slits by CVD.
A hydroxide solution method for etching the silicon substrate was developed by Ultramet,
equally or more efficient than the silane chloride gas method developed by JPL.
A substrate material that will not react with the deposition gases is recommended in order to
maintain better control over the final dimensional tolerances.
From the literature search and the experience obtained in this project, X-ray lithography is
recommended for manufacturing of fine collimators (e.g. 0.035-mm pitch).
T
820 I_m
l
84 I_m
cm
Side view
5.0 cm ..=1
v I
Top view
Figure 1. Schematic of micromachined silicon negative replica
j
4.75"
:k
Side view
Graphite
top holder
0.005"
clearance
egative
replica
,Graphite
bottom holder
Cross-sectional
view
Graphoil
Carbon felt
0.005" gap
Negative
replica
Thermocouple
orifice
Two-piece holder
Figure 2. Schematic of negative replica/graphite holder setup
Negative
replica _ •
WF6 + H2 + Ar
Vacuum
Thermocouple
Quartz fT/C
chamber
Two-piece
graphite
,,j holder
WF6 + H2 + Ar
Vacuum
Resistance
heater
Figure 3. Schematic of CVD apparatus for tungsten deposition
6
ORIGINAL PAGE IS
OE.POORQUALITY
Figure 4. Final tungsten collimator grid structure (top: full part,
actual size _2" diameter; bottom: optical micrograph, 10x)
ORIGINAL PAGE t'.S
OF POOR QUALr_
Figure 5. Optical micrograph showing etching
of silicon substrate by WF 6
8
OIPJGtNAL PAGE iS
OF pOOR QUALITV
Figure 6. Scanning electron micrograph showing uniform
tungsten coating distribution (660x)
9
ORIGINAL PAGE IS
OF POOR QUALITY
A
t _
Figure 7. Optical micrograph showing incomplete filling
of slits (voids within tungsten)
10
Form Approved
REPORT DOCUMENTATION PAGE oMa.o. olo4-o188
Public repOrtbng burden foe gh._. collection Ol' informatnon _s est.mated to 3_erage 1 hour per r_spOr'se, including the time for rewew,ng ir_sttud_on$, seaf(:,i%lt_ e_,$tk/'.g data $O_rce$,
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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE
February, 1995
4. TITLE AND SUBTITLE
High "Energy Collimating Fine Grids
Robert H. Tuffias, and
6. AUTHOR(S}
Victor M. Arrieta,
Raffaele La Ferla
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)
Ultramet
12173 Montague Street
Pacoima, CA 91331
3. REPORT TYPE AND DATES COVERED
Final (May 1993-March 1994)
5. FUNDING NUMBERS
NAS5-32530
8. PERFORMING ORGANIZATION
REPORT NUMBER
ULT/TR-94-6505
9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)
NASA Goddard Space Flight Center
Engineering Directorate
Greenbelt, ME) 20771
10. SPONSORING / MONITORING
AGENCY REPORT NUMBER
11. SUPPLEMENTARY NOTES
NASA Goddard COTR: Charles Katz, Code 704.2
12a. DISTRIBUTION/AVAILABILITY STATEMENT
Unclassified/Unlimited
12b. DISTRIBUTION CODE
13. ABSTRACT (Maximum2OOwords)
Several upcoming missions will deploy X-ray and gamma-ray imaging systems that
image specific wavelengths of high-energy EM radiation. Accurate imaging systems
require collimating grids that eliminate off-axis radiation and provide imaging
of an extremely small arc angle. Fabrication techniques investigated previously
have not been able to meet the demanding tolerances (typically 0.0001" or less)
required to meet grid performance goals. The objective of this project was to
demonstrate the fabrication of extremely tight tolerance collimating grids using
a high-Z material, specifically tungsten. The approach taken was to fabricate
grids by a replication method involving the coating of a silicon grid substrate
with tungsten by chemical vapor deposition (CVD) . A negative of the desired grid
structure was fabricated in silicon using wafering techniques developed for the
semiconductor industry and able to produce the required tolerances. A network
of accurately spaced slots was machined into a single-crystal silicon surface.
These slots were then filled with tungsten by CVD, via the hydrogen reduction of
tungsten hexafluoride. Following tungsten deposition, the silicon negative was
etched away to leave the tungsten collimating _rid structure.
14. SUBJECT TERMS 15. NUMBER OF PAGES
collimation, collimating grids, tungsten, X-rays, ii
high-energy EM radiation, chemical vapor deposition (CVD) 16. PRICE CODE
17. SECURITY CLASSIFICATION
OF REPORT
Unclassified
NSN 75_0-01-280-5500
18. SECURITY CLASSIFICATION
OF THIS PAGE
Unclassified
19. SECURITY CLASSIFICATION 20. t IMITATION OFA_STF,',CT
OF ABSTRACT
Unqlassified SAR
S'arda,d _orm 293 _=ev 2-89!
;'_'<"b"d by _%1 ",l_ 119-'9
2'_9. ! :,2


High energy collimating fine grids

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