JPH0983281A - Manufacture of crystal vibrator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To attain a thin thickness by using a mask member with high immunity against a plasma of a fluoride gas and suitable for reactive ion etching so as to apply etching to a crystal substrate. SOLUTION: A crystal vibrator/filter made of an AT-cut crystal or the like having a high frequency fundamental wave is manufactured by thin processing by parallel flat plate type reactive ion etching. The latched portion indicates a region of the surface of the crystal substrate masked by a mask. A mask material using Al2 O3 or Ni as the major component is preferred to form windows for a thin substrate. Since the material Al2 O3 has sufficient etching immunity in the case of reactive ion etching, the material is useful when the tolerance of processing precision up to ±10μm is allowed. When a high etching speed is adopted or high processing precision of ±2μm is a requirement, a thin film whose major component is Ni is preferred. A sufficient immunity for etching a crystal substrate of 70μm is warranted by a material Ni or chromium.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a crystal resonator manufacturing method, and more particularly to a crystal resonator manufacturing method using a reactive ion etching method.

[0002]

2. Description of the Related Art Conventionally, quartz, which is known as a typical piezoelectric crystal, is artificially mass-produced by a hydrothermal synthesis method, and products utilizing its characteristics are used in various fields.

For example, a crystal oscillator is a product utilizing the characteristic that when a voltage is applied to a thin plate of quartz by applying a voltage, the crystal vibration is excited by the piezoelectric inverse effect of the quartz.

With the increase in frequency, downsizing, precision, and stability of crystal units (including those used as filters), the method of manufacturing crystal units using photolithography etching, which is a semiconductor manufacturing process, has been adopted. This is becoming popular, instead of mechanical cutting of crystals with a conventional blade saw or wire saw, photolithography is performed using a mask of any shape, and unnecessary crystal parts are melted by wet etching. It is a method of removing.

In particular, since the crystal resonator based on the AT substrate uses the thickness shear vibration, which has stable temperature characteristics in a wide range and is in high demand, ultrathin processing is required to cope with high frequency. .

When the crystal is thinned by mechanical processing, there is a limit to the thinning due to handling, and there is a problem in strength. However, if only the vibrating portion is thinned by etching, it is possible to obtain a high frequency of 100 MHz or higher with the fundamental wave.

Such a processing method is superior in machining accuracy to the conventional machining, can be processed according to a mask having an arbitrary shape, and can obtain a high frequency as a fundamental wave by thinning etching. Is advantageous.

The wet etching solution used at this time is generally hydrofluoric acid (HF) or ammonium bifluoride (N).
An aqueous solution of H ₃ HF ₂ ) is used. The etching reaction between these and quartz is shown below.

[0009]

Embedded image SiO ₂ + 6HF → SiF ₆ ^{2 −} +2 ⁺ H + 2H ₂ O SiO ₂ + 3NH ₄ HF ₂ → SiF ₆ ^{2 −} + 2H ⁺ + 3NH ₃ +2
H ₂ O These acids are strong acids and the temperature is high (40 ° C or higher)
There is a risk that the peeling phenomenon of the resist or the like may occur because the treatment is carried out at.

Therefore, at present, a two-step punching process using a stable metal mask is employed. This processing method is
The crystal substrate is processed by photolithography technology.
The part diagram is shown in FIG. As shown in this figure,
In the process of thinning / splitting by etching, there is a two-step process in which the resist is microfabricated, a gold vapor deposition mask is processed in that form, and the quartz substrate of the main body is processed using the gold vapor deposition mask. Has been done.

In the present manufacturing process of the crystal unit,
Wet etching of quartz is indispensable because not only the chemical polishing process but also the mechanical polishing process uses a process using hydrofluoric acid for final finishing.

However, when performing wet etching with AT-cut quartz, the etching rate depends on the crystal axis and Z
As a result, the etching progresses quickly in the direction along the axis, so that the finished surface becomes uneven as shown in FIG. In addition, for the same reason, the side wall portion becomes slanted due to anisotropy even in the portion corresponding to the contour of the vibrating surface,
It has the drawback that it cannot be machined to size.

On the other hand, when the finished surface roughness is prototyped and evaluated as a vibrator, the smaller the surface roughness, the lower the equivalent series resistance (hereinafter referred to as Ci) and the less loss the vibrator can obtain. It is known. Therefore,
If there is no distortion during processing, the mirror finished product is the most desirable.

However, although it is possible to carry out mirror finishing by mechanical polishing, in the method of thinning using wet etching, a mirror-like surface state or a surface state close to it is obtained due to the above-mentioned etching anisotropy. Things are difficult.

For this reason, for example, a crystal thinning method by dry etching using a reactive gas is used. According to this method, it is possible to perform processing that does not depend on the crystal axis for etching (no anisotropy), the accuracy of the etching shape is increased, and the finish roughness of the etching surface can be controlled.

In order to remove the thermal oxide film of the conventional Si semiconductor by dry etching, a fluorocarbon gas or SF ₆ gas is generally used. However, in the conventional etching, the film thickness is at most about 1 to 3 μm, and the purpose is to remove the oxide film. Therefore, the crystal of quartz in the present method is deeply etched up to 70 μm. Then the situation is different.

For example, when performing oxide film etching,
A resist used for photolithography is often applied as it is as a mask used for dry etching.

[0018]

However, the etching rate differs depending on the material, and in the resist and SiO ₂ ,
The resist is easier to etch. Therefore, Si
When etching O ₂ , a resist thickness of about 3 times the SiO ₂ film thickness is required.

Therefore, when the quartz is deeply etched, the resist film thickness must be very large.
If the resist film thickness is increased, the exposure degree of photolithography will decrease. Therefore, when the quartz is deeply etched, it is necessary to use another mask material.

The present invention has been made in view of the above background, and an object of the present invention is to provide a mask material having high resistance to the plasma of a fluorine gas and suitable for reactive ion etching.

[0021]

SUMMARY OF THE INVENTION In order to solve the above problems, the present invention provides a crystal resonator having a crystal substrate thinning process for masking the crystal substrate and performing parallel plate type reactive ion etching. In the manufacturing method, the mask material of the quartz substrate is characterized by containing Al ₂ O ₃ or Ni as a main component.

As the mask material, nichrome having a Ni content of 80% or more is used.

Hereinafter, the present invention will be described in more detail. A
A crystal oscillator / filter having a high-frequency fundamental wave such as a T-cut crystal can be manufactured by a parallel plate type reactive ion etching thinning process.

As the mask material for forming the window for the thin plate,
It is preferable that Al ₂ O ₃ or Ni be the main component. In particular, Al ₂ O ₃ is suitable as a mask because it has sufficient etching resistance when performing reactive ion etching. Especially, the etching rate of crystal is 3
It is preferable to use Al ₂ O ₃ when the processing accuracy is 000 A or less and the processing accuracy is up to ± 10 μm.

However, if the etching rate of quartz becomes high, the Al ₂ O ₃ mask may crack due to thermal strain. Further, when high processing accuracy of about ± 2 μm is required, Al ₂ O ₃ is not suitable. In such a case, it is preferable to use a thin film containing Ni as a main component. Even when a thin film containing Ni as the main component is used as a mask, sufficient etching resistance can be obtained.

Preferably, Ni alone or Nichrome having a Ni content of 80% or more is used as the mask.

[0027]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below with reference to the drawings.

In this embodiment, instead of the step of etching the surface of the AT-cut quartz crystal by wet etching to manufacture the vibrator and the filter, RIE (Reactive I
A dry etching method was carried out in which the crystal was thinned by plasma treatment by on etching. At this time, a mask material that enables deep etching was selected.

Table 1 shows a mask material used in each test example and its resistance, which was obtained by performing a test for thinning an AT-cut substrate by dry etching. As shown in this table, in each test example, the Al ₂ O ₃ template and Cr were used.
/ Au laminated film, Cr film, Ni film, Nichrome (Ni80-Cr20
And Ni90-Cr10) film was used as a mask material.

[0030]

[Table 1] Test Example Mask material Quartz etching depth Mask material etching amount selection ratio Condition 1 Al ₂ O ₃ 11.4 μm to 0.11 μm 100 I 2 Al ₂ O ₃ 3.6 μm to 0.04 μm 100 II 3 Cr / Au 11.3 μm> 0.5 μm <23 I 4 Cr 10.2 μm> 0.5 μm <20 I 5 Ni 12.4 μm 0.30 μm 41 I 6 Nichrome 12.6 μm 0.32 μm 39 I (Ni90-Cr10) 7 Nichrome 12.1 μm 0.35 μm 35 I (Ni80-Cr20) Tables 2 and 3 show details of the dry etching conditions under Conditions I and II, respectively.

[0031]

[Table 2] Dry etching conditions I Etching gas SF ₆ 25sccm + O ₂ 25sccm Etching pressure 50mmTorr RF input 200W Peak voltage 2000V Self-bias voltage 950V Etching time 1hr.

[0032]

[Table 3] Dry etching condition II (Al ₂ O ₃ template only) Etching gas SF ₆ 25sccm + O ₂ 25sccm Etching pressure 115 mmTorr RF input 400 W Peak voltage 2600 V Self-bias voltage 1250 V Etching time 10 min. As the Al ₂ O ₃ template, a laser-processed one having a thickness of 0.5 mm was adopted, and a Cr / Au laminated film, a Cr film, a Ni film
The film and the nichrome film were formed with a thickness of 5000 Å by electron beam evaporation.

FIG. 3 shows a schematic explanatory view of the mask. In this figure, the shaded area indicates the area where the crystal surface is covered with a mask. The etching conditions were as shown below. The etching depth of the crystal was calculated from the frequency of the exposed portion of the crystal substrate of the sample after the experiment, and the etching depth of the metal film thickness was measured by the SEM photograph.

As described above, the mask material preferably has a higher etching resistance than quartz. Therefore, the ratio of the etching depth of the crystal to the etching depth of the mask material is defined as the selection ratio and is also shown in Table 1. It was judged that the material having the larger selection ratio is more suitable for the mask.

The Al ₂ O ₃ template showed excellent resistance when etched at 200W. But 400
When RF power of W was applied, the mask itself cracked after 10 minutes due to thermal strain. Further, it was found that the accuracy of the laser processing was inferior to the accuracy of the shape of the photolithography by about 1/5.

Next, as a result of using the Cr / Au laminated film and the Cr film, as shown in the table, the etching rate selection ratio was small, and after the above experiment, the metal mask material was etched and the quartz crystal was removed. It was exposed.

On the other hand, in the etching experiment using Ni and nichrome as masks, both have sufficient resistance as etching masks, and the metal film remains with respect to the etching depth of quartz. When the thickness of the remaining mask was measured by an SEM photograph, the thickness of the etched metal mask was 0.3 to 0.35 μm.

From the above results, the film thickness of the metal mask necessary for etching quartz to 70 μm is 3.5 μm or more for Cr / Au and Cr, and 1.7 to 2.2 for Ni and Nichrome.
It can be estimated to be 0 μm or more.

However, metals such as Au, Cr, Ni and nichrome give rise to compression on a quartz substrate, and experience shows that a film of 2.5 μm or more causes a phenomenon of peeling from the substrate. Therefore, Ni and nichrome are most suitable as a metal mask having a sufficient resistance to etch quartz 70 μm and having a thickness that does not cause peeling.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a manufacturing process of a crystal unit.

FIG. 2 is an explanatory diagram of crystal anisotropy.

FIG. 3 is an explanatory diagram of crystal masking.

Claims (2)

[Claims] 1. A method of manufacturing a crystal resonator, comprising a step of thinning a crystal substrate by performing parallel plate type reactive ion etching by masking the crystal substrate, wherein the mask material of the crystal substrate is Al ₂ O ₃ Alternatively, a method of manufacturing a crystal unit, characterized in that one of Ni is a main component. 2. The method of manufacturing a crystal unit according to claim 1, wherein the mask material is nichrome having a Ni content of 80% or more.