CA1087384A

CA1087384A - Method and apparatus for producing nozzle arrays for ink jet printers

Info

Publication number: CA1087384A
Application number: CA309,385A
Authority: CA
Inventors: Erik R. Solyst
Original assignee: International Business Machines Corp
Current assignee: International Business Machines Corp
Priority date: 1977-12-23
Filing date: 1978-08-15
Publication date: 1980-10-14
Also published as: US4157935A; GB2011645B; JPS5842031B2; IT7830728A0; JPS5488126A; IT1160327B; FR2412410A1; DE2855080A1; GB2011645A

Abstract

METHOD OF ETCHING UNIFORM HOLES IN SUBSTRATES FOR THE
FABRICATION OF INK JET NOZZLES AND THE LIKE
Abstract of the Disclosure Nozzle arrays for ink jet recording are produced by preferred chemical etching of a substrate material which frequently has a non-uniform thickness. The preferred substrate is a monocrystalline silicon wafer and the 100 plane surface of the wafer is coated with etchant masking material and the resist coated wafer is held in close physical contact with a base member. A suitable mask member which defines a nozzle array pattern is spaced a predetermined distance from the base member and is positioned parallel to the base member. The wafer is then exposed through the mask by a suitable light source arranged at a suitable angle while the wafer is simultaneously rotated about an axis perpendicular to the wafer. The wafer is then exposed to anisotropic etching to produce a uniform array of nozzles in the wafer wherein the lateral walls of the nozzles are substantially in the "111" plane of the wafer. The masking material is then stripped from the wafer.

Description

Background of the Invention This invention relates to a method for simultaneously chemically etching an array of uniform through holes in a substrate and more particularly, the invention relates to the fabrication of nozzles for ink jet printers.
In prior art ink jet printing systems, it is known that an array of closely spaced nozzles is required. During ink jet printing with this system, drops are simultaneously ejected by all the nozzles and a charge electrode is arranged in front of each nozzle in the area where the drops are formed. A constant deflecting field is operable to deflect all the charged drops from the ink stream so that only the uncharged drops continue to the paper to form printed characters. An ink jet printer with an array of several nozzles is described in U.S. patent ; 3,373,437.

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` iO87384 1 ~t has also been known in the prior art that a nozzle array suitable

2 for use in ink jet printing can be fabricated by etching a semiconductor

3 chip. One example of such an ink jet nozzle is described in U.S. patent

4 4,007,464 to Bassous et al, which patent is assigned to the assignee of the present invention. In this process one surface of the semiconductor 6 chip is initially photoresist coated and then exposed and developed so 7 that the etchant attacks only predetermined regions to produce the nozzles.
.
8 Those nozzles have a pyramidally tapered cross-section.
g Unfortunately, it is very difficult to produce substrates having a totally uniform th;ckness from semiconductor materials. Non-uniform ~`
11 thicknesses are particularly detrimental when several tapered nozzles 12 are to be etched simultaneously. In this case, a thin area of the ,il , .. .
~ 13 substrate wil1 have a larger orifice than in a thicker section, if both , . . .
14 nozzles are etched simultaneously. Because of their improved characteristics, tapered such as conical or pyramidal nozzles such as described in the 16 above-mentioned Bassous et al patent, are desirable for ink jet printers.
17 IBM*technical disclosure bulletin Vol. 17 No. 11 April 1975 pages 18 3450-52 describes a method for simultaneously etching several nozzles in 19 a silicon plate which produces uniform orifice widths irrespective of 20 differences in the thickness of the silicon plate. Differences in the 21 thickness of the plate are compensated for during the exposure of the 22 photoresist layer by changing the size of the exposure area or, in the ~
23 case of square exposure surfaces, by tilti~g them in relation:to the -24 direction of.the silicon plate. These known methods of changing the 25 surface attacked ~y the etchant necessitate that the photoresist free 26 area associated with each nozzle be produced separately and that the 27 thickness of the silicon plate be measured in those areas where a 28 no.zle is to be etched, These factors make this method unsllitable for 29 use in a production fabrication technique-for ink jet nozzles.
* Registered Trade Mark of International Business Machines Corporation, Armonk, New York , ~, .

1 Another method for compensating for the non-uniform thickness of the semiconductor substrate is described in U.S. Patent No. 4,066~491, issued January 3, 1978, and assigned to the assignee of the present invention. The holes are first chemically pre-etched until the first hole is about to penetrate through the substrate and subsequently sputter etching is utilized until all holes have penetrated through the substrate. This method requires two separate etching steps which adds ;
to the complexity of the method.
A further method for solving the problem of non-uniform semi-conductor substrates in the production of ink jet nozzles is describedin Canadian Application No. 300,057 filed on March 30, 1978 and assigned to the assignee of the present invention. In this application, the substrate is anisotropically etched from one side of the wafer until the openings are through in the thin section of the wafer. Apertures are then made in the masking material on the other side of the wafer in the locations where the nozzles are desired. The substrate is then aniso-; tropically etched again so that the array of nozzles is then formed to the size of the apertures opened on the other side of the substrate.
This method also requires two etching steps and for this reason has not proved entirely satisfactory in making nozzle arrays for ink jetprinters.
Summary of the Invention It is therefore the principal object of this invention to provide an improved method which permits a plurality of tapered through holes to be simultaneously produced in a substrate by means of chemical etching whereby the holes have identical orifice widths irrespective of differences in the thickness of the substrate.
Briefly, according to the invention, the substrate is first coated with an etchant masking material on one surface. The wafer is then mounted in contact with a base member and a mask is mounted on a suitable support which positions the mask parallel to the base member spaced a r, ~

~87384 1 predeternlined distance from the base member. The warer is then exposed 2 through the mask by a suitable light source arranged at a predetermined 3 angle relative to the wafer and, simultaneous with the exposure, the light 4 source is rotated around an axis perpendicular to the wafer. The exposed wafer is then anisotropically etched to produce uniform orifices from 6 the wafer including a wafer having non-uniform thickness.
7 Brief Description of the Drawings FIGURE l is a perspective view of a silicon wafer which has been g anisotropically etched to form a plurality of ink jet nozzles;
FIGURE 2 is a cross-section view along lines 2-2 of Figure l;
11 FIGURE 3a-d represent sequential cross-section views of a silicon 12 wafer processed to form the ink jet nozzles of Figure l; -- ~
13 FIGURE 4 is a diagrammatic schematic view showing apparatus for ~-accomplishing the exposure step according to the invention; m ~, 15 FIGURE 5 is a diagrammatic schematic viéw showing the exposure step `` 16 according to the invention as applied to a wafer of non-uniform thickness;
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17 FIGURE 6 is a diagramnlatic schematic view showing alternate apparatus 18 for accomplishing the exposure step according to the invention; ;~
19 FIGURE 7 is a diagram showing an enlarged~view of the nonexposed ~
area of the photoresist when exposed utilizing the apparatus shown in ~- -21 Figure 6. ~ ;
22 Detailed DescriDtion of the Preferred Embodiments . _ 23 In principle my invention is applicable to the production of an ink 24 jet nozzle array from substrates which have or can be made to have vastly 25 different etching rates in one area compared to the etch rate in surround-26 ing areas. Examples of substrates which have different etch rates comprise 27 single crystal materials such as silicon and sapphire which have vastly 28 different etching rates between an exposed and unexposed area when 29 subjected to suitable anisotropic etching. An example of a sub-30 strate material which can be made to have vastly different etching rates .~

1 is a selectively crystallizable photosensitive glass which has a greater etching rate in an exposed and heat treated area compared to an unexposed area. My invention will be described in terms of its application to a specific embodi-ment in which the substrate is silicon, but it will be recognized that the invention is not so limited.
A perspective view of a small portion of a nozzle plate of the type formed by a process according to the invention is shown in Figure 1. A wafer 10 of silicon or like material is etched or otherwise processed subtractively to form openings 12 and 14. These openings serve in ink jet applica-tions as nozzles where it is important that the orifices of `~
all nozzles have the same critical dimensions within a close tolerance. A cross-section of a small portion of the nozzle plate of Figure 1 is shown in Figure 2. For example, the wafer may have a thickness b typically about 7 mils and the desired orifices dimension s is typically 800 ~ in. and the width of the nozzle d is typically 11 mils. To achieve uniform orifice dimensions in a nozzle array, it is necessary that the openings in the photoresist be uniform in size and ; the wafer thickness be uniform. The requirement for uniform openings in the photoresist can easily be met by using state of the art photolithographic semiconductor techniques. The requirement for uniform thickness in the wafer is a very difficult problem, since the tolerances required on the nozzle size dimension far exceed the current state of the art in wafer technology. Non-uniformity of the thickness results in non-uniformity of the other dimensions in subtractive material processing, but the process according to my invention is substantially insensitive to variations in thickness as will be seen from the description of my invention.

The stages in the process, according to my invention, for 1~8738~
1 forming an array of nozzles of uniform dimension are shown in Figure 3. A layer of silicon dioxide 16, 18 is produced on both faces of the wafer SA9-77-035 -5a-~ 87384 1 by thermal oxidation as shown in Figure 3a. A suitable photoresist layer 2 20 is applied on the face 22 (lO0 crystal plane for silicon) of the 3 wafer that will eventually form the base of the pyramid shaped hole.
The photoresist is then exposed through a suitable mask and this exposure is preferably done at an exposure station which is shown in Figure 4.
6 The photoresist is then developed to remove the non-exposed area and 7 the silicon dioxide underlying the non-exposed area is etched to form 8 an opening 26 as shown in Figure 3b. The remainder of the photoresist 9 is then stripped from wafer lO and the wafer is then subjected to anisotropic etching to form an array of openings along the 111 11 crystal plane 28 in the wafer as shown (one opening only) in Figure 3c.
12 As can be seen with reference to the dashed circle 24 in Figure l, the '`
13 photoresist left on the wafer after exposure and development has circular 14 openings and the sides of the base of the pyramid shaped hole will be tangent to the circular opening after the anisotropic etching process.
1~ The silicon dioxide layers 16, l~ are then stripped from the wafer to 17 form an array of ink jet nozzles.
18 The exposure station comprises a flat platen 30 containing vacuum 19 ports 32 on which the photoresist coated wafer 34 is held in physical contact. The vacuum supplied to vacuum ports 32 is of such a level that 21 it can pull a relatively thin wafer into physical contact over the 22 entire surface. The platen 30 may conveniently be fabricated from an 23 optical flat. In this case, it is possible' to view the wafer through the optical flat so that it can readily be determined whether physical contact has been achieved. A suitable mask support melllber 36 is provided 2G on the flat platen so that the mask substrate 3~ can be supported parallel 27 to the platen within a very small tolerance such as lO microinches, and 28 the spacing between thermask and the platen is greater than the maxlnlum 29 wafer thickness. The masking pattern consists of a plurality of opaque circular areas 40 deposited on the surface of the masking substrate 3~

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1 in the desired nozzle array pattern.
The diameter d of the opaque circular area 40 is determined as shown in Figure S.
d = s + 2 b tan~
where:
b = spacing between platen and mask;
d = diameter of opaque circular mask, s = square orifice side dimension, and ~ = angle between crystalographic planes 100 and 110 (approximately 53.7 for silicon).
After the photoresist application and with the wafer 10 located on platen 30 as described above, the wafer is exposed by a suitable light source 44. Light source 44 may comprise any suitable source which pro-duces radiation at a wavelength to which the photoresist is sensitive.
Mercury and Xenon light sources are suitable for many of the photoresist materials. These sources may be accompanied by suitable filters if ., necessary and a lens system to collimate the light if necessary. Light source 44 is a columnar beam covering the full wafer area and directed onto the wafer at an angle ~ which is approximately 53.7 for silicon.
The columnar light beam is rotatable around an axis 42 perpendicular to the wafer 10. Alternatively the relative motion can be produced by keeping light source 44 fixed and rotating the wafer about axis 42.
The relative motion can be by continuous rotation in which case the unexposed area for an orifice is circular (as shown by dashed circle 24 in Figure 1). alternatively the relative motion can be by a plurality of increments, for example, four increments each comprising 90 degrees movement. In this case, the unexposed area comprises four elliptical curved sections as shown in Figure 7.
Figure 5 shows an enlarged section of a wafer array, and it can be seen that after a full 360 rotation of the columnar beam, the non-` - -~87384 1 exposed areas on the photoresist will be a circular area and that the 2 diameter is exactly equal to that which will produce the desired orifice 3 size after the anisotropic etching, and this is independent of the wafer ~`~
thickness.
After exposure of the photoresist, the non-exposed area is removed 6 by a suitable solvent that will dissolve the non-exposed portion of the 7 photoresist, and the wafer is then processed through the chemical etching8 process. The chemical etching process can be carried out according to g the process described and claimed in the above-mentioned patent 4,007,464to Bassous et al. This etching is carried out by exposing the silicon ~
11 wafer to a solution containing ethylene diamine, pyracatechol and water ~ ;
12 at 110-120C to form the tapered openi-ngs in the wafer. Etching is 13 stopped when orifices appear on the lower side of the wafer. The etching14 period is generally on the order of three to four hours for a substrate on the order of 7 mils thick. The oxide layer is then stripped from 16 both sides of the wafer. An alternate embodilnent of the exposure step 17 is shown in Figure 6. In this embodiment, four separate light beams 30, 18 32, 34 and 36 are used to expose the wafer 10' simultaneously. In this 19 case, it is not necessary to rotate the light source. As shown in Figure 7, the exposed area is not circular, but an area comprising four 21 elliptical curved sections 4v, 50, 52 and 54. This shape, however, 22 produces the same orifice size and shape as the circular area produced , 23 in the previously described embodinlent when used with anisotropic etching of a single crystal material.
While the invention has been particularly shown and described with 2G reference to preferred embodilllents thereof, it will be understood by ~ 27 those skilled in the art that various changes in the form and details : 28 may be made therein wit~hout departing from the spirit and scope of the 29 invention. ~ ~.
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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows.

1. The method for producing nozzle arrays for ink jet printers comprising the steps of:
mounting a photoresist coated wafer in physical contact with a base member; said wafer comprising a crystaline material having greatly different anisotropic etch rates in different crystallographic direc-tions;
positioning a mask structure having a predetermined pattern of orifice masks thereon parallel to the base member;
exposing the photoresist through the mask by a columnar light source of radiation directed at a predetermined acute angle to said wafer and simultaneously producing relative rotational motion between the radiation source and said wafer around an axis perpendicular to the base member;
treating the wafer to render it subject to anisotropic etching only in the non-exposed areas of the wafer; and anisotropically etching the wafer to produce uniform orifices corresponding to said predetermined pattern from said wafer.

2. The method of claim 1 wherein the step of producing relative motion between said light source and said wafer comprises rotating the light source around an axis perpendicular to the base member.

3. The method of claim 1 wherein the step of producing relative motion between said light source and said wafer comprises producing said motion in a plurality of equally spaced steps.

4. The method of claim 1 wherein the step of producing relative motion between said light source and said wafer comprises indexing said base member to a plurality of equally spaced positions.

5. The method of claim 1 wherein said wafer comprises a mono-crystalline silicon wafer and said predetermined angle is about 54.7 degrees.

6. The method for producing nozzle arrays for ink jet printers comprising the steps of:

mounting a photoresist coated silicon wafer in physical contact with a base member;
positioning a mask support structure having a predetermined pattern of orifice masks thereon parallel to the base member;
exposing the photoresist through the mask by a suitable columnar light source directed at an angle of about 54.7 degrees to the wafer and simultaneously producing relative rotational motion between the light source and the wafer around an axis perpendicular to the base member;
treating the wafer to remove the photoresist in the non-exposed area of the wafer; and anisotropically etching the wafer to produce uniform orifices from said wafer.

7. The method of claim 6 wherein the step of producing relative motion between said light source and said wafer comprises rotating the light source around an axis perpendicular to the base member.

8. The method of claim 6 wherein the step of producing relative motion between said light source and said wafer comprises producing said motion in a plurality of equally spaced steps.

9. The method of claim 6 wherein the step of producing relative motion between said light source and said wafer comprises indexing said base member to a plurality of equally spaced positions.

10. The method for producing nozzle arrays for ink jet printers comprising the steps of:
mounting a photoresist coated wafer in physical contact with a base member; said wafer comprising a crystalline material having greatly different anisotropic etch rates in different crystallographic direc-tions;
positioning a mask structure having a predetermined pattern of orifice masks thereon parallel to the base member;
exposing the photoresist through the mask by a plurality of colum-nar light sources spaced around said wafer, each of said light sources being directed at a predetermined acute angle to the wafer;

treating the wafer to render it subject to anisotropic etching only in the non-exposed areas of the photoresist; and anisotropically etching the wafer to produce uniform orifices corresponding to said predetermined pattern from said wafer.

11. The method according to claim 10 wherein said wafer comprises a monocrystalline silicon wafer and said predetermined angle is about 54.7 degrees.