JPH08109500A - Fine working of silicon - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily produce a silicon fine structure by forming a pattern of an electropositive and electronegative-doped region in the surface of a silicon substrate and selectively removing an anode region with an oxidative acid etchant.

SOLUTION: A prescribed pattern consisting of a p-dope region 102 by B, etc., and an n-dope region 103 by P, etc., is formed on the surface of a silicon substrate 101 consisting of a single crystal or a multicrystal silicon. When this surface comes into contact with an electrolyte, electrons are supplied from the p-dope 102 to the n-dope 103, thereby turning the former relatively electropositive and the latter electronegative. The silicon substrate 100 thus obtd. is etched with the oxidative acid etchant. This etchant is preferably constituted of a hydrofluoric acid and an oxiding agent such as chromic acid, nitric acid and hydrogen peroxide. Thus, the electropositive p-dove region 102 is removed, thereby obtaining the fine structure consisting of the electronegative n-dope region 103, and if necessary, undercut is further applied, thereby forming a bridge part, a cantilever, etc.

COPYRIGHT: (C)1996,JPO

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a microfabrication method, and more particularly to a method for microfabrication silicon by selective etching.

[0002]

2. Description of Related Art Microfabrication refers to the production of very small mechanical structures, usually on the order of a few microns. Applications for microfabricated structures include, for example, strain gauges and inkjet printheads with tiny channels for liquid flow. Silicon is typically used as the fabrication material.

One method of micromachining silicon is the wet etch method using potassium hydroxide (KOH) or similar type alkaline solutions. However, this method is limited by the anisotropy of the etch rate on the {100} and {111} planes of crystalline silicon. Alkaline etching is limited by the crystal planes.

US Pat. No. 4,995,954 discloses another method of wet etching silicon by applying a voltage to the silicon in an electrochemical cell containing an aqueous hydrofluoric acid electrolyte. There is. The disclosed process creates a porous region in the silicon,
It then dissolves in the alkaline bath.

Another method of micromachining silicon is dry etching, which uses a plasma to etch a precise profile in the silicon.

[0006]

SUMMARY OF THE INVENTION It has been discovered that silicon can be microfabricated by using patterned doping. Specifically, the method of the present invention is based on the relatively different resistance to etching between n-doped and p-doped regions of silicon. Accordingly, the methods of micromachining silicon described herein include forming a pattern of differentially doped regions in the surface of silicon, at least one of the regions being another negative region. And etching the surface with an acid-oxidizing etchant. The positive region is selectively p-doped and the negative region is selectively n-doped. Doping is done using standard photolithographic techniques. The acid is preferably a mixture of chromic acid and hydrofluoric acid.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The method of the present invention uses selective doping to produce regions on silicon that are etched at different rates. With proper patterning of the doped regions, the silicon can be microfabricated by simple chemical etching.

Referring to FIG. 1, a silicon substrate 100 is provided, which is a p-doped side-by-side region 1.
02, and a substrate 101 of crystalline silicon having a shaped region 103 that is n-doped. The silicon substrate may be a wafer of single crystal silicon or polycrystalline silicon deposited, for example by chemical vapor deposition. By using standard techniques well known to those skilled in the art (eg, photolithography and ion implantation), patterning produces differentially heavily doped regions. The doping level is preferably such that an electron density exceeding 10 ¹⁸ / cc can be obtained. phosphorus for the n-region,
Standard dopants such as boron may be used for the p-region. Specifically, the desired shape of the microfabricated structure is patterned with n-doping. The regions to be etched off are p-doped.

The wafer is then selectively etched by immersion in an oxidizing etchant. The preferred oxidizing etchant is an aqueous solution of chromic acid and hydrofluoric acid. The preferred composition is a 1: 1 mixture of 1.2M chromic acid and 49 weight percent hydrofluoric acid. Other concentrations of chromic acid and hydrofluoric acid can be used effectively. For example, the ratio of chromic acid to hydrofluoric acid can range from about 0.5: 1.5 to about 1.5: 0.5. The concentration of chromic acid can range from about 0.5M to about 1.5M. The hydrofluoric acid concentration can range from about 30 weight percent to about 60 weight percent. In addition, other oxidants such as nitric acid and hydrogen peroxide can replace chromic acid. The temperature of the etchant solution and the etching duration can be adjusted so as to obtain a desired degree of etching.

The presence of the oxidizing electrolyte induces an internal bias between the p-doped and n-doped regions. p-
The doped region supplies electrons to the n-doped region. Therefore, the p-doped region is positively biased, while the n-doped region is negatively biased. There is no need for external bias by applying voltage from an external power supply. The positive p-doped regions are selectively etched.

While not wishing to be limited to any particular theory of the chemical or electrical mechanism by which the invention is realized, it is suggested that the cathodic reaction is as follows. _{^{Cr 2 O 7 - + 6e -}} + 14H → 2Cr +3 + 7H 2 O (1) anode reaction is as follows. 2H ₂ O + Si → SiO ₂ + 2H ₂ (2) SiO ₂ + 6HF → H ₂ SiF ₆ + 2H ₂ O (3)

As electrons are withdrawn from the p-doped region, highly reactive sites are created in the structure of the silicon atom, which causes a reaction between silicon and the electrolyte.
Believable.

FIG. 2 shows the structure obtained after etching.
The p-doped region 103 remains on the substrate 101 as a support. The n-doped region has been etched away.

Although there is some degree of crystal plane selectivity, the method of the present invention can be used to micromachine relatively complex and irregular shapes in silicon. Also, conventional undercutting techniques such as controlled etching can be used with the methods described herein. For example, FIG. 3 shows a silicon wafer 200 including a substrate 201 having two microfabricated regions 202 and 204 connected by a bridge portion 203. In another structure shown in FIG. 4, a silicon wafer 300 has a substrate 301 having a protruding microfabricated region 302 and a cantilever beam 303 extending laterally therefrom.
including. Such microfabricated structures can be further undercut by conventional techniques to completely separate them from the silicon substrate.

While the present invention has been described by way of preferred embodiments, those skilled in the art will readily appreciate that variations and modifications may be made without departing from the spirit or scope of the invention as defined by the claims. Would be obvious.

[Brief description of drawings]

FIG. 1 is a schematic side view showing a patterned doped region in silicon.

FIG. 2 is a side view showing silicon finely processed by etching.

FIG. 3 is a side view showing a bridge structure formed by microfabrication.

FIG. 4 is a side view showing a cantilever structure formed by microfabrication.

[Explanation of symbols]

 100 Silicon Substrate 101 Substrate 102, 103 Region 200 Silicon Wafer 201 Substrate 202 Microfabrication Region 203 Bridge Part 204 Microfabrication Region 300 Silicon Wafer 301 Substrate 302 Microfabrication Region 303 Cantilever Beam

Claims (10)

[Claims] 1. A step of preparing a silicon substrate; b) forming a pattern of differently-doped regions of a predetermined shape in the surface of the silicon substrate, and contacting the surface with an electrolytic solution. At least one of the shaped regions is relatively positive and another at least one of the shaped regions is relatively negative when placed on a substrate; and c) with an oxidative acid etchant. A method for producing a silicon fine structure, comprising the steps of etching the surface and selectively removing the anode region with an etchant. 2. The relatively positive shaped area is
The method of claim 1 which is p-doped. 3. The method of claim 2, wherein the relatively negative region is n-doped. 4. The method of claim 1, wherein the oxidizing acid etchant comprises an aqueous solution of hydrofluoric acid and an oxidizing agent. 5. The method of claim 4, wherein the oxidizing agent is chromic acid. 6. The oxidizing acid etchant is 1.2M.
The method of claim 1 comprising an essentially 1: 1 mixture of chromic acid and 49 weight percent hydrofluoric acid. 7. The method of claim 4, wherein the oxidant is nitric acid. 8. The oxidant is hydrogen peroxide.
The described method. 9. The method of claim 1, wherein the silicon is single crystal. 10. The silicon is polycrystalline.
The described method.