JPH07109200A - Method for etching quartz - Google Patents

Abstract

PURPOSE:To uniformly flatten the uneven surface of a quartz substrate at a high etching speed by employing a buffer solution containing HF and CaF2 as an etching liquid. CONSTITUTION:A teflon-coated magnetic rotator 6 is disposed on the bottom of a teflon-made reaction container 4, and a basket 3 is also disposed in the container. Quartz substrates are vertically disposed on the bottom surface of the basket 3, and 400cc of a mixture solution prepared by dissolving 240-480g of calcium fluoride (or chromium fluoride) in 400 cc of 50% hydrofluoric acid comprising a HF-CaF2 (or HF-CrF2) mixture solution is added as an etching liquid 2 to the reaction container 4. A constant temperature tank 8 with a thermometer 5 is disposed on a magnetic stirrer 9 with a heater, and a magnetic rotator 6 is disposed on the bottom of the tank 8. (Hot) water 7 is charged in the constant temperature tank 8. The reaction tank 4 is hung in the hot water 7 from a stand, and the quartz substrates are etched with stirring, while maintaining the temperature of the etching solution 2 at approximately 40 deg.C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a quartz etching method, and more particularly to a technique for uniformly etching a quartz substrate used for a quartz oscillator in a short time.

[0002]

2. Description of the Related Art A crystal oscillator utilizing the electrostrictive effect of α-quartz has an electrode attached to a thin crystal plate and an AC electric field of a frequency equal to a natural frequency depending on the direction and size of the crystal cut. When applied, it resonates and continues to vibrate at a stable single frequency. It is used in many oscillator circuits including frequency meters.

In order to miniaturize a crystal unit having a high mechanical quality factor and a large resonance frequency, it is extremely important to carry out etching processing of the crystal substrate with high accuracy and reproducibility.

Etching is one of the processing techniques indispensable for manufacturing a crystal device. There are two purposes for which etching is used.

The first purpose is to clean the surface by removing surface roughness (concavities and convexities) and distortion due to mechanical processing such as polishing and cutting of the crystal surface.

The second is the application of etching to fine processing, which is carried out for the purpose of separating and dividing the quartz substrate when producing a quartz resonator by using photolithography (so-called small splitting). In this case, it is necessary to form a small pattern of about 1 μm on the crystal substrate using a resist or a metal mask, and etch the crystal substrate thereunder with high precision.

Etching is mainly based on a chemical reaction, and selection of an etching solution is important. Etching is a process of removing atoms one by one from the surface of the substrate by exchanging energy by a chemical reaction mechanism when viewed microscopically, but it is a kind of processing when viewed macroscopically, and it is precisely the desired shape. The most important point is to be able to finish it.

[0008]

The rate of etching of quartz (expressed by the thickness of the substrate material removed in a certain period of time or its weight) is uncontrollable if it is too fast, and vice versa. There is a problem as a means. Generally, the processing depth is indirectly controlled by the etching time,
In order to improve the processing accuracy, it is necessary that the etching rate be within a certain range and that the reproducibility be good.

Generally, the etching rate of quartz is proportional to the temperature of the etching solution. Therefore, by etching at a high temperature, it is possible to shorten the time required to etch the crystal, but by increasing the temperature of the etching solution, its evaporation becomes remarkable, and in the case of an open system, the etching liquid composition is improved. There is a problem that it cannot be kept uniform.

A change in the composition of the etching solution leads to a change in the etching rate, and the etching conditions cannot be kept constant in the mass production process. Therefore,
Changes in the composition of the etching solution due to evaporation cause variations in etching accuracy.

Further, since the etching rate of quartz largely depends on the etching solution used, in the case of etching in an open system at a high temperature, even if the etching solution evaporates, there is no variation in the etching accuracy. Development is required.

Although a large number of crystal oscillators are simultaneously formed in a single crystal substrate and the crystal oscillators are divided into small pieces to be assembled into individual crystal oscillators, the production efficiency is essentially increased. On the other hand, on the other hand, there is a problem that it is difficult to obtain the uniformity of the process conditions in the substrate, and it is difficult to create devices having the same characteristics.

The cutting surface or polishing surface of the quartz substrate, which is obtained by cutting (polishing) α-quartz crystal according to the purpose of use of the oscillator and polishing it to a predetermined size, is the grain size of the abrasive grains used for processing. It is strained by processing up to a depth of several times to 10 times, and it cannot be used as a substrate unless this layer is removed.

The depth of the layer subjected to this processing strain is about 10 μm including the partially strained layer due to microcracks on the polished surface, and the surface irregularities can be removed quickly and uniformly, and the finished surface can be removed. Etching is required to be a mirror surface.

The reason is as follows. In order to produce the characteristics of the crystal unit with good reproducibility, high yield, and at the lowest possible cost, the crystal unit is manufactured from a crystal substrate using photolithography. This is because a smooth crystal substrate is required in order to form electrodes and metal masks that are uniform and have good adhesion in the process.

Further, in the fine cutting of the quartz substrate, a high etching rate is required in order to shorten the processing time.

Therefore, it is a practically important task to find out the composition of the etching solution and the optimum etching conditions.

In the etching process, the reaction product formed by the reaction between the reaction species and the atoms on the surface of the substrate is dissolved and removed in the etching solution. If this reaction product is not removed properly, there is a problem that the etching rate becomes non-uniform in time or place. Furthermore, not only reaction products but also insoluble reaction products are formed by side reactions of etching, and if these cover the surface of the substrate material, etching does not occur.

The present invention has been made on the basis of the above-mentioned background, and in the technique of etching a quartz substrate by using a wet etching process, the unevenness of the quartz substrate surface is made uniform within the plane at a high etching rate. An object of the present invention is to provide an etching processing method for flattening.

[0020]

In order to solve the above-mentioned problems, the invention according to claim 1 is to perform an etching process for a crystal by bringing the crystal and an etching solution containing HF into contact with each other. In the method, there is provided a quartz crystal etching method characterized in that a buffer solution containing CaF ₂ together with HF is used as the etching solution.

According to a second aspect of the present invention, in the method of etching a crystal in which the crystal and the etching solution containing HF are brought into contact with each other to etch the crystal, the etching solution contains CrF ₂ together with HF. Provided is a quartz crystal etching method characterized by using a buffer solution.

The wet etching technique is, of course, determined by the combination of crystal and etching solution.
It depends on the relationship between the etching solution and the amount of the quartz substrate, the temperature of the solution, and parameters such as stirring.

In the present invention, 50% hydrofluoric acid which forms an azeotropic solution at 40 ° C. under normal pressure
By using a calcium fluoride mixed solution or a 50% hydrofluoric acid-chromium fluoride mixed solution as the etching solution for the quartz substrate, the vapor has the same composition as the liquid even if the etching solution evaporates. Does not change the etchant composition.

Therefore, the etching conditions can be made constant. As a result, the variation caused by the change in the composition of the etching solution caused by the evaporation of the etching solution in the open system as in the past was solved.

Anhydrous HF has no electrical conductivity (1.6 × 10
^-6 Ω ^-1 cm ^-1 ) undergoes acid dissociation in an aqueous solution as shown below.

[0026]

HF + H ₂ O → H ₃ O ⁺⁺ F ⁻ K ₁ = 7.2 × 10 ⁻⁴ F ⁻ + HF → HF ₂ ⁻ K ₂ = 5.1 However, the free energy of bond cleavage between H and F is The degree of difficulty is small in an aqueous solution because it is exceptionally large as compared with that between H-halogens and hydrogen bonds between HF2 molecules are large.

Further, it is characterized in that a linear polymer is formed by the hydrogen bond between HFs and the boiling point is remarkably higher than that of other hydrogen halides. In a 5 to 15M solution, H ₃ O ⁺ and H
Ionizing to F ₂ ⁻ and more complex species, HF becomes a strong acid.

Etching of quartz with hydrofluoric acid proceeds based on the following chemical reaction formula.

[0029]

[Equation 2] SiO ₂ + 6F ⁻ + 4H ⁺ → SiF ₆ ^{2 −} + 2H ₂ O Reaction formula 3 consists of redox reactions of the following reaction formulas 4 and 5.

[0030]

[Equation 3] SiO ₂ + 4H ⁺ + 4e ⁻ = Si + 2H ₂ O (E °
(V) (standard oxidation-reduction potential) = -0.86: 25 ° C)

[0031]

[Equation 4] SiF ₆ ²⁻ + 4e ⁻ = Si + 6F ⁻ (E ° (V)
= −1.24: 25 ° C.) Since the change in standard free energy of reaction formula 3 is negative as shown in the following formula, the reaction spontaneously proceeds.

[0032]

[Expression 5] ΔG ° = −nFE ° = −4 × 96500C ×
(-0.86- (1.24)) V = -146.7 kJ =
-35 kcal The free energy change is represented by the following equation.

[0033]

## EQU6 ## ΔG = ΔG ° + RTln {[SiF ₆ ²⁻ ] /
[F ⁻ ] ⁶ [H ⁺ ] ⁴ } Therefore, as the F ⁻ ion or H ⁺ ion concentration increases, the negative free energy change increases and the etching of the quartz crystal proceeds.

Further, HF is an excellent ionizing solvent having a large relative dielectric constant and dissolves many compounds to form a solution having high conductivity. Ionic fluorides such as calcium fluoride and chromium fluoride are easily dissolved in hydrofluoric acid as shown by the following formula, and these are reactive groups (F) in the mixed solution.
^- ions, H ⁺ ions, HF ₂ ^- ions) increasing the concentration.

[0035]

[Equation 7] CaF ₂ → Ca ²⁺ + 2F ⁻ , CaF ₂ + HF → C
a ²⁺ + F ⁻ + HF ₂ ⁻ CrF ₂ → Cr ²⁺ + 2F ⁻ , CrF ₂ + HF → Cr ²⁺ + F ⁻
+ HF ₂ ^- therefore 50% hydrofluoric acid - calcium fluoride mixed solution and 50% hydrofluoric acid - by use of chromium fluoride solution mixture etchant, etching of quartz proceeds as follows chemical equation To do.

[0036]

Equation 8] _{^{SiO 2 + 6F - + 4H +}} → SiF 6 2+ 2H 2 O SiO 2 + 3HF 2+ H + → SiF 6 2+ 2H 2 O SiO 2 + 3CaF 2 → SiF 6 2+ Ca 2+ + 2CaO SiO 2 + 3CrF 2 → SiF ₆ ^2- + Cr ²⁺ + 2CrO In each of the above reactions, hexafluorosilicic acid (H ₂ Si
F ₆ (aq)), its calcium hexahydrate (CaSiF
₆ (aq)) and its chromium hexahydrate (CrSiF ₆ (a
q)) is produced and the SiO ₂ is etched.

CaO and C which are by-products of the etching reaction
rO reacts with water to dissolve water-soluble calcium hydroxide or [C
It becomes r (H ₂ O) ₆ ] ²⁺ ions, which are dissolved and removed in the etching solution.

By using a 50% hydrofluoric acid-calcium fluoride mixed solution or a 50% hydrofluoric acid-chromium fluoride mixed solution as an etching solution, the etching of the quartz crystal is continued based on the reaction formulas 10 to 13. The etching rate can be made higher than that of an etching solution containing hydrofluoric acid, calcium fluoride, or chromium fluoride alone.

In the case of an aqueous solution of hydrofluoric acid, ionization ions such as H ⁺ (HF) _x F ⁻ are difficult to move due to hydrogen bonds between HFs, so that etching locally progresses and the entire surface of the quartz substrate is covered. Not evenly etched.

Since etching is a reaction in which atoms of the substrate material are removed from the surface by a chemical reaction, in order for the etching to be carried out smoothly, the reactive groups easily reach the surface of the substrate, and the reaction product is on the surface. It must be easy to remove from. These can be controlled by agitating the solution, raising the temperature, or the like, and activating the movement of the reactive groups (F ⁻ ions, H ⁺ ions, HF ₂ ⁻ ions) in the solution.

Further, in the present invention, by adding calcium fluoride or chromium fluoride to hydrofluoric acid, the conductivity is remarkably increased and the reactive groups (F ⁻ ion, H ⁺ ion, HF ₂ ⁻ ion) in the solution are added. Made it easy to move. As a result, the etching progressed over the entire surface of the substrate, and the irregularities and strains on the surface of the quartz substrate were uniformly removed within the surface to flatten the surface of the quartz substrate. Therefore, in the process of manufacturing the crystal unit by photolithography, it is possible to form electrodes and metal masks that are uniform and have good adhesion by using the finished surface obtained by such etching.

Further, a salt such as CaF ₂ or CrF ₂ is mixed with HF (a plurality of kinds of salts may be mixed), and this mixed solution serves as a buffer solution, so that the p
The effect of keeping the concentration of ions such as H constant is obtained, and as a result, uniform etching is performed.

As a result of the above-mentioned actions, it is considered that the etching progresses over the entire surface of the substrate, and the irregularities and strains on the surface of the quartz substrate are uniformly flattened within the surface.

As described above, by the crystal etching method according to the present invention, it is possible to form an electrode and a metal mask that are uniform and have good adhesion. For example, it can be applied to a method of manufacturing a crystal unit.

[0045]

EXAMPLES Examples of the present invention will be described in detail below.
The present invention is not limited to the examples below.

FIG. 1 shows a wet etching apparatus used in the present invention. This apparatus has a function of etching the quartz substrate 1 while stirring the etching liquid 2 in the reaction container 4. Stirring is one of the important operations in carrying out the reaction experiment, and promotes the smooth progress of the reaction. A magnetic stirrer is very effective for stirring in a closed system. A thermostatic chamber 8 with a thermometer 5 is installed on a magnetic stirrer 9 with a heater. The magnetic rotor 6 is installed on the bottom of the constant temperature bath 8, and (warm) water 7 is put into the constant temperature bath 8.

The Teflon-coated magnetic rotor 6 is installed on the bottom of the reaction container 4, the basket 3 is installed in the reaction container, and the quartz substrate 1 is installed vertically on the bottom surface of the basket.
Add. Since the hydrofluoric acid-based solution melts the glass, a reaction container made of Teflon was used as the etching reaction container. The temperature of the etching solution in the reaction container is measured by a thermometer 5 incorporated in the reaction container.

The reaction vessel is suspended in warm water using a stand, and two magnetic rotors 6 in the reaction vessel and in the constant temperature bath are rotated by a magnetic stirrer 9 with a heater.

The temperature of the hot water 7 is adjusted to keep the temperature of the etching solution in the reaction vessel at a constant temperature of 40.degree. Like this 4
The quartz substrate is etched while stirring the etching solution at 0 ° C.

The composition of the etching solution used in the present invention is as follows.

For the mixed solution system of HF (hydrogen fluoride) -CaF ₂ (calcium fluoride), the following four sets of etching solutions were adopted.

As the etching solution of Example 1, a mixed solution of HF (hydrogen fluoride) -CaF ₂ (calcium fluoride) was used. CaF _{2 in} 400 cc of 50% HF hydrofluoric acid
CaF ₂ supersaturated hydrofluoric acid in which 480 g was dissolved was prepared, and the mixing ratio based on the weight percentage was HF: CaF ₂ : H ₂ O =
It was set to 25:50:25. This HF (hydrogen fluoride) -C
A mixed solution 400 cc of aF ₂ (calcium fluoride) was used as an etching solution.

As the etching liquid of Example 2, a mixed solution of HF (hydrogen fluoride) -CaF ₂ (calcium fluoride) having a different mixing ratio from that of Example 1 was used. A mixed solution of 240 g of CaF ₂ dissolved in 400 cc of 50% HF hydrofluoric acid was prepared, and 400 cc thereof was used as an etching solution.

The etching liquid of Comparative Example 1 was 50% HF.
400 cc of hydrofluoric acid was used. As the etching solution of Comparative Example 2, 400 cc of CaF ₂ aqueous solution was used.

For the mixed solution system of HF (hydrogen fluoride) -CrF ₂ (chromium fluoride), the following four sets of etching solutions were adopted.

As the etching liquid of Example 3, a mixed solution of HF (hydrogen fluoride) -CrF ₂ (chromium fluoride) was used. CrF ₂ 4 on 400 cc of 50% HF hydrofluoric acid
CrF ₂ saturated hydrofluoric acid in which 80 g was dissolved was prepared, and the mixing ratio by weight percentage was HF: CrF ₂ : H ₂ O = 2.
It was set to 5:50:25.

This mixed solution 400 cc of HF (hydrogen fluoride) -CrF ₂ (chromium fluoride) was used as an etching solution.

As the etching liquid of Example 4, a mixed solution of HF (hydrogen fluoride) -CrF ₂ (chromium fluoride) having a different mixing ratio from that of Example 3 was used. 50% HF hydrofluoric acid 4
A mixed solution of 240 g of CrF ₂ dissolved in 00 cc was prepared, and 400 cc of the mixed solution was used as an etching solution.

The etching solution of Comparative Example 3 was 50% HF.
400 cc of hydrofluoric acid was used, and 400 cc of CrF ₂ aqueous solution was used as the etching liquid of Comparative Example 4.

In all of the examples and comparative examples, the surface roughness of the quartz substrate before etching was evaluated using a three-dimensional surface roughness measuring machine, and then several quartz substrates were placed in a basket, The quartz substrate was etched while stirring the etching solution at 40 ° C. using each etching solution.

After 30 minutes and 45 minutes from the start of etching,
After 60 minutes, the quartz substrate was taken out, and the etching depth and the surface roughness after etching were examined using a three-dimensional surface roughness measuring machine.

Examples 1 and 2 of Comparative Solution System of HF (hydrogen fluoride) -CaF ₂ (calcium fluoride) and Comparative Example 1
Table 1 and FIG. 2 show the etching time dependence of the etching depth by wet etching using the etching solutions of Nos. 2 and 2.

[0063]

[Table 1]

Examples 3 and 4 of Comparative Solution System of HF (Hydrogen Fluoride) -CrF ₂ (Chromium Fluoride) and Comparative Examples 3 and 4
Table 2 and FIG. 3 show the etching time dependence of the etching depth by wet etching using the above-mentioned etching solution.

[0065]

[Table 2]

It was found that the etching depth was proportional to the etching time in all the etching solutions.

When the mixed solution system of HF (hydrogen fluoride) -CaF ₂ (calcium fluoride) was used as the etching solution, the etching rate, that is, the magnitude of the gradient of the etching depth-etching time straight line, is shown in Comparative Example 2 < Comparative Example 1 <Example 2 <Increasingly in the order of Example 1, the mixed solution of Example 1 in which calcium fluoride is supersaturatedly dissolved in hydrofluoric acid is the best as an etching solution for quartz.

When the mixed solution system of HF (hydrogen fluoride) -CrF ₂ (chromium fluoride) was used as the etching solution, the etching rate, that is, the magnitude of the gradient of the etching depth-etching time straight line, was measured by Comparative Example 4 < Comparative Example 3 <Example 4 <
As the crystal etching solution is increased in the order of Example 3, the mixed solution of Example 3 in which chromium fluoride is saturated and dissolved in hydrofluoric acid is the best.

The surface roughness (irregularities) of the quartz substrate after etching with each etching solution was examined using a three-dimensional surface roughness measuring instrument, and the result was HF (hydrogen fluoride) -CaF.
Table 2 shows the mixed solution system of ₂ (calcium fluoride).
Table 4 shows a mixed solution system of HF (hydrogen fluoride) -CrF ₂ (chromium fluoride).

[0070]

[Table 3]

[0071]

[Table 4]

The surface roughness (unevenness) of the mixed solution system of HF (hydrogen fluoride) -CaF ₂ (calcium fluoride) by etching is Comparative Example 2> Comparative Example 1> Example 2> Example 1
Is decreasing in order. Particularly in Example 1, the crystal surface is about 9 times flatter than in Comparative Example 2.

The surface roughness (irregularities) of the mixed solution system of HF (hydrogen fluoride) -CrF ₂ (chromium fluoride) by etching is in the order of Comparative Example 4> Comparative Example 3> Example 4> Example 3>. is decreasing. In particular, Example 3 is about 6 than Comparative Example 4.
The double crystal surface is flat.

Therefore, from the both sides of the etching rate and the flattening of the quartz surface by etching, as the etching solution for the quartz substrate, the mixed solution of Example 1 in which calcium fluoride was supersaturated dissolved in hydrofluoric acid or chromium fluoride in hydrofluoric acid was used. It was found that the mixed solution of Example 3 in which the above was saturated dissolved was the best.

[0075]

As described above, according to the present invention, a mixed solution in which calcium fluoride is supersaturated in hydrofluoric acid or a mixed solution in which chromium fluoride is saturated in hydrofluoric acid is used as an etching solution for a quartz substrate. By doing so, the following various effects are achieved.

(1) Etching with a hydrofluoric acid-calcium fluoride mixed solution has a higher etching rate than an etching solution of HF or CaF ₂ , so that the etching time can be shortened.

(2) Since the etching rate with the hydrofluoric acid-chromium fluoride mixed solution is higher than that with the HF or CrF ₂ etching solution, the etching time can be shortened.

(3) By adding calcium fluoride or chromium fluoride to hydrofluoric acid, the ionization of fluorine ions during etching can be increased.

(4) An azeotropic solution is formed at 40 ° C. under normal pressure.
% Hydrofluoric acid-potassium hydrogen fluoride mixed solution or 50%
By using a mixed solution of hydrofluoric acid-potassium fluoride as an etching solution and etching the quartz substrate while constantly stirring the etching solution, it is possible to perform etching with a uniform etching solution composition that does not change with time.

(5) Since the conductivity is remarkably increased by adding potassium hydrogen fluoride or potassium fluoride to hydrofluoric acid, the reactive groups (F ⁻ ion, H ⁺ ion,
HF ₂ ⁻ ions) become easy to move, and as a result, etching progresses over the entire surface of the substrate, and unevenness and strain on the surface of the quartz substrate can be planarized uniformly within the surface.

(6) Since the surface of the quartz substrate formed by the etching method of the quartz substrate of the present invention is smooth, it is possible to form an electrode and a metal mask having uniform and high adhesion on the surface.

(7) The present invention can be applied to the fine processing of a crystal unit using photolithography.

[Brief description of drawings]

FIG. 1 is a diagram of a wet etching apparatus for quartz.

FIG. 2 is a correlation diagram between the etching time and the etching depth of a crystal with an etching solution of a mixed solution system of HF (hydrogen fluoride) -CaF ₂ (calcium fluoride).

FIG. 3 is a correlation diagram between the etching time and the etching depth of the crystal by each etching solution of the mixed solution system of HF (hydrogen fluoride) -CrF ₂ (chromium fluoride).

[Explanation of symbols]

 1 ... Quartz substrate 2 ... Etching solution 3 ... Basket 4 ... Reaction vessel 5 ... Thermometer 6 ... Magnetic rotor 7 ... Hot water 8 ... Constant temperature bath 9 ... Magnetic stirrer with heater

Claims (2)

[Claims] 1. A method of etching a crystal in which an etching solution containing HF is brought into contact with a crystal to etch the crystal, wherein a buffer solution containing CaF ₂ together with HF is used as the etching solution. Characteristic quartz etching method. 2. A method for etching a quartz crystal in which an etching solution containing HF is brought into contact with quartz to etch the quartz crystal, wherein a buffer solution containing CrF ₂ together with HF is used as the etching solution. Characteristic quartz etching method.