Note: Descriptions are shown in the official language in which they were submitted.
7~)~0
TlTLE
Silicon nozzle Structures and Method of Manufacture
TECHNICAL FIELD
Monocrystalline silicon bodies with passages.
5 BACRGROUND OF THE INVENTION
In the prior art and specifically in united States
Patent 3,921,91~ it is suggested that a monocrystalline,
crystallographically oriented silicon wafer may oe selectively
etched to form one or more reproducible channels of a specific
10 form in the wafer body. The specific type of the channel
described in that patent has a rectangular entrance cross-
section which continues to an intermediate rectangular
cross-section, smaller than the entrance cross-section, and
then to an exit cross-section which has a shape other than
15 rectangular. A channel of this specific type is established
by either of two disclosed processes, both of which utilize
a heavily doped p+ layer (patterned in the one process and
unpatterned in the other as an etchant barrier. In the two
processes, a siliccn wafer is heavily doped to place it near
20 or at saturation from one major face to form the p+ etchant
barrier. Thereafter, patterned anisotropic etching from the
opposite major face proceeds until the p+ barrier is reached.
The anisotropic etching resu}ts in a rectangular entrance
cross-section and a rectangular intermediate cross-section
25 defining a membrane smaller in size than the entrance
cross-section.
In the application of one process, the etching
process ls contin-led ft-om the entrance side until an opening
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is made through the membrane. The other process utilizes
patterned isotropic etching prom the opposite side (exit side)
of the nozzle to complete a passage through the membrane to
the intermedlate cross-section.
Although these prior art processes may provide
satisfactory ink jet nozzle structures, both of the described
processes and the resulting structures have inherent problems.
For example, due to inherent wafer thickness variations and
isotropic etch nonuniformities, these processes require
10 extensive mechanical and/or chemical polishing of both major
surfaces of the wafer to improve dimensional control of the
esulting nozzle structures. This is a costly processing
step. Additionally, the nozzle structures produced by these
processes have heavily satnrated p+ regions surrounding the
15 exit openings, and these regions tend to be brittle and thus
subject to failure when exposed to high fluid pressures or
pressure transients typically present in ink jet printing
systems.
DISCLOSURE OF TOE INVENTION
In accordance with the present inventlon, a
standard commercially available semi-conductor wafer of
crystallographically oriented, monocrystalline p-type sillcon
is used to produce a single fluia nozzle or an array ol
nozzles directly and without the need for mechanical or
25 chet~ical polishing of the two major surfaces of the wafer by a
process wherein a low saturation n surface layer is formed on
at least one major s~1rface of the wafer. materials resistant
to an an{sot~op~c etchant, later employed, are then deposited
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on both surfaces of the wafer. Thereafter, aperture masks
defining the entrance and exit areas of a nozzle are formed on
these major surfaces and the exit area is coated with a
material whieh is both resistant to an etching solution and
5 whieh provides an electrical connection to the n layer. A
cavity is anisotropicallY etched from the entranee area of the
wafer through to the n layer at the exit side by immersing the
wafer in a caustic etching solution. A potential applied
across the p/n junction at the exit side of the wafer
10 electroehemically stops the etching action leaving a membrane
having a thickness substantially equal to the n-layer. A
passage is then anisotropieally etched through the membrane
from the exit side to complete the nozzle structure.
THE DRAWINGS
FIG. 1 shows a perspeetive view of a portion of the
nozzle strueture in aeeordanee with the present invention.
FIG. 2 is a cross-sectional view of the nozzle
structure taken along line 2-2 of FIG. 1.
FIG, 3 through 8 illustrate sequential cross-
20 sectional views of a sllicon wafer processed in accordancewith the present invention.
: DETAILED DESCRIPTlON
In multi-nozzle inX jet printing systems utilizing
nozzles made of semi-conductor materlal, some of the more
25 important charaeteristics required of the nozzle are the
uniforc~ity in the sfze of each respective nozzle, spatial
distribution of the nozzles in an array, their resistance to
craekln~ uncler the ~lu;c11c pressures encountered ln the
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system, provision of an efflcient mechanical impedance match
between the fluid supply and the exit opening, as well as,
their reslstance to wear caused by the high velocity fluid
flow through the nozzle structure.
Referrlng now to FIG. 1, there Is shown a portion of
the nozzle structure made in accordance with the present
invention. Specifically a substrate 10 is shown having an
array of uniform openings 11 therein. Each opening 11 starts
with an initial, substantially square area and tapers to and
10 terminates in a substantially square area smaller than the
initial square area defining a membrane 12. As shown in
Fig. 2, each membrane 12 in turn has an opening 13 extending
therethrough which starts in a substantially square area
smaller than the square area of each respective membrane 12
15 and terminates in a substantially square area larger than the
starting square area of said openlng. Both horizontal axes of
- the openings 13 in the membrane 12 are substantially aligned
with the horizontal axes of each corresponding opening 11 in
the main body of the wafer 10 by virtue ox the wafer 10
20 crystallography.
~lGS. 3 through 8 illustrate a sequence of process
steps for production of an aperture in a single crystal
silicon wafer 10 for forming one fluid nozzle or an array of
nozzles. It is to be understood that the following process
25 steps may be Ised in a different sequence`and that other film
materials for performing the same functions described below
mag be used. F-lrthermore, fllm formatlon, size, thickness and
thc llke, may also bc varled. The wafer l is of single
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crystal (100) oriented p type silicon with electrical
resistivity of .5 to 10 ohm-cm, approximately 19.5 to 20.5
mils thick having front 14 and back 15 surfaces. The (]00)
planes are parallel to surfaces 14 and 15~ As shown in FIG.
5 3, phosphorous is diffused into the front 14 and back 15
surfaces of the silicon wafer 10 to a depth of about 5 microns
forming n type layers 16 and 17. As will become obvious later
only one diffused layer is required to form a nozzle structure
by the process (exit side). The diffusion is accomplished in
10 a well-known manner by hazing a gas mixture containing 0.75%
P~3, 1X20 , and the make-up of Ar and N2 flow for 30 minutes
past the silicon wafer 10 which is maintained at 950 C. This
is followed by a long drive-iD period (1~50 C for 22 hours) to
achieve a thick layer (about 5 microns). Since the final
15 coDcentration of-phosphorous in the n layers 16 and 17 is very
low, this diffusion step introduces very little stress into
the silicon wafer 10, and consequently the silicon structtlre
~etai~s its strength.
Next as shown in FIG. 4, both front 14 and back 15
20 surfaces of the wafer 10 are coated with a protective material
sigh as LPCVD silicon nitrlde forming layers 18 and 19 which
can resist a long etching period in a caustic (ROW) solution.
One of the ways to accomplish this is to utilize a low
pressure chemlcal vapor deposition of silicon nitride
25 deposited at about 800 C. Oxide layers (not shown) less than
0.5 mlcrons thick may be grown on both sides of layers 18 and
19 to reduce the effect of stress between nitride and silicon
end to l~prove adhesion of photoresist to nitride. To promote
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ease of photoshaping it i6 recommended that the wafer 10 when
procured have its back surface 15 etched in an acidic rather
than caustic solution.
Thereafter, masks are prepared corresponding to the
5 desired entrance 20 and exit 21 areas of the nozzle. The
masks for both entrance 20 and exit 21 areas are made circular
in shape since the openings in the silicon wafer 10 defined by
circular masks wlll etch out to squares parallel to the 100
planes, each square circumscribing its respective circle. Use
10 of circular masks eliminates possible error due to the theta
misalignment which may occur when a square shaped mask is
used. The silicon nitride layers 18 and 19 are photoshaped
simultaneously on both sides using a two-sided photospinner
tnot shown) and a two-sided aligner (not shown). The
lS resulting structure after etching away of portions of layers
18 and 19 defining the entrance 20 and exit 21 areas, is shown
in FIG. 5.
The exit area 21 is then protected from the etching
solution by covering it with a metallic layer 22, as shown in
20 FIG. 6, or by use of a hermetic mechanical flxture (not
shown). Thereafter the waxer is submerged in a hot (80-85C)
ROW solution (not shown) and a potential is placed across the
pJn junction at the back side 15 by connecting the positive
side of an electrical power source (not shown) with the
25 metallic layer 22 protecting the exit area 21. Other alkaline
etch solutions such as metal hydroxides of the Group I-A
elements of the Pc~iodic Table, for example, ~aOH, N1140H, or
others, may ye use~l. The Ise of electrochemically controlled
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thinning process for semi-conductors is well known in the art
and is described in detail in U.S. Patent 3,689,389 granted to
one of the applicants in the present application.
The opening 11 in the monocrystalline silicon wafer
5 10 is etched anisotropically until the diffused layer 17 at
the back side ~5 is reached, at which time the etching action
stops due to an oxide layer (not shown) which is caused to
grow at the p/n junction due to the applied potential across
the junction. It is well known in the art that the (111)
10 plane is a slow etch plane in monocrystalline silicon material
when a K0~ etching solution is used. Thus, the etching step
produces a pyramidal opening in the wafer 10 which opening
truncates in a membrane 12 wben it encounters the
electrochemical etch barrier set up at the silicon and
15 diffused layer 17 interface (p/n junction.
Thereafter, the wafer 10 is removed from the etching
solution, the protective metallic layer 22 and associated
- electrical connection on the exit side are removed, and the
entrance side 20 is protected from the etching solution
20 usually by a layer 24 formed by air oxidation. The wafer 10
; is then re-submersed into the etching solution and a pyramidal
passage is etched anisotropically from the back surface 15 to
form the exit opening 13. The resulting structure is shown in
FIG 7.
If desired, the protective coat1ngs 18, 19 and 24
are then removed leaving a completed pure silicon nozzle
str~'cture as shown in FlG. 8. Typically the initial opening of
the entrance 20 is about 35 mils wide and the smallest portion
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of the exlt opening 13 i about 1.5 to 4 mils wide.
Since the etch rate perpendicular to the (111)
planes is very low compared to the vertical etch rate (100),
overetch does not mitigate against the high aLcuracy defined
5 by the exit mask. To prevent ink from wetting the surface of
the waEer on the exit side, the back surface 15 of the wafer
10 may be coated with a material of low surface energy such as
Teflon.
. . .
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