Directable aperture etched in silicon

Abstract: A method is described for anisotropically etching an aperture into a monocrystaline substrate such that the angle of the aperture with respect to the nominal crystaline planes of the substrate may be preselected. Etchant pits through opposing planar surfaces are offset from one another by a preselected longitudinal offset trigonometrically related to the desired aperture angle. Other selected parameters trigonometrically related to the aperture angle include the angle of intersection of the intersecting crystaline planes, the substrate thickness and the depth of each of the etchant pits.

Description: Field of the invention The field of the invention relates to etching apertures in monocrystalline substrates such as silicon. Background of the invention It is known to etch apertures in silicon substrates by etchant processes commonly referred to as micro-machining. These apertures have been used in optics and also as fluid flow nozzles. Examples of these apertures are enumerated below. U.S. Patent 4,269,653, issued to Wada et al discloses an aperture etched through silicon for use as an electron beam exposure device. The resulting aperture axis is perpendicular to the nominal plane of the silicon substrate. U.S. Patents 4,628,576 and 4,647,013, both issued to Giachino et al, disclose apertures etched through silicon for use as the nozzle portion of a fuel injector valve. Again, the aperture axis is perpendicular to the nominal plane of the substrate through which the aperture is etched. IBM Technical Disclosure Bulletin, Volume 14, No. 2, July 1971, Pages 417-418, entitled "Fabricating Shaped Grid and Aperture Holes", discloses a method for etching an array of apertures for use as illumination apertures. More specifically, a silicon substrate is shown having an epitaxial layer grown thereon. A desired array or pattern of apertures is etched through the epitaxial layer. An etchant which attacks the silicon substrate, but not the epitaxial layer, is then used to etch the substrate off from the backside thereby exposing the epitaxial layer and the pattern of apertures etched therethrough. Each aperture has an aperture axis perpendicular to the epitaxial layer. IBM Technical Disclosure Bulletin Volume 25, No. 10, Page 5041, entitled "Silicon Ion-Beam Extraction Grids", which constitutes the most relevant prior art, discloses the fabrication of extraction grids for ion-beam sources from silicon which involves anisotropically etching pits in the opposite surfaces of a silicon substrate. Apertures having one or two aperture planes are formed in the substrate where the pits intersect. The etchant pits on opposite sides of the substrate are generally symmetrically aligned with one another and the resultant apertures lie in planes parallel to the surfaces of the substrate. U.S. Patent 4,169,008 discloses a method of etching apertures for use as nozzles in ink-jet printers. Etchant pits in both the top and bottom surfaces form an opening having an aperture axis perpendicular to the nominal plane of the wafer. By etching from the bottom side, is alleged that variations in the aperture openings are avoided which would otherwise occur due to variations in the wafer thickness. Stated another way, the dimensions of the openings in the bottom planar surface are determined by the bottom mask openings, rather than by the intersection of the top etchant pit with the bottom planar surface. Each aperture axis is perpendicular to the nominal plane of the substrate. A problem with the above approaches is that the angle of the aperture plane is not selectable. The aperture axis is constrained to a direction perpendicular to the nominal plane of the substrate. Additional structure is needed to direct the fluid flow in a direction other than perpendicular to the substrate plane which may be desired for a particular application. Object of the invention It is therefore an object of the present invention to provide a method for etching apertures from a single monocrystalline substrate wherein the angle of the aperture plane may be preselected. Summary of the invention According to the present invention, there is proovided a method for anisotropically etching a first aperture into a monocrystalline substrate such that the plane of the first aperture is inclined at a selectable angle (β) with respect to the nominal crystalline planes of the substrate, said method comprising the steps of -: a) dimensioning said substrate of thickness (t) with top and bottom planar surfaces substantially parallel to said nominal crystalline planes; b) anisotropically etching a first etchant pit through said top planar surface and against said nominal crystalline planes to a first preselected distance (d₁) from said bottom planar surface, said first etchant pit having plane surface sides along intersecting crystalline planes to said nominal crystalline planes, the intersection of said plane surface sides with said top planar surface defining an angle of intersection (), a first pair of longitudinal edges and a first pair of transverse edges of said first etchant pit, and c) anisotropically etching a second etchant pit through said bottom planar surface and against said nominal crystalline planes to a second preselected distance (d₂) from said top planar surface, whereby the areas of the first and the second etchant pits vertically overlap each other, the centre of the second etchant pit being laterally offset from that of the first etchant pit and the sum of said first and second preselected distances (d₁) and (d₂) is larger than the thickness (t) of said substrate so that a-portion of said second etchant pit intersects a portion of said first etchant pit to form said first aperture, said second etchant pit having plane surface sides along said intersecting crystalline planes, the intersection of said plane surface sides with said bottom planar surface defining a second pair of longitudinal edges and a second pair of transverse edges of said second etchant pit, the plane of the first aperture being defined as that plane which occurs between a first protruding edge, defined by the intersection of the first etchant pit with the top wall of the second etchant pit, and a second protruding edge, defined by the intersection of the second etchant pit with the bottom wall of the first etchant pit, one edge of said second pair of transverse edges being longitudinally offset from an opposing one of said first pair of transverse edges by a preselected longitudinal offset (ΔL), said opposing offset edges belonging to those intersecting crystalline planes which include said first and second protruding edges, the angle of inclination (β) of the plane of the aperture relative to the top and bottom planar surfaces being related to the longitudinal offset (ΔL), the thickness (t) of the substrate, the first preselected distance (d₁) of the first etchant pit from the bottom planar surface, the second preselected distance (d₂) of the second etchant pit from the top planar surface and the angle of intersection () between said intersecting planes and said nominal planes by the trigonometric relationship: β = tan⁻¹ [{t-(d₁+d₂)}/{ΔL - (d₁+d₂)/(tan )}] In accordance with the above, an advantage of the invention is that the designer may select any aperture angle desired by appropriate selection of the above described values (ΔL, d₁, d₂ and the type of monocrystalline material ()). Another advantage, is that the desired aperture may require only a single etchant step wherein top and bottom planar surfaces are concurrently etched to the same depth. Still another advantage, is that a directable aperture is formed in only a single substrate whereas multiple substrates were heretofore required. An additional advantage is that multiple apertures may be formed, each having a different aperture axis, in a single substrate. Brief description of the drawings The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a perspective view of an embodiment in which the invention is used to advantage; Figure 2 is a cross-sectional view taken along lines 2-2 of Figure 1; Figures 3A through 3C illustrate various process steps in fabricating the embodiment illustrated in Figure 2; and Figure 4 is an expanded view of a portion of the embodiment illustrated in Figure 2. Description of the Preferred Embodiment Referring first to Figure 1, aperture assembly 10 is shown for the purposes of illustrating an example in which the invention is used to advantage. Aperture assembly 10 is here shown fabricated from a {100} silicon substrate 12 which has been sawed from a wafer, by conventional means, such that top planar surface 14 and bottom planar surface 16 lie substantially in the {100} nominal crystalline plane. First etchant pit 20 is shown formed through top planar surface 14 with side walls 22, 24, 26, and 28, and bottom wall 30. Side walls 22, 24, 26, and 28 lie in the {111} intersecting planes which intersect the {100} nominal plane at an angle  of approximately 54.7°. As shown in more detail later herein with particular reference to Figure 2, aperture 40 and aperture 42 are symmetrically positioned in substrate 12. It is to be understood that although a {100} silicon substrate is shown, the invention may be used to advantage with substrates having major surfaces lying in other planar directions such as, for example, {110} silicon. In general, the invention may be used to advantage with any monocrystalline substrate. Referring now to Figure 2, aperture 40 and aperture 42 are generally defined by the intersection of top etchant pit 20 with respective bottom etchant pit 44 and bottom etchant pit 46. As described in greater detail later herein with particular reference to Figures 3A through 3C, bottom etchant pit 44 is defined by side walls 48, 50, 52 (not shown) and 54 (not shown) lying in the {111} intersecting planes, and top wall 56 lying in the {100} plane. Side walls 48, 50, 52, and 54 are formed at an angle  with respect to bottom planar surface 16 or the {100} planes in general. Aperture plane 62 of aperture 42 is shown in Figure 2 as a plane between protruding edges 64 and 66 of aperture 42. Protruding edge 64 is defined by the intersection of top etchant pit 20 with top wall 56 of bottom etchant pit 44. Protruding edge 66 is defined by the intersection of bottom etchant pit 44 with bottom wall 30 of top etchant pit 20. Aperture axis 68 of aperture 42 is shown perpendicular to aperture plane 62 midway between protruding edges 64 and 66. The angle of the aperture plane with respect to bottom planar surface 16, or the {100} planes in general, is herein designated as angle β. Aperture 40 is shown in Figure 2 as being symmetrically formed in substrate 12 with respect to aperture 42. Bottom etchant pit 46 is having top wall 76 lying in the {100} plane and side walls 78, 80, 82 (not shown), and 84 (not shown) lying in the {111} planes. Side walls 78, 80, 82 and 84 are etched at an angle  with respect to bottom planar surface 16 or the {100} planes in general. Aperture plane 92 of aperture 40 is defined as a planar surface between protruding edges 94 and 96 of aperture 40. Protruding edge 96 is defined by the intersection of top etchant pit 20 with top wall 76 of bottom etchant pit 46. Protruding edge 94 is defined by the intersection of bottom etchant pit 46 with bottom wall 30 of top etchant pit 20. Aperture axis 98 of aperture 40 is shown perpendicular to aperture plane 92 midway between protruding edges 94 and 96. The angle of aperture plane 92 with respect to top planar surface 14, or the {100} planes in general, is herein designated as angle β. Referring now to Figures 3A through 3C, wherein like numerals refer to like parts shown in Figures 1 and 2, the fabrication of aperture assembly 10 is now described. The fabrication steps described herein utilize photolithographic and etching techniques well known in the microelectronics industry. Substrate 12 is illustrated in Figure 3A as a silicon substrate, preferably having been sawed to a thickness of approximately 200 to 500 microns with major surfaces lying substantially in the {100} planes. Top and bottom silicon oxide layers 102 and 104, respectively, are grown on substrate 12 using conventional techniques well known in the semiconductor industry. Layer of silicon nitride 106 and layer of silicon nitride 108 are deposited over respective silicon oxide layers 102 and 104, respectively, using conventional vapor deposition techniques. Silicon oxide layer 102 and silicon nitride layer 106, together define passivation layer 107. Similarly, the combination of silicon oxide layer 104 and silicon nitride layer 108 define passivation layer 109. With particular reference to Figure 3B, photo-resist layers 112 and 114 are deposited over respective passivation layers 107 and 109. Rectangular etchant pattern 116 is delineated in photo-resist layer 112 by conventional photolithographic techniques. Rectangular etchant pattern 116 is etched through passivation layer 107 thereby exposing a corresponding rectangular portion of top planar surface 14 for etching top etchant pit 20 in a subsequent etching step. In a similar manner, bottom rectangular pattern 126 and bottom rectangular pattern 128 are delineated through bottom photo-resist layer 114 and passivation layer 109. Referring now to Figure 3C, a wet anisotropic etchant solution such as aqueous-potassium-hydroxide solution or ethylene-diamine-pyrocatecol solution, is applied to top rectangular pattern 116, bottom rectangular pattern 126, and bottom rectangular pattern 128 to form respective top etchant pit 20, bottom etchant pit 44 and bottom etchant pit 46. For the {100} silicon wafer illustrated herein, the wet etchant acts against the {100} planes at a rate approximately 100 times greater than against the {111} planes. The etchant steps are timed such that top etchant pit 20 is etched against the {100} planes to a distance d₁ before bottom planar surface 16. Stated another way, top etchant pit 20 is etched to a depth t-d₁ where t is the thickness of substrate 12. During the etching of the planes to a distance of d₁, there will be relatively little etching against the {111} planes. Accordingly, sidewalls 22, 24, 26 (Figure 1) and 28 (Figure 1), which lie in the {111} planes, are slightly offset from rectangular pattern 116. More specifically, the intersection of sidewalls 22, 24, 26 and 28 with top planar surface 14 defines, respectively, left transverse edge 118, right transverse edge 120, longitudinal edge 122 (Figure 1), and longitudinal edge 124 (Figure 1) of etchant pit 20, all of which are slightly offset from rectangular pattern 116. Both bottom etchant pit 44 and bottom etchant pit 46 are etched to a depth d₂ before top planar surface 14. That is, bottom etchant pit 44 and bottom etchant pit 46 are etched to a depth t-d₂. With respect to bottom etchant pit 44, the intersection of sidewalls 48, 50, 52 (not shown) and 54 (not shown) with bottom planar surface 16 defines, respectively, left transverse edge 130, right transverse edge 132, longitudinal edge 134 (not shown), and longitudinal edge 136 (not shown), all of which are slightly offset from rectangular pattern 126. Referring to bottom etchant pit 46, the intersection of sidewalls 78, 80, 82 (not shown) and 84 (not shown) with bottom planar surface 16 defines, respectively, left transverse edge 138, right transverse edge 140, longitudinal edge 142 (not shown), and longitudinal edge 144 (not shown), all of which are slightly offset from rectangular pattern 128. Top etchant pit 20 and bottom etchant pits 44 and 46 are here shown etched to the same depth (d₁=d₂) such that the etchant steps are performed concurrently. That is, by etching to the same depth, the timing of the etching steps is substantially the same thereby enabling the etching steps to be performed concurrently. The trigonometric relationship of the various etching parameters described herein above with respect to the desired or preselected angle of the aperture plane is now described with reference to Figure 4. For clarity of illustration, the following description is with particular reference to aperture 42. Those skilled in the art, however, will recognize that the description herein is also applicable to aperture 40, and is further applicable to other apertures desired by a designer which are not specifically illustrated herein. It is to be further noted that the description herein relates to a {100} silicon wafer. The teachings described, however, are applicable to other monocrystaline substrates such as, for example, {110} crystaline structures wherein the {111} planes intersect the planes at an angle of 90°. As previously described herein with reference to Figure 2, aperture plane 62 extends through protruding edges 64 and 66 forming angle β with respect to bottom planar surface 16. Side walls 48 and 50 of etchant pit 44 lie along the {111} planes at an angle  to the nominal {100} plane of 54.7°. Preselected angle β is related to the longitudinal distance between left transverse edge 130 of bottom etchant pit and the opposing right transverse edge 120 of top etchant pit 20 (ΔL), the preselected distance of top etchant pit 20 from bottom planar surface 16 (d₁), the preselected distance of bottom etchant pit 44 from top planar surface 14 (d₂), the angle between the {111} planes and the nominal {100} plane () and the thickness of substrate 12 (t) by the following trigonometric relation: Thus, it is seen that the preselected angle β may be changed by the designer by changing any of the parameters shown in the above equation. The designer is therefore able to select a desired aperture angle by simply selecting from: the type of monocrystaline substrate (); wafer thickness (t); longitudinal offset of opposing etchant pits (ΔL); and the etchant time or thickness (d₁ + d₂).

Claims: A method for anisotropically etching a first aperture into a monocrystalline substrate such that the plane of the first aperture is inclined at a selectable angle (β) with respect to the nominal crystalline planes of the substrate, said method comprising the steps of -: a) dimensioning said substrate of thickness (t) with top (14) and bottom (16) planar surfaces substantially parallel to said nominal crystalline planes; b) anisotropically etching a first etchant pit (20) through said top planar surface (14) and against said nominal crystalline planes to a first preselected distance (d₁) from said bottom planar surface (16), said first etchant pit (20) having plane surface sides (22, 24) along intersecting crystalline planes to said nominal crystalline planes, the intersection of said plane surface sides with said top planar surface defining an angle of intersection (), a first pair of longitudinal edges (122, 124) and a first pair of transverse edges (118, 120) of said first etchant pit (20), and c) anisotropically etching a second etchant pit (44) through said bottom planar surface (16) and against said nominal crystalline planes to a second preselected distance (d₂) from said top planar surface (14), whereby the areas of the first (20) and the second (44) etchant pits vertically overlap each other, the centre of the second etchant pit (44) being laterally offset from that of the first etchant pit (20) and the sum of said first and second preselected distances (d₁) and (d₂) is larger than the thickness (t) of said substrate so that a portion of said second etchant pit (44) intersects a portion of said first etchant pit (20) to form said first aperture (42), said second etchant pit (44) having plane surface sides (48, 50) along said intersecting crystalline planes, the intersection of said plane surface sides with said bottom planar surface (16) defining a second pair of longitudinal edges and a second pair of transverse edges (130, 132) of said second etchant pit (44), the plane of the first aperture (42) being defined as that plane which occurs between a first protruding edge (64), defined by the intersection of the first etchant pit (20) with the top wall (56) of the second etchant pit (44), and a second protruding edge (66), defined by the intersection of the second etchant pit (44) with the bottom wall (30) of the first etchant pit (20), one edge (130) of said second pair of transverse edges (130, 132) being longitudinally offset from an opposing one (120) of said first pair of transverse edges (118, 120) by a preselected longitudinal offset (ΔL), said opposing offset edges (130, 120) belonging to those intersecting crystalline planes which include said first and second protruding edges (64, 66), the angle of inclination (β) of the plane (62) of the aperture relative to the top and bottom planar surfaces (14, 16) being related to the longitudinal offset (ΔL), the thickness (t) of the substrate, the first preselected distance (d₁) of the first etchant pit (20) from the bottom planar surface (16), the second preselected distance (d₂) of the second etchant pit (44) from the top planar surface (14) and the angle of intersection () between said intersecting planes and said nominal planes by the trigonometric relationship: β = tan⁻¹ [{t - (d₁+d₂)}/{ΔL - (d₁+d₂)/(tan )}] A method as claimed in claim 1, wherein the intersection of said plane surface sides (22, 24) with said top planar surface (14) defines a rectangle having a left transverse edge (118), a right transverse edge (120) and said first pair of longitudinal edges (122, 124); and wherein the intersection of said plane surface sides (48, 50) with said bottom planar surface (16) defines a rectangle having a left transverse edge (130), a right transverse edge (132,) and said second pair of longitudinal edges. A method as claimed in claim 1 or 2, wherein said first preselected distance (d₁) of said first etchant pit (20) from said bottom planar surface (16) is substantially equal to said preselected distance (d₂) of said second etchant pit (44) from said first planar surface (14), said first and second etchant pits (20, 44) being concurrently formed in a single etchant step. A method as claimed in claim 2, for anisotropically etching a second aperture (40) into said substrate such that the plane of said second aperture is inclined at a selectable angle (beta) with respect to said nominal crystalline planes of said substrate, the method further comprising the step of anisotropically etching a third etchant pit (46) through said bottom planar surface (16) and against said nominal crystalline planes to said second preselected distance (d₂) from said top planar surface (14), a portion of said third etchant pit (46) intersecting a portion of said first etchant pit (20) to define said second aperture (40), symmetrically with respect to said first aperture (42). A method as claimed in any preceding claim, wherein said substrate comprises a silicon substrate. A method as claimed in any preceding claim, wherein said nominal crystalline planes comprise (100) planes and said intersecting crystalline planes comprise (111) planes and said angle of intersection () is approximately 54.7°. A method as claimed in any one of claims 1 to 5, wherein said nominal crystalline planes comprise (110) planes and said intersecting crystalline planes comprise (111) planes and said angle of intersection () is approximately 90°.

Inventors: MIKKOR MATI, SICKAFUS EDWARD NATHAN, MIKKOR, MATI, SICKAFUS, EDWARD NATHAN
Applicants: FORD FRANCE, FORD MOTOR CO, FORD WERKE AG, FORD FRANCE S. A., FORD MOTOR COMPANY LIMITED, FORD-WERKE AKTIENGESELLSCHAFT
Application year: 1989
Main classification: H01L21
IPC codes: H01L21, B41J2, F02M61