JPS60211945A - Method for formation of thin film - Google Patents

Abstract

PURPOSE:To enable to control film thickness accurately by a method wherein the non-etching part of the double layer silicon substrate consisting of a P type conductive layer and an N type conductive layer is coated by a silicon oxide film, and while a DC positive voltage is being applied to one conductive layer, an etching is performed on the other conductive layer using an etchant consisting of hydrazine. CONSTITUTION:A double layer silicon substrate 12 is composed of a P type conductive layer 13 and an N type conductive layer 14. A silicon oxide film 15 is formed on the surface of said double layer silicon substrate by performing a thermal oxidization or a CVD method. As the silicon oxide film 15 is formed on both sides of the double layer silicon substrate 12, a part of the silicon oxide film located on the side of the N type conductive layer 14 is removed, and an electrode is connected to the N type conductive layer. The silicon oxide film on the side of the P type conductive layer 13, excluding the region where an etching is not performed, is removed. Hydrazine is filled up in a container 16 as an etchant 17, it is heated up using a heater 18, and when a DC voltage power source 20 is connected in such a manner that platinum is used as a cathode electrode 19 and the N type conductive layer 14 is used as an anode, the P type conductive layer 13 which is not coated by the silicon oxide film is etched. The etching is stopped when the P type conductive layer is removed and the N type conductive layer comes in contact with the etchant 17.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to thinning a silicon substrate, and particularly to a method of forming a silicon thin film by electrochemical line engraving.

(Prior art and its problems) The technology of engraving the entire silicon substrate with an engraving solution to form a thin film is used in the manufacture of diaphragm-type silicon pressure sensors, acceleration sensors, and thermopile infrared sensors. For example, a diaphragm type silicon pressure sensor has a structure in which a diffused resistor 2 is formed in a silicon thin film sl, as shown in FIG. The diffused resistor 2 is formed by introducing impurities of a conductivity type opposite to that of the silicon substrate 3 by ion implantation, thermal diffusion, or the like.

The silicon thin film portion 1 is thinned so that it deforms greatly when the pressure to be measured is applied, and generates a large force. The resistance value of the diffusion resistor 2 changes due to the generation of stress, and pressure is detected. The silicon thin film portion 1 is formed as a layer by electrical discharge machining or chemical etching. For example, if potassium hydroxide KOH, ethylene diamine pyrocatechol FDP, or hydrazine is used, silicon is anisotropically etched to form an inclined portion 4 as shown in FIG. If a (ioo) plane silicon wafer is used, the inclined portion 4 forms an angle of 54.7° (111
) surface. Or nitric acid HNO, or hydrofluoric acid HF' ((
- If used, the isotropic line will be carved and the slope will be vertical. The output of the pressure-to-electrical signal conversion of the diaphragm type silicon pressure sensor shown in Fig. 1 is proportional to the stress, but since the stress is inversely proportional to the square of the thickness of the silicon thin film portion 1, even a small film thickness variation causes a large sensitivity variation. occurs. In order to reduce sensitivity variations, it is necessary to accurately control the entire film thickness. Conventionally, the film thickness has been controlled by the front side time, but since the silicon substrate itself has thickness variations, accurate film thickness control has been difficult. For this reason, it is difficult and rare to produce a diaphragm type pressure sensor with uniform sensitivity, complete safety, and large waves.

Various methods have been proposed to accurately control the total film thickness, but there are problems in terms of performance, productivity, and cost.

First, the method using a high concentration boron layer is shown in Figure 2.

A high concentration boron layer 5 of 5 x 101 gcm-'' or more is formed on the surface of a silicon substrate 3 to a thickness of several μm by epitaxial growth or thermal diffusion.Furthermore, a low concentration epitaxial layer 6 is grown on it to form a diffused resistor 2. do.

The reason why the low-concentration epitaxial layer 6 can be grown is that the impurity concentration of the high-concentration boron layer 5 is extremely high, and it is impossible to form a medium-concentration (approximately 3 x 10 cm) diffusion resistor 2 therein. That's why. The marking liquid is KOH and FDP mentioned above.
, hydrazine. The disadvantages of this method are that the cost of the silicon substrate consisting of a high concentration boron layer and a low concentration epitaxial layer is high, and that the boron in the high concentration boron layer 5 is redistributed during the thermal process and does not come into contact with the diffused resistor 2. , the thickness of the low concentration epitaxial layer 6 must be made sufficiently thick. Next, a method for forming the entire thin film by electrochemical engraving on the PN junction silicon substrate is shown in FIG. PM conductive layer 7
If a two-layer silicon substrate is formed from the N-type conductive layer 8 and the N-type conductive layer 8, the surface of the N-type conductive layer 8 will be covered with a peritoneal membrane 9 and electrode wiring. give The cathode electrode 10 is platinum P. A two-layer silicon substrate and a cathode electrode 10 are immersed in an engraving solution 11 for engraving. The marking solution is hydrofluoric acid HF, and when a positive DC voltage of about 0.5 V is applied to the N-type 4'tjL layer 8, the P-type conductive layer 7 is marked. When the Pffl conductive layer 7 is completely removed, the engraving stops and only the N-type conductive layer 8 remains on the two-layer silicon substrate. By this electrochemical line engraving method, it is possible to form a thin film of a diaphragm type silicon pressure sensor using a dielectric film or a metal evaporated film as a total mask material. However, since phylic acid I (Fk) is used as the engraving solution, the engraving is isotropic.
In the sloped portion 4 shown in FIGS. 1 and 2, stress is concentrated at the vertical edge of the thin film and the fracture strength is low.

(Objective of the Invention) An object of the present invention is to provide a method of forming a thin film that can eliminate all of the above-mentioned defects and accurately control the film thickness.

(Structure of the Invention) According to the present invention, 2 consisting of a P-type conductive layer J@ and an N-type conductive layer
All non-lined areas of the silicon substrate are covered with a silicon oxide film and one conductor F4. A method for forming a thin J'gi is obtained, which is characterized in that the other conductive layer is marked with a marking solution made of hydrazine while applying a positive voltage directly to the J'gi.

(Examples) Next, the present invention will be described with reference to all drawings showing examples. FIG. 4 is a block diagram of a line engraving device showing one embodiment of the present invention. In particular, a method for forming a thin film of a diaphragm type silicon pressure sensor is shown. 2) The fi silicon substrate 2 is composed of a P-base conductive layer 13 and an N-type conductive layer 14. Pm, the thickness of the conductive layer 13 is, for example, 350 μm and the specific resistance is 10
The thickness of the N-type conductive layer 14 is, for example, 20 μm and the specific resistance is 3 to 5 Ω-m. 2 like this
The layered silicon substrate can be obtained by thermally diffusing phosphorus P or arsenic Aa' onto a P-type silicon substrate, or by depositing N on a P-type silicon substrate by epitaxial growth techniques.
It can be easily formed using a mold silicon layer as a core layer. Thermal oxidation or CVD (Chemistry) is applied to the surface of the two-layer silicon substrate on the image side.
A disilicon oxide film 15 is formed by a cal vapor deposition method. Silicon oxide film 1
5 is formed on both sides of the two-layer silicon substrate 12, but a portion of the silicon oxide film on the N-type 4-electrode MH4 side is completely removed, and electrodes are connected to the N-type conductive layer. The silicon oxide film on the P-type conductive layer 13 side is removed except for the region where the Pm conductive layer is not etched. Container 16 (c) Filled with hydrazine as a marking solution 17 and heated by a heater 18 to bring the temperature of the solution to about 90°C. When a DC voltage source 20i of about 3 V is connected using platinum Pt as a cathode electrode 19 and the N-type conductive layer 14 as an anode, the portion of the P-type thick conductive layer 13 not covered with the silicon oxide film is carved. Now, if the crystal plane orientation of the P-type conductive layer 13 is 'r(100) plane, since hydrazine has anisotropy, the inclined part has (1
11) A plane appears with an angle of 54.7°. If the opening in the portion not covered by the silicon oxide film is sufficiently large compared to the thickness of the P-type trench conductive layer 13, the line engraving progresses gradually until it reaches the N-type conductive layer 14. When the P-type groove conductor layer is removed and the N-type quaternary conductor layer comes into contact with the engraving dense liquid 17, the engraving stops. This is considered to be because a thin silicon oxide film is formed on the bottom surface of the di-N type four-electrode layer by anodic oxidation. The reason why the P21 conductive layer 13 is marked is probably because the applied voltage is in the opposite direction to the PN junction, so no current flows through the P-type conductive layer and no oxidation phenomenon occurs.

The magnitude of the applied voltage is 2V or more, and the upper limit is up to the junction breakdown voltage.

FIG. 4 is a simple configuration diagram showing the concept of the present invention. During actual cutting, a reflux device is used to prevent the engraving bath liquid 17 from evaporating and changing its composition, and the engraving progresses uniformly. It is desirable to stir the pattern marking solution using a Star 2 or the like. Also, although not clearly shown in the figure, both sides of the two-layer silicon substrate 12 are mirror-polished, and the thickness is Nm.

P-type impurities are diffused into the surface of the conductive layer 14 to form a pressure-sensitive resistor. Further, in the electrode connection portion, N-type high concentration impurity is diffused to form an ohmic contact.

Although the above description has been made using a (100) plane two-layer silicon substrate as an example, the plane orientation is of course not limited to the (100) plane, and other plane orientations may be used.

Further, the thickness and specific resistance of the P-type conductive layer and the N-type conductive layer are not limited to the above-mentioned values. Furthermore, although the polarity of the applied voltage has been described as being in the opposite direction, it is also possible to perform line engraving with the polarity in the forward direction. In this case, an electrode is connected to the P-type conductive layer and an N-type conductive layer is marked. However, since the N-type four conductive layer is also biased and a forward current flows, there is a limit to the magnitude of the applied voltage, and the Nm conductive layer is etched without being anodized and only the PM conductive layer is anodized. value.

(Effects of the Invention) If all diaphragm silicon pressure sensors are manufactured using the thin film forming method of the present invention, the thickness of the thin film portion will depend on the thickness accuracy of the epitaxial layer or the depth accuracy of the diffusion layer, regardless of the marking time. Settled. The thickness of the epitaxial layer and the depth of the diffusion layer can be controlled with high precision and uniformity using current integrated circuit technology, making it possible to obtain highly precise and uniform thin films that cannot be obtained with conventional line marking and time-based methods. This makes it possible to provide high-precision diaphragm-type silicon pressure sensors in large quantities at low cost.

The thin film forming method of the present invention is not only applicable to diaphragm type silicon pressure sensors, but can also be applied to forming thin films of silicon acceleration sensors and thermopile infrared sensors. Furthermore, it can be applied not only to sensor devices but also to other thin film functional devices.

[Brief explanation of drawings]

FIG. 1 is a structural sectional view of a conventional diaphragm type silicon pressure sensor. FIG. 2 is a structural cross-sectional view of a diaphragm type silicon pressure sensor whose film thickness is controlled by a highly concentrated poron layer. FIG. 3 is a block diagram of a conventional electrochemical line engraving method. FIG. 4 is a block diagram of a thin film forming method showing an embodiment of the present invention. 1...Silicon thin film part 2...Diffused resistor 3...Silicon substrate, 4...Slanted part 5...High concentration boron layer 6.Low concentration epitaxial layer 7...P type trench conductor layer 8...N
Mold conductive layer 9..protective film 10.polarized electrode 11..engraving solution 12.two-layer silicon substrate 13..P
Type conductive layer 14... N-type conductive layer 15... Silicon oxide film 16... Container 17... Engraving solution 18. Heater. 19... Cathode electrode 20... DC voltage source 71 Figure 2 Figure 3 Figure 9 8 r 10 Figure 4 0 \ 8

Claims (1)

[Claims] All non-lined areas of a two-layer silicon substrate consisting of a P-type conductive layer and an N-type conductive layer are covered with a silicon oxide film, and while a full DC positive polarity voltage is applied to one conductive layer, the other conductive layer is etched with lines made of hydrazine. A method of forming a thin film by marking a line in a solution.