JP4638682B2 - Manufacturing method of crystal unit - Google Patents

Description

  The present invention relates to a method for manufacturing a crystal resonator used as a reference signal source for a timepiece or a mobile communication device.

  A so-called tuning-fork type crystal resonator having two vibrating legs is widely used as a reference signal source for a timepiece or a mobile communication device. As shown in FIG. 7, the tuning fork type crystal resonator includes one base portion 5 and two vibrating legs 7 extending from the base portion 5. The short side of the tuning-fork type crystal unit having the leg width 9 of the vibrating leg 7 coincides with the X axis (electrical axis) which is one of crystal axes of the crystal, and the long side of the leg length 11 is a crystal. It is directed to the Y ′ axis, which is a direction rotated by 0 ° to 5 ° around the X axis from the Y axis (mechanical axis) of the crystal axis. The Z ′ axis, which is the thickness direction, is a direction perpendicular to the X axis and the Y ′ axis. Since the resonance frequency of such a crystal resonator is determined by the leg width 9 and leg length 11 of the vibrating leg 7, it is manufactured by a photolithography technique in order to obtain a uniform leg width 9 and leg length 11 (for example, a patent). Reference 1).

  FIG. 8 is a flowchart showing an outline of the manufacturing process of the crystal unit. The quartz oscillator is cut out from a quartz crystal, and is formed by using a photolithography technique sequentially starting from a rectangular quartz wafer that has been polished and adjusted to a predetermined thickness. In step (a) of FIG. 8, a quartz wafer is first formed by sputtering on both sides of the front and back with a metal film as a Cr film and an Au film as a base. In step (b), a photoresist is applied on both sides of the Au film. For example, a spin coater is used for coating. In step (c), after the step of drying the photoresist at a predetermined temperature (pre-baking), in step (d), the completed pattern of the crystal unit is exposed. In step (e), the pattern exposed to the photoresist is developed in a developer. In step (f), the metal film (Au, Cr) is etched using the developed photoresist pattern as a mask. The pattern of the metal film (Au, Cr) remaining at this time becomes the outer shape of the crystal resonator. In step (g), the photoresist is stripped. In step (h), a new photoresist is applied on both surfaces, and in step (i), the photoresist is pre-baked, and then in step (j), the pattern of the gold plating film used for frequency adjustment is exposed and developed. In step (k), gold plating is performed.

  In the step (l), the photoresist on which the gold plating film is formed is peeled off, and in the step (m), a new photoresist is applied to both surfaces. After pre-baking in the step (n), in the step (o), the photoresist film is exposed with a pattern serving as an electrode. After the development in the step (p), in the step (q), the crystal is made of hydrofluoric acid and ammonium fluoride solution using the pattern of the metal film (Au film with Cr as a base) previously formed in the step (f) as a mask. The outer shape of the crystal unit is formed by etching with the mixed solution. In the step (r), the metal film (Au, Cr film) is removed by etching while leaving the portion covered with the electrode pattern photoresist film formed in the steps (o) and (p). The In step (s), Pd is vapor-deposited using Ti as a base while rotating the wafer. As a result, the front and back and side electrode films are formed. In the step (t), the photoresist film for the electrode is peeled off, and the Ti and Pd films remain only in the portion that becomes the electrode by so-called lift-off. In step (u), unnecessary Au and Cr films are removed by etching. At this time, in order to leave the Pd film, an etching solution is used in which iodine is dissolved in an etching solution, but iodine that does not dissolve Pd is dissolved in a potassium iodide solution. Thus, a crystal resonator having electrodes and a tuning fork-shaped outer shape is completed.

  As described above, the pattern used as a mask when etching the crystal wafer is a pattern of a metal film (Au, Cr film) formed on the crystal wafer, and the metal film pattern formation is performed on the crystal resonator. The finished pattern is exposed and developed with a photoresist as a mask, and etching is performed using Au and Cr etching solutions. Since the quartz wafer is etched not only in the thickness direction but also in the lateral direction (side surface) by etching, the width of the metal film pattern is slightly increased in consideration of the decrease due to the etching. However, the dimension of the pattern of the formed metal film does not necessarily match the dimension of the exposure mask due to the temperature, time, the state of liquid flow at the time of developing the photoresist, and the side etching at the time of Au and Cr etching. Further, the difference between the dimension of the formed metal film pattern and the dimension of the exposure mask is not always constant, and varies depending on the state during exposure and development, the state during etching of Au and Cr, and the like. Therefore, measure the pattern width of the formed metal film, especially the leg width, which has a large influence on the resonance frequency, and compare the measured value of this leg width with the design value so that the leg width becomes the design value. The etching time of the crystal wafer is determined based on the difference between the measured value and the design value. Measurement of the pattern dimension of the metal film is usually performed after (g) photoresist stripping.

By the way, quartz crystal is an anisotropic single crystal belonging to the trigonal system. Therefore, the etching rate differs depending on the crystal orientation, and the cross-sectional shape of the legs after etching is not rectangular, but the + X direction as shown in FIG. It becomes the shape which has the convex part 3. The post-etching leg width W ′ measured by the above experiment is the leg width in the vicinity of the surface as shown in the figure. This is because it is difficult to measure the convex portion in the depth direction because the measurement is performed using a metal microscope.
JP-A-5-315881 (page 3-4, Fig. 1-2)

  However, in the above-described conventional technique, only the leg width is managed, so even if the completed leg width is the same, the size of the convex portion is different due to the difference in etching time. There was a problem that it was large and the yield was poor.

  For example, as shown in FIG. 11, if the leg widths of the two legs A and B on which the metal film pattern 1 is formed are W1 and W2 and W1> W2, the conventional technique uses the etching time t1 of W1, The etching time t2 for W2 is t1> t2. Even if the etching is ideally performed and both the leg widths after etching become W as designed as shown in FIG. 11, the sizes δ1 and δ2 of the respective protrusions 3 are longer as the etching time is longer. Since it becomes smaller, δ1 <δ2, and the relationship between the effective leg widths W + δ1 and W + δ2 becomes W + δ1 <W + δ2, and the effective leg width becomes smaller when the etching time is longer. Therefore, in this case, when the conventional technique is used, the completed frequencies f1 and f2 are different from each other as f1 <f2. Therefore, there are problems that the frequency variation of the completed crystal resonator is large and the yield is poor. Note that the amount of reduction from the + X side is larger due to anisotropy, and the amount of reduction from the −X side is very small due to anisotropy, so the figure shows that there is no reduction from the −X side. The same applies to other figures.

  In order to solve the above-described problems, an object of the present invention is to provide a method for manufacturing a crystal resonator with a small frequency variation and a high yield.

In order to achieve the above object, a method for manufacturing a crystal resonator according to the present invention is a method for manufacturing a crystal resonator having a vibration leg that vibrates at a predetermined frequency. Metal film forming step of forming a metal film having a different shape, a measuring step of measuring the dimension of the metal film at a location corresponding to the leg width of the vibrating leg, and a vibrating leg previously obtained based on a crystal resonator for a sample The first correlation between the leg width and the frequency of the etching, the second correlation between the decrease in the leg width by etching and the frequency, and the third correlation between the etching time and the decrease in the leg width are used. Te, a calculation step of calculating an etching time and a dimension measured in the measuring step and the predetermined frequency, the metal film as a mask, the etching time calculated by the calculating step, an etching step of etching the quartz wafer, Characterized in that it has.

The method for manufacturing a crystal resonator according to the present invention is characterized in that a short side direction of the vibration leg coincides with an X axis of the crystal wafer .

  According to the above manufacturing method, not only the vibration leg width but also the size of the convex part of the vibration leg cross section can be considered in determining the etching time, so the frequency variation of the crystal unit immediately after manufacturing by etching is small. There is an effect of becoming.

  Further, according to the above manufacturing method, the relational expression between the etching time, the vibration leg width, and the frequency can be easily obtained, and the etching time calculation formula can be derived.

The best mode for carrying out the present invention will be described below with reference to the drawings. First, based on a plurality of sample crystal resonators manufactured in advance, a first correlation indicating a relationship between a size and a frequency of a predetermined portion of the crystal resonator, a reduction amount and a frequency due to etching of the predetermined portion, Steps for obtaining a second correlation indicating the above relationship and a third correlation indicating the relationship between the etching time and the amount of decrease due to etching of the predetermined portion will be described.
2 to 4 are plotted using data in mass-produced products as sample crystal resonators. FIG. 2 is a graph plotting the relationship between the leg width W ′ after etching and the frequency f. There is a correlation that can be approximated by a straight line between the leg width W ′ after etching and the frequency f. FIG. 3 is a graph plotting the relationship between the amount d of decrease in leg width etching and the frequency f. Although the leg width on the metal film pattern before the etching is not constant, there is a correlation that can be approximated by a substantially straight line between the decrease d due to the etching of the leg width and the frequency f. This is presumably because the decrease d due to the etching of the leg width reflects the size of the convex portion, and when d increases, the convex portion becomes smaller and the frequency decreases. FIG. 4 is a graph plotting the relationship between the etching time t and the decrease d due to the etching of the leg width. There is a correlation that can be approximated by a straight line between the etching time t and the decrease d due to the etching of the leg width.

  The solid line shown in the graph of FIG. 2 is a regression line obtained from each measurement point by the least square method and can be expressed as Equation (1). Here, g is a coefficient indicating the magnitude of frequency change when the leg width changes by unit length, and h is an intercept serving as an adjustment term. This equation (1) represents the first correlation between the leg width, which is the dimension of the predetermined part of the crystal resonator, and the frequency.

    f = gW ′ + h (1)

  The solid line shown in the graph of FIG. 3 is a regression line obtained from each measurement point by the least square method, and can be expressed as Equation (2). Here, i is a coefficient indicating the magnitude of frequency change when the amount of decrease due to etching of the leg width changes by a unit amount, and j is an intercept serving as an adjustment term. This mathematical formula (2) represents the second correlation indicating the relationship between the amount of decrease due to etching of the predetermined portion and the frequency.

    f = id + j (2)

  The solid line shown in the graph of FIG. 4 is a regression line obtained from each measurement point by the least square method, and can be expressed as Equation (3). Here, k is a coefficient indicating the amount of change in the amount of decrease due to the etching of the leg width when the etching time changes by a unit amount, and m is an intercept serving as an adjustment term. This mathematical formula (3) represents the third correlation indicating the relationship between the etching time and the amount of decrease due to etching of a predetermined portion.

    d = kt + m (3)

  Further, the decrease d due to the etching of the leg width can be defined as Equation (4) using the measured leg width W0 on the metal film pattern and the leg width W ′ after the etching.

    d = W0−W ′ (4)

  Equations (1) and (2) are multiplied by s times and u times, respectively, and a so-called linear combination is performed, and W ′ is eliminated using the relationship W ′ = W0−d obtained from Equation (4). Further, when formula (3) is substituted for d and arranged, a formula for obtaining the etching time t from the frequency f and the leg width W0 on the metal film pattern is obtained as in formula (5). Here, p is a coefficient indicating the magnitude of change in etching time when the measured value of the leg width on the metal film pattern changes by unit length when the frequency is constant, and q is the leg on the metal film pattern. This is a coefficient indicating the magnitude of change in etching time when the frequency is changed by a unit amount when the measured value of the width is constant, and r is an intercept.

    t = pW0 + qf + r (5)

  s and u have a meaning of weighting for the mathematical formulas (1) and (2), and can take arbitrary values. However, according to experiments, it was most preferable when s and u were set to 1, respectively.

  As an example of manufacturing a crystal resonator having a leg length of 2250 μm, a leg width of 200 μm, a plate thickness of 100 μm, and a frequency of 32.768 kHz, when μm is used as a unit of leg width and a minute is used as a unit of etching time t, formulas (1) to (5) ) Take the following values. g = 117.3 (Hz / μm), h = 107155.1 (Hz), i = −91.4 (Hz / μm), j = 33816.6 (Hz), k = 0.044 (μm / min) ), M = −3.4 (μm), p = 12.8 (min / μm), q = −0.22 (min / Hz), and r = 4926 (min). The value of each coefficient mentioned here is an example, and it goes without saying that it varies depending on the leg length, leg width, etching conditions, and the like.

  Next, the manufacturing process in this invention is demonstrated. FIG. 5 is a flowchart of the manufacturing process according to the present invention. The quartz oscillator is cut out from a quartz crystal, and is sequentially formed using a photolithography technique starting from a rectangular quartz wafer that has been polished and adjusted to a predetermined thickness. In addition, the parenthesized alphabet described in the following description represents steps (a) to (u) in FIG. First, a quartz wafer having a predetermined shape is prepared. (A) First, an Au film is formed on a quartz wafer by sputtering as a metal film on both sides of the front and back surfaces with Cr as a base. (B) A photoresist is applied on both sides of the Au film. For example, a spin coater is used for coating. (C) After the step of drying at a predetermined temperature (pre-baking), (d) the completed pattern of the crystal resonator is exposed. (E) The pattern exposed to the photoresist is developed in a developer. (F) The metal film (Au, Cr) is etched using the developed photoresist pattern as a mask. The pattern of the metal film (Au, Cr) remaining at this time becomes the outer shape of the crystal resonator. (G) Thereafter, the photoresist is peeled off.

  After step (g), (A1) the leg width on the metal film pattern is measured. The number of wafers to be processed simultaneously, that is, the number of one lot is about 30, and it is desirable to measure the total number of wafers. However, since it takes time, measurement can be performed by extracting any 3 to 5 wafers. This is because the processing conditions such as temperature are the same in the same lot, so that a representative value may be obtained even with about 3 to 5 samples. Each wafer is measured about 5 points per sheet. An average value of these measurement data is obtained and set as the leg width W0 on the metal film pattern for calculating the etching time t. (A2) The etching time t is calculated by substituting the obtained leg width W0 on the metal film pattern and the predetermined frequency f into the above equation (5). (H) A new photoresist is applied on both sides, (i) after pre-baking, (j) a pattern of a gold plating film used for frequency adjustment is exposed and developed, and (k) gold plating is performed. (L) The photoresist on which the gold plating film is formed is peeled off, and (m) new photoresist is applied on both sides. (N) After pre-baking, (o) the photoresist film is exposed to a pattern to be an electrode.

  (P) After development, (q) the quartz is etched with a mixture of hydrofluoric acid and ammonium fluoride solution using the pattern of the metal film (Au film with Cr as a base) previously formed in (f) as a mask. . The etching time is the etching time t calculated in step (A2). In this step, the outer shape of the crystal resonator is formed. (R) The metal film (Au, Cr film) is removed by etching, leaving the portion covered with the electrode pattern photoresist film previously formed in (o) and (p). (S) While rotating the wafer, Pd is deposited on Ti as a base. As a result, the front and back and side electrode films are formed. (T) The photoresist film for the electrode is peeled off, and the Ti and Pd films remain only in the portion that becomes the electrode by so-called lift-off. (U) Unnecessary Au and Cr films are removed by etching. At this time, in order to leave the Pd film, an etching solution is used in which iodine is dissolved in an etching solution, but iodine that does not dissolve Pd is dissolved in a potassium iodide solution. Thus, a crystal resonator having electrodes and a tuning fork-shaped outer shape is completed.

Next, the state of the vibrating leg of the tuning fork type crystal resonator manufactured by the above manufacturing method will be described with reference to FIG. FIG. 1A is a cross-sectional view showing a vibrating leg of a crystal wafer having a metal film 1, and the width W of the metal film 1 represents a design value of the vibrating leg having a frequency f1. 1B and 1C are cross-sectional views showing vibration legs of a quartz wafer having metal film widths W20 and W10, respectively, where metal film width W10 <metal film width W20. FIG. 1D is a cross-sectional view showing the vibrating leg after the crystal wafer of FIG. 1A is etched by the above manufacturing method, and FIG. 1E is the cross section of the crystal wafer of FIG. 1B by the above manufacturing method. It is sectional drawing which shows the vibration leg after etching.
For example, if an attempt is made to manufacture a vibrating leg with a frequency f1 in both FIGS. 1B, 1C, 1A, and 1B, conventionally, both quartz wafers are etched until the width of the vibrating leg becomes W. However, according to the manufacturing method of the present embodiment, the etching times calculated by substituting W20 and W10 into W0 and substituting f1 into f in the above equation (6) are different, and FIG. d) As shown in FIG. 1 (e), since the etching time takes into account the finally remaining convex portion 3, the portion corresponding to the metal film 1 having the width W20 has a width longer than the designed width W. The portion corresponding to the metal film 1 having a width of 10 becomes W20 ′, indicating that the width W10 ′ is shorter than the width W of the design value. Although the finally produced vibration legs have different widths, both of the frequencies can be close to the desired frequency f1.
FIG. 1 represents the concept of the present invention, and is merely an example. As described above, the desired frequency and the width of the metal film at a predetermined portion (the width of the metal film 1 corresponding to the vibration leg) are shown. ), An appropriate etching time can be derived, so that the manufacturing yield of the crystal resonator can be improved.

In the above-described embodiment, the example in which the steps (A1) and (A2) in FIG. 5 are added immediately after the step (g) has been shown. However, as is clear from the above description, the steps (A1) and (A2) ) May be performed anywhere after the step (f) in which the metal film pattern is formed and before the step (q) in which the crystal wafer is etched.

  FIG. 6 is a graph comparing the frequency of the lot average value of the frequency of the crystal resonator manufactured by the manufacturing method of the present invention with the frequency of the lot average value of the crystal resonator manufactured by the conventional method. According to the manufacturing method of the present invention, the frequency variation is smaller. In the conventional manufacturing method, the standard deviation of the frequency distribution was about 140 Hz, whereas in the manufacturing method according to the present invention, it was about 100 Hz, and a remarkable improvement of about 30% was observed.

(A) is sectional drawing showing the vibration leg of the crystal wafer which has the metal film 1, (b), (c) is sectional drawing showing the vibration leg of the crystal wafer which has the metal film width W20 and W10, respectively, (d) is FIG. 5B is a cross-sectional view showing the vibration leg after etching the crystal wafer of FIG. 5B, and FIG. 5E is a cross-sectional view showing the vibration leg after etching the crystal wafer of FIG. It is explanatory drawing concerning this invention, and is a figure which shows the relationship between the leg width of a crystal oscillator, and a frequency. It is explanatory drawing concerning this invention, and is a figure which shows the relationship between the reduction | decrease amount of the leg width by an etching, and a frequency. It is explanatory drawing concerning this invention, and is a figure which shows the relationship between etching time and the reduction | decrease amount of the leg width by etching. It is a flowchart which shows the manufacturing process in this invention. It is the figure which compared the frequency of the lot average value of the frequency of a crystal oscillator with the prior art and the method by this invention. It is a figure explaining the structure of a tuning fork type crystal oscillator. It is a flowchart which shows the manufacturing process of the crystal oscillator in a prior art. It is a figure which shows the relationship between the etching time, the leg width on a metal film pattern, and the difference of the leg width after an etching. It is a figure which shows the leg cross section of a crystal oscillator. It is sectional drawing which shows the relationship between the leg width on the metal film pattern before the etching of a crystal oscillator, the leg width after an etching, and a convex part.

Explanation of symbols

1 Metal film pattern 3 Convex part 5 Base part 7 Vibrating leg 9 Leg width 11 Leg length

Claims (2)

In a method of manufacturing a crystal unit having a vibrating leg that vibrates at a predetermined frequency,
A metal film forming step of forming a metal film having a shape corresponding to the outer shape of the crystal resonator on both surfaces of the crystal wafer;
A measuring step of measuring the dimension of the metal film at a location corresponding to the leg width of the vibrating leg;
The first correlation between the leg width and the frequency of the vibration leg obtained in advance based on the crystal resonator for the sample, the second correlation between the amount of decrease in leg width by etching and the frequency, the etching time and the leg width. Using the third correlation with the amount of decrease in the calculation step of obtaining the etching time from the dimension measured in the measurement step and the predetermined frequency ,
A method of manufacturing a crystal resonator, comprising: using the metal film as a mask; an etching time obtained in the calculation step; and an etching step of etching the crystal wafer. The method for manufacturing a crystal resonator according to claim 1, wherein a direction of a short side of the vibrating leg coincides with an X axis of the crystal wafer .

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