Note: Descriptions are shown in the official language in which they were submitted.
4~3 : '
The invention parallels the inventions described in
u.S~ Patent No. 4,059,480 issued November 22nd, 1977 and U.S.
Patent No. 4,066,491 issued January 3, 1978 of Gerhard Trippel
and Wolf-Dieter Ruh for "Method of Forming Viaducts in
Semiconductor Material" and "Method of Simultaneously Etching
Tapered Viaducts in Semi-conductor Material" respectively.
The invention relates to a methOd of making small
viaducts or "through-holes" in semiconductor plates, and it
particularly pertains to the making of such viaducts to be
used particularly as nozzles for ink jet printers where the
dimensions of the orifices are required to be substantially
uniform.
In ink jet printing, a series of ink droplets is projected
against paper or some other record medium. The ink is
projected through a nozzle or several closely adjacent
nozzles arranged in a wall of a container of ink by vibrating
the ink, for example, by means of piezo electric crystal.
Thus ink droplets are formed at a distance outside the
nozzle which are selectively charged at the moment of their
generation by an electric charging electrode. When one
single nozzle is used~ the drops are deflected more or less
strongly by a constant deflection field in that the charge
which is applied to the drops by a means of the charging
electrode varies in its amplitude. In ink jet printing with
a multiple of nozzles, all nozzles project droplets simul-
taneously, and a charging electrode is arranged in front of
each nozzle in the droplet formation area. A constant
deflection field deflects the droplets that have received a
charge from the charging electrode, in such a manner that
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1 they do not reach the paper. Only the uncharged droplets
2 fly in a straight line towards the paper and form the
3 characters. Ink jet printers of the first type with one
4 nozzle are described, for example, in U.S. Patent 3,569,275.
Ink jet printers with several parallel nozzles are described
6 in U.S. Patent 3,373,437.
7 High quality ink jet printing demands that the individual
8 droplets and thus the spots made when the drops impinge on
g the paper, are sufficiently small and closely adjacent so
10 that they are no longer recognizable as individual droplets. .
11 In order to obtain this result, 80 drops or more are required
1~ for each centimeter of length, and each drop is to have a
13 diameter not greater than 0.175 mm. In order to achieve
14 this, éach nozzle orifice should have a diameter of not more
than 0.05 mm, and with multiple nozzles the distance from
16 orifice to orifice should be in the order of 0.25 mm or
17 smaller if possible.
18 The prior art contains suggestions for making ink jet
19 nozzle plates of semiconductor material by etching processes.
The closest art probably is found in the following U.S.
21 patents:
22 3,921,916 11/1975 Bassous 239/601
23 4,007,464 2/1977 Bassous et al 239/601
24 And in the published literature:
R. A. Leone and C. H. Ting, "Fabricating Shaped Grid and
26 Aperture Holes", IBM Technical Disclosure Bulletin Vol. 14,
27 No. 2, July 1971, pp. 417-418.
28 The patent to Bassous is directed to a nozzle for ink
29 jet printing and a method of making it is described. The
front side of a silicon wafer is first anisotropically
SA9-76-044 -3-
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etching from the front side of silicon wafer straight through
the back side thereof. The process according to the invention
and its advantageous accomodation of uneven semiconductor
wafers is clearly not found.
The publi~ation is directed to a nozzle plate made by
fabricating holes in a silicon substrate by first anisotropi-
cally etching from the front side of the substrate to a selected
depth therein, and then anisotropically etching from the back
side thereof to the hole formed by the f1rst etching step.
~This represents probably the closest art, however, the sïmple
process açcording to the invention and the unobvious operation
thereof appears to have been completely overlooked.
According to the invention, the objects indirectly
referred to hereinbefore and those that will appear as the
sp,ecification progresses obtain in a process for fabricating
nozzle plates and the like $rom mono-crystalli~e semiconductor
wafers with substantially uniform nozzle or'ifices despite
variations in thickness of the ~afers.
Thus, the invention in one aspect provides a process for
producing an ink jet nozzle plate in the form of a mono-
crystalline silicon wafer having a multiple of nozzles arranged
between obverse and reverse surfaces of said wafer which are
subject to non-uniform thickness therebetween and having orifices
of substantially uniform dimensions in said obverse surface which
lies substantially in a 100 plane of said wafer comprising the
steps of: coating said wafer with etchant masking material in-
cluding said obverse and reverse surfaces, forming apertures in
said masking material on said reverse surfaces at locations
corresponding to the desired locations of the nozzles to be formed,
etching said wafer from said reverse surface with anisotropic
etchant in the areas within said apertures to points near, at, or
effectively beyond said obverse surface as dictated by variations
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in said non-uniform thickness, forming apertures in said masking
material on said obverse surface smaller than the apertures
formed in masking material on the reverse surface substantially
in registry with said apertures on said reverse surface at
locations corresponding to the desired locations of the orifices
of said nozzles to be formed and subtending those apertures in
said reverse surface, and etching said wafer from said obverse
surface with anisotropic etchant in the areas within said
apertures on said obverse surface and continuing the etching as
originating from the reverse, thereby producing frustro-pyramidal
nozzles with plane surface sides extending entirely through said
wafer in 111 planes of said wafer with orifices of substantially
uniform dimensions in said obverse surfaces determined by said
apertures formed on said obverse surface.
In a further aspect the invention provides a process for
producing an ink jet nozzle plate in the form of a mono- ¦
crystalline silicon wafer having a multiple of nozzles arranged
between obverse and reverse surfaces of said wafer which are
subject to non-uniform thickness therebetween and having orifices
of substantially uniform dimensions in said obverse surface which
lies substantially in a 100 plane of said wafer comprising the
steps of: coating said wafer with insulating material including
said obverse and reverse surfaces, coating said insulating
material with etchant masking material, forming apertures in
said masking material on said reverse surface at locations
corresponding to the desired locations of the nozzles to be formed,
forming apertures in said insulating material on said reverse
surface through said apertures in the masking material, etching
said wafer from said reverse surface with anisotropic etchant
in the areas within said apertures to points near, at, or
effectively beyond said obverse surface as dictated by variations
in said non-uniform thickness, lighting said apertures and said
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wafer as etched for locating smaller apertures in said masking
material on said obverse surf~ce at locations corresponding to
the desired locations of the orifices of said nozzles to ~e
formed, forming apertures in the masking material on ~he obverse
surface smaller than the apertures formed in the masking material
on the reverse surface and subtending the apertures on the
reverse surface, forming apertures in the insulating material on
the obverse surface through said smaller apertures and etching
said wafer ~rom said obverse surface with anisotropic etchant
in the areas within said apertures in said insulating material on
said obverse surface and continuing the etching as originating
from the reverse, thereby producing frusto-pyramidal nozzles with
plane surface sides extending entirely through said wafer with
orifices of substantially uniform dimensions in said obverse
surface determined by said apertures as formed on said obverse
surface despite any variations in thickness of said wafer.
The first step, in a typical process according to the
invention, usually performed by a vendor, is the production of a
semi-conductor wafer with the major surfaces lying substantially
in the "100" plane. The semiconductor crystalline structure
then has internal "111" planes at a convenient angle with
respect to the "100" planes, for example, in crystalline
silicon at an angle of 54.7. A suitable anisotropic etchant
is used to etch pyramidal cavities in the semiconductor
wafer. An anisotropic etchant works much more normally to
the "100" pla~e than it does laterally or parallel to the
"100" plane and thus it works very much less at the "111"
plane. Hence, the action of the etchant leaves pyramidal
nozzle surfaces. In accordance with the process according
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1 to the invention, the wafer is first coated with etchant
2 masking material on both the obverse and reverse major
3 surfaces. Apertures are then made in the masking material
4 on ~he reverse in the locations where the nozzles are desired
and etching begun as described above. The etching is halted
6 at a time when the etchant is well along toward the obverse
7 surface, and indeed some of the formative nozzles may be
8 through the obverse. A~ this time apertures are opened in
9 the masking material on the obverse. The apertures opened
are dimensioned to form the orifices of the nozzles. These
11 apertures are smaller than those opened on the reverse and
12 also smaller than the desired orifices by a predetermined
13 amount predictable on-a degree of under-cutting in the
14 etching process. T~e smaller apertures are readily aligned
with the larger apertures by shining light through the
16 partially etched nozzles and merely placing the mask for the
17 orifice apertures in reasonable alignment. Anisotropic
18 etching next results in frustro-pyramidal cavities extending
19 from the obverse meeting the frustro-pyramidal pits already
extending from the reverse at levels intermediate of the
21 obverse and reverse surfaces of the wafer. Although it
22 might be expected that the anisotropic etching process
23 should stop at this stage because the "100" plane is no
24 longer exposed to the etchant, the etching proceeds to the
final stage wherein the nozzles are substantially pyramidal
26 in configuration; the orifices are the only feature limiting
27 the pyramidal,configuration. Thus this unobvious progress
28 of the process according to the invention results in a much
29 improved nozzle plate made by a novel process of relaxed
requirements.
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1 In order that full advantage obtain in the practice of
2 the invention, a preferred embodiment thereof, given by way
3 of example only, is described in detail hereina~ter with
4 reference to the accompanying drawing, forming a part of the
specification, and in which:
6 FIG. 1 depicts a cross-section of a nozzle plate made
7 by a process according to the invention; and
8 FIG. 2 -- sections (a), (b) and (c) being taken together --
g show three stages of the nozzle plate development as it
evolves by the process according to the invention.
11 A cross-section of a small portion of a nozzle plate of
12 the type formed by a process according to the invention is
13 shown in FIG. 1. A wafer 10 of silicon, or like material is
14 etched, or otherwise processed subtractively, to form viaducts
12, 14. These viaducts will serve in ink jet applications
16 as nozzles where it is important that the orifices of all
17 nozzles have the same critical dimension 0 within a close
18 tolerance and the spacings S between orifices have the same
19 critical dimensioning. For example, in one application thé
wafer is 1.15 mm (0.045 in) thick in which the desired
21 orifice dimension is 0.020 mm. (0.0008 in.) and the spacing
22 is 0.200 mm. (0.0080 in). Although the other dimensions are
23 important, they are much less critical in most cases. Non-
24 uniformity of the thickness results in non-uniformity o~ the
other dimensions in subtractive material processing, but the
26 process according to the invention is substantially insensitive
27 to variations,in thickness as will be seen hereinafter.
28 Three stages in a process according to the invention for
29 forming an array of nozzles of uniform orifice and inter-
orifice spacing dimensions in FIG. 2 (a), (b) and (c) are
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1 -illustrated. A small portion 20 of a wafer of silicon, or
2 like material, having variations in thickness inadvertently
3 resulting from slicing, lapping and polishing operations is
4 shown in cross-section. In the interest of clarity, irregularity
in thickness is illustrated only by the cuniform configuration.
6 Those skilled in the art will readily recognize the approach
7 to be taken in applying the teaching of the invention to the
8 application at hand.
9 The wafer 20 is supplied with the obverse surface 22
lying within il of a plane known to the artisan as the
11 "100" plane. This "100" plane is simply defined in terms of
12 mono-crystalline silicon electro-physical geometry as a
13 plane parallel to surfaces of the parallel-piped structure
14 of the crystal. The reverse surface 24 is likewise within
il of another "100" plane or very close thereto in view of
]6 the skew due to the cuniform configuration. Another plane
17 of crystalline silicon, diagonally of the "100" plane, and
18 known to the artisan as the "111 plane", lies at an angle,
19 for silicon, of 54.7 to the "100" plane. The relationship
of this "111" plane to the "100" plane in accordance with
21 the invention will become more apparent hereinafter.
22 Silicon-dioxide is grown, or otherwise deposited, on
23 both obverse and reverse surfaces of the wafer 20. The SiO2
24 layers 26 in turn are covered by photolithic etchant masking
material 27. The SiO2 layers are interposed because in most
26 applications the photoresist alone is unable to stand up to
27 the etchant; otherwise the SiO2 layer is omitted. The
28 obverse is considered the side on which the orifices of the
29 nozæles are to appear; otherwise there is little difference.
A pattern of apertures 28, substantially as the pattern of
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l orifices desired, is photolithograpllically produced on the
2 reverse leaving the surface 24 of the silicon wafer 20
3 exposed. An anisotropic etchant is applied and pyramidal
4 nozzles 31, 32, 33 are begun. The anisotropic etchant is
chosen from a num~er commercially available that etch silicon
6 much faster in the direction normal to the "100" plane than
7 in the lateral direction toward the "lll" plane. Thus as
8 the etchant works a four-sided pyramidal opening appears
9 with the sides 41, 42, 43 (and 44 not shown because it is in
the foreground) each substantially lying in a "lll" plane.
ll The etching is fairly predictable and fairly uniform
12 with a given supply of wafers, however, irregularities,
13 especially in thickness of the wafer, bring some uncertainty
14 into the process. The etching from the reverse is usually
allowed to proceed partially until a cross-section such as
16 that shown in FIG. 2 (a) obtains. A pattern of apertures 48
17 is photolithographically produced in the obverse. The
18 apertures 48 in this pattern are smaller than the apertures
l9 28. Precision alignment in registering the aperture patterns
is thus obviated. The patterns are readily registered by
21 shining light through the wafer and aligning the mask for
22 the apertures 48 so that three apertures 48 are within the
23 corresponding apertures 28 as determined by the light coming
24 through the wafer. These apertures are dimensioned to allow
for some undercut in the etching process. For example,
26 apertures 28 of 0.015 mm (0.0006 in) are made for a resulting
27 nozzle orifice of 0.020 mm (0.0008 in3.
28 From the obverse, the anisotropic etching process is
29 set up as before. The nozzles now take the shape as illustrated
by F~G. 2 (b) wherein two four-sided frustro-pyramidal pits
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1 intesect wi-thin the wafer 20. The etching does not stop,
2 however, but continues, with the "111" planes of the silicon
3 material and the etchant masking material being the major
4 limiting factors, to the full etch phase. The masking
material and the SiO2 layers are then stripped off, leaving
the structure much as depicted in FIG. 1 except for irregular
7 thickness but with orifices of uniform dimensions.
8 The materials used for etching, for masking and for
9 stripping and the like are fully described in the prior art
hereinbefore mentioned, although the advantageous simplicity
11 brought about by the novel process according to the invention
12 is obscured somewhat by the marked detail. Those skilled in
13 the art will readily apply the teaching of the invention to
14 the application at hand using only.the necessary teaching of
the prior art discussed hereinbefore.
16 While the invention has been described in terms of a
17 preferred process and alternatives have been suggested, it
18 should be clearly understood that those skilled in the art
19 will make further changes without departing from the spirit
and scope of the invention as defined in the appended claims
21 concluding the specification.
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