Patent Application
20140290019
This application claims the priority benefits of Japanese application serial no. 2013-069428, filed on Mar. 28, 2013. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification.
This disclosure relates to a method for fabricating a vibrator that includes a vibrating plate and an electrode disposed around of this vibrating plate.
As an electrostatic drive type vibrator, there is known, for example, a disc resonator that has a disc (circular plate)-shaped vibrating plate and an electrode for vibrating this vibrating plate, disposed on a base substrate. To fabricate this vibrator, the so-called Micro Electro Mechanical System (MEMS) method is used, for example. This is a method for dry etching a polysilicon (Si) film and a silicon oxide (SiO2) film while alternately laminating them. The polysilicon (Si) film constitutes a vibrating plate and an electrode, and the silicon oxide (SiO2) film forms a sacrifice film. Wet etching the sacrifice film with the use of Hydrogen fluoride (HF) water solution or similar after the film formation process and dry etching process have been completed separates the vibrating plate and the electrode from each other as well as supports the vibrating plate in a state where the vibrating plate floats above the base substrate. In order to configure such a vibrator to run at a voltage as low as possible, that is, to facilitate an electrostatic bond between the electrode and the vibrating plate, it is preferred to bring the electrode and the vibrating plate as close to each other as possible.
The polysilicon that constitutes the above-described vibrating plate has an approximately 20 to 40 ppm/° C. frequency variation against a temperature change. Constituting the vibrating plate with polysilicon will resultantly produce more variable frequency characteristics depending on the temperature of an atmosphere where the vibrator is placed. For the above reason, combining silicon oxide with a vibrating plate as an auxiliary film for reducing the frequency variation in constituting the vibrating plate is being examined. Silicon oxide has temperature characteristics that cancel the temperature characteristics of polysilicon. Specifically, the frequency of silicon oxide decreases as the frequency of polysilicon increases in response to a temperature change, while the frequency of silicon oxide increases as the frequency of polysilicon decreases.
However, forming an auxiliary film made up of silicon oxide on a surface of the vibrating plate after conducting, for example, the above-described wet etching process will make it harder to constitute a low voltage drive type vibrator because the electrode and the vibrating plate need to be separated from each other in advance for the auxiliary film. On the other hand, forming the auxiliary film before forming and dry etching a polysilicon film and a sacrifice film are complete will remove the whole or a part of the auxiliary film in a subsequent wet etching process. For a vibrating plate that vibrates in a lateral direction, a varied film thickness dimension of the auxiliary film in a height direction may possibly result in a non-uniform vibration inside the vibrating plate. On the other hand, using a material such as Germanium (Ge), which is insoluble to hydrogen fluoride and removable with a flux that does not dissolve polysilicon, as the above-described sacrifice film would increase the material cost.
Japanese Unexamined Patent Application Publication No. 2010-35144 (paragraph 0070) discloses a technique for forming a silicon oxide film at a vibrating portion. Japanese Unexamined Patent Application Publication No. 2009-160728 (paragraph 0034) discloses a technique for filling a groove 3 with a fixing material such as SiN or SiO2 in forming a machine component in the MEMS method. Japanese Unexamined Patent Application Publication No. 7-122791 discloses a technique for etching SrTiO3 in isopropyl alcohol-diluted HF solution. Japanese Unexamined Patent Application Publications No. 2002-353443 (paragraphs [0006] and [0007]) and 2011-228338 (paragraphs 0076 and 0077) disclose that an etching rate is different between an NSG film and a BPSG film when such solution is used. However, the above-described problems are not examined in these patent documents.
A need thus exists for a method for fabricating a vibrator which is not susceptible to the drawback mentioned above.
A method for fabricating a vibrator with a vibrating plate and an electrode for vibrating the vibrating plate. The method includes: laminating a first sacrifice film including silicon and oxygen, a conductive film including silicon, and a second sacrifice film including silicon and oxygen on a base substrate from a lower side in this order; forming a through hole that passes through the conductive film and the second sacrifice film and reaches at least a middle portion in a film thickness direction of the first sacrifice film; filling in an auxiliary film for adjusting temperature characteristics including silicon and oxygen from a lower end position to an upper end position of the through hole; patterning a resist on the conductive film and the second sacrifice film so as to have a shape of the vibrating plate in plan view; subsequently laminating a third sacrifice film including silicon and oxygen and a conductive film for forming an electrode on an upper layer side of the second sacrifice film from the lower side in this order, and patterning a resist on the conductive film for forming an electrode so as to form an electrode in a region outside a vibrating plate; and subsequently etching an upper end portion of the auxiliary film, a lower end portion of the auxiliary film, the first sacrifice film, the second sacrifice film, and the third sacrifice film, while leaving the auxiliary film inside the through hole on the vibrating plate, with a use of an etching fluid including hydrogen fluoride.
The foregoing and additional features and characteristics of this disclosure-will become more apparent from the following detailed description considered with reference to the accompanying drawings, wherein:
A description will be given for an exemplary embodiment of a method for fabricating a vibrator according to the disclosure with reference to
On an outer periphery side of the vibrating plate 10, supporting portions 30 for supporting the vibrating plate 10 are disposed. The supporting portions 30 support the vibrating plate 10 in a state where the vibrating plate 10 floats above the base substrate 1 (specifically, a first polysilicon film 41, which will be described below). These supporting portions 30 are each disposed between the electrodes 20 and 20 adjacent to each other. As illustrated in
Between: the vibrating plate 10, the supporting portions 30, and the electrodes 20; and the silicon nitride film 4, a conductive film 5 is formed. The conductive film 5 is patterned to correspond to shapes of the vibrating plate 10, the electrodes 20, and the supporting portions 30. Each of the supporting portions 30 is supported by the conductive film 5. Each of the electrodes 20 is also supported by the conductive film 5. As illustrated in
A beam portion 10a is disposed on an upward side of the vibrating plate 10. The beam portion 10a connects two electrodes 20 and 20, which face each other, of the four electrodes 20.
The vibrating plate 10 is formed to be disc-shaped, and is constituted by a conductive film that is made up of polysilicon (specifically, a polysilicon compound doped with impurities such as phosphorus). As illustrated in
In this example, each of the auxiliary films 12 is disposed to avoid a region between a center point O of the vibrating plate 10 and each of the supporting portions 30, that is, so as to face each of the electrodes 20. The center point O serves as a center in plan view. As illustrated in
Here a height position of a top surface of the vibrating plate 10 and a height position of a top surface of each of the auxiliary films 12 are aligned with each other. A height position of a bottom surface of the vibrating plate 10 and a height position of a bottom surface of each of the auxiliary films 12 are also aligned with each other. The “Being aligned” here refers specifically to the following range; the height levels of the top surface and the bottom surface of the auxiliary film 12 when, for example, viewed from a side of the base substrate 1, are respectively denoted by “L1” and “L2”, as illustrated in
With regard to a difference between the height levels L1 and L3, or Δt1 (L1−L3), the “being aligned” means that Δt1 is equal to or less than one third of a thickness dimension of the vibrating plate 10. Similarly with regard to a difference between the height levels L2 and L4, or Δt2 (L4−L2), “being aligned” means that Δt2 is equal to or less than one third of the thickness dimension of the vibrating plate 10. Note that the above-described
A method of forming an auxiliary film 12 with such a shape will be described below with reference to
Subsequently, the first polysilicon film 41, which is made up of polycrystalline silicon, is formed on an upper layer side of the silicon nitride film 4 and phosphorus is diffused into the first polysilicon film 41. Then, a resist pattern is formed on the first polysilicon film 41 by the photolithography method so as to correspond to shapes of the vibrating plate 10, the supporting portion 30, the electrode 20, and the extraction electrode 6, as illustrated in
Subsequently, a first sacrifice film 61, a second polysilicon film 42 made up of polycrystalline silicon (specifically, a conductive film doped with phosphorus after forming a polycrystalline silicon film), and a second sacrifice film 62 on the upper layer side of the first polysilicon film 41 are laminated from the lower side in this order using, for example, the CVD method. The sacrifice films 61 and 62 are silicon oxide films containing impurities such as phosphorus or boron, in addition to silicon and oxygen. This means that these sacrifice films 61 and 62 are thin films doped with the impurities at the time of formation of a silicon oxide film in the CVD method. The film thickness dimension of each of the sacrifice film 61 and 62 is, for example, 1 μm. The sacrifice films 61 and 62 may be subject to an annealing process after being doped with impurities.
Subsequently, a photoresist mask 71 is formed on an upper layer side of the second sacrifice film 62, and a resist pattern corresponding to the through hole 11 is formed on the photoresist mask 71, as illustrated in
After the photoresist mask 71 is removed, a thin film 13, which is made up of an silicon oxide film, is formed on a surface of the second sacrifice film 62, which includes an inside of the through hole 11, by the CVD method. Specifically, an organic processing gas, which contains silicon, and an oxidizing gas for oxidizing the processing gas are supplied onto the surface of the second sacrifice film 62. The processing gas and the oxidizing gas come into contact with exposed surfaces of the second sacrifice film 62 and similar, whereby, as illustrated in
Subsequently, as illustrated in
After the above-described etching process, the film thickness dimension d2 of the second sacrifice film 62 will be, for example, 0 μm to 1.0 μm. In other words, the auxiliary film 12 will protrude from the top surface of the second polysilicon film 42 by the film thickness dimensions d2. Then, as illustrated in
Subsequently, a third sacrifice film 63 is formed so as to cover the structure formed in the above processes. The third sacrifice film 63 is made up of silicon oxide and the structure is made up of the second sacrifice film 62 and the second polysilicon film 42. The third sacrifice film 63 is formed so as to have the same film thickness dimension as a clearance dimension between the vibrating plate 10 and the electrode 20. The third sacrifice film 63 may be a silicon oxide film containing impurities but does not need to contain impurities because this film has a small film thickness dimension. In a wet etching process conducted after a vibrator is formed, the sacrifice films 61 to 63 are made to be easier to be etched compared with the thin film 13, as described below. Specifically, the sacrifice films 61 and 62 are doped with impurities. In contrast, the third sacrifice film 63 is etched quickly even if it is not doped with impurities and is etched even more quickly if doped with impurities, because the film has a small film thickness dimension, as described above. Then, a resist pattern is formed on the sacrifice films 61 and 63 such that the conductive film 5 (the first polysilicon film 41) on a lower side of the opening 32 and on a lower side of the electrode 20 will be exposed.
As illustrated in
Then, a resist film (not shown) having an opening in a region other than the regions for the electrode 20, the support pillar 33, and the beam portion 10a is formed, the third polysilicon film 43 is dry etched as illustrated in
The sacrifice films 61 and 62 are silicon oxide films (phosphorus-doped silicate glass: PSG) containing impurities such as phosphorus, as described above, and easily dissolve in the etchant, as illustrated in later-described
Therefore, the sacrifice films 62 and 63 are etched more quickly than the auxiliary film 12 in an upper region of the vibrating plate 10, for example, when the structure is dipped in an etchant, as illustrated in
In other words, in this disclosure, the period required to wet etch the sacrifice films 61 to 63 is obtained in advance through, for example, an experiment, and the dimensions d1 and d2 are each set such that the upper and lower ends of the auxiliary films 12 will be removed during this etching period. As a result, the above wet etching process removes the sacrifice films 61 to 63 and forms a vibrator, as illustrated in the above-described
An example of specific dimensions d1 and d2 will be described below. Assume that a vibrating plate 10 with a radius dimension of 30 μm is to be formed, and compounds whose etching rates in a hydrogen fluoride water solution have the values listed in the following table will be used as the respective sacrifice films 61 to 63. The following table also illustrates the etching rate of the compound used as the auxiliary film 12. Note that these etching rates are merely exemplary and vary depending on, for example, the amount of impurities or the state of an annealing process (heat treatment) and the concentration or composition of the hydrogen fluoride water solution to be used.
This example shows that removing (wet etching) the first sacrifice film 61 having the above-described film thickness dimension (1 μm) takes 12.5 minutes. Thirteen minutes, including a margin, will be accordingly assigned to remove each of the sacrifice film 61 to 63. From the fact that the auxiliary film 12 is etched 780 nm in 13 minutes, it can be seen that the above-described dimensions d1 and d2 may well be both set to 780 nm.
Going through the above-described film formation process and etching (dry etching and wet etching) process spaces the vibrating plate 10 and the electrode 20 apart from each other by the film thickness dimension of the third sacrifice film 63 (for example, 0.01 to 1.0 μm, 0.1 μm in this example). The vibrating plate 10 is supported via the supporting portion 30 in a state where the vibrating plate 10 floats above the first polysilicon film 41. The beam portion 10a is put into a state where the beam portion 10a floats above the vibrating plate 10 by the film thicknesses of the second sacrifice film 62 and the third sacrifice film 63. Then, performing a sealing process and cutting the base substrate 1 along a dicing line that partitions the vibrator separates each vibrator into individual pieces.
For this vibrator, an oscillation frequency of the vibrating plate 10 hardly vary even if the temperature of the atmosphere where the vibrator is placed changes while an output signal is being output. In other words, as already described in the background paragraph, silicon and silicon oxide have a frequency versus temperature characteristic that cancel each other. The auxiliary film 12, which is made up of silicon oxide, is filled in the vibrating plate 10, which is made up of silicon, as described earlier. As a result, the oscillation frequency of the auxiliary film 12 will decrease even if the oscillation frequency of the vibrating plate 10 attempts to increase according to a change in temperature outside the vibrator. On the other hand, the oscillation frequency of the auxiliary film 12 will increase even if the oscillation frequency of the vibrating plate 10 attempts to decrease. The constitution made up of the vibrating plate 10 and the auxiliary film 12 hence provides an almost constant value of oscillation frequency regardless of a change in outside temperature.
According to the aforementioned embodiment, the vibrating plate 10 is formed from silicon (polycrystalline silicon) with the use of the sacrifice films 61 to 63 as a silicon oxide film. At the time of formation of the vibrating plate 10, the through hole 11 is provided in the vibrating plate 10, and the auxiliary film 12, which is made up of silicon and oxygen, is filled in the through hole 11. The auxiliary films 12 are each made to protrude upward and downward in advance from the top and bottom surfaces of the vibrating plate 10, respectively, assuming that a thickness of the auxiliary film 12 will be reduced when the sacrifice films 61 to 63 are wet etched. Hence the top and bottom surfaces of the vibrating plate 10 and the top and bottom surfaces of the auxiliary film 12 can be aligned with one another when the sacrifice films 61 to 63 have been etched. In other words, unless the auxiliary film 12 is made to protrude from the vibrating plate 10 upward and downward, the auxiliary film 12 will be immersed from both of the upper and lower sides in the subsequent wet etching process. As a result, the auxiliary film 12 will not remain in the through hole 11 or leave the effect of correcting the temperature characteristics insufficient even if the auxiliary film 12 remains.
On the other hand, having the auxiliary film 12 to protrude from the vibrating plate 10 upward and downward can leave the auxiliary film 12 in the through hole 11 even after the wet etching process. This makes it possible to form the auxiliary film 12 in the course of the process of fabricating the vibrator, and thus eliminating the need for interposing the auxiliary film 12 between the vibrating plate 10 and the electrode 20. As a result, a vibrator having a satisfactory frequency versus temperature characteristic can be obtained by combining the auxiliary film 12, which is made up of silicon oxide film, with the vibrating plate 10, which is made up of silicon, while bringing the vibrating plate 10 and the electrode 20 closer to each other to constitute a low voltage drive type vibrator.
The sacrifice films 61 and 62 are doped with impurities, and the auxiliary film 12 is constituted as a silicon oxide film without being doped with impurities. As can be seen from the above-described
As illustrated in
Now, a description will be given for the reason why the width dimension t of the auxiliary film 12 has been set within the above-described range. The auxiliary film 12 needs to have a smaller solubility in hydrogen fluoride than the sacrifice films 61 and 62. Hence, at the time of formation of the auxiliary film 12, it is preferred that the auxiliary film 12 not only be free of impurities such as phosphorus but also be formed into a fine film.
As described in the above-described
Other examples of the vibrator described above will be listed below.
For such a vibrator, supplying an input signal to the electrodes 20 and 20 causes each vibrating plate 10 to generate flexure vibrations. Specifically, the vibrating plates 10 and 10 generate vibrations between a position where the vibrating plates 10 and 10 come close to each other and a position where the vibrating plates 10 and 10 separate away from each other. In
For a vibrator including the vibrating plate 10 having a small width dimension like this, the sacrifice films 61 to 63 each have a relatively small area in plan view. Consequently, it takes only a short time for a wet etching process. If hydrogen fluoride diluted with water is used, the film amount to be reduced (dimensions d1 and d2) of the auxiliary film 12 will be equal to or less than 80 nm.
Now a preferred range of the above-described differences Δt1 and Δt2 after a vibrator is fabricated (wet etched) will be described. Specifically, if the differences Δt1 and Δt2 both have a negative value, that is, an upper end portion or a lower end portion of an auxiliary film 12 intrudes into the vibrating plate 10, as can be seen from the above-described description, the auxiliary film 12 provides lower capability of adjusting temperature characteristics and could also possibly provide unstable vertical vibration. Hence, the differences Δt1 and Δt2 should preferably have zero or a positive value. In contrast, if the auxiliary film 12 protrudes above and below excessively from the vibrating plate 10 after the vibrator is fabricated, that is, the differences Δt1 and Δt2 both have a large positive value, vibrations of the vibrating plate 10 may be possibly hindered. Summarizing the above, the differences Δt1 and Δt2 should preferably fall within the following range: 0 μm≦Δt1(Δt2)≦(one-third of the thickness dimension of the vibrating plate 10).
As can be also seen from the above-described
In the above-described
Again in
In the method for fabricating a vibrator, a height position of a bottom surface of the through hole may be set at a position downward away from a bottom surface of the vibrating plate by a film thickness dimension to be etched at a lower end portion of the auxiliary film in the etching such that a bottom surface of the vibrating plate and a lower end position of the auxiliary film inside the through hole are aligned with each other after the etching process. In the method for fabricating a vibrator, the second film thickness dimension may be set to have a same dimension as a film thickness dimension to be etched at an upper end portion of the auxiliary film in the etching such that a top surface of the vibrating plate and an upper end position of the auxiliary film inside the through hole are aligned with each other after the etching process.
In the method for fabricating a vibrator, the filling-in may form the auxiliary film inside the through hole and on a surface of the second sacrifice film by supplying a processing gas including silicon and oxidizing gas for oxidizing the processing gas onto a superficial layer side of the second sacrifice film. The method may further include etching an excess of the auxiliary film formed on a surface of the second sacrifice film and an upper layer portion of the second sacrifice film after the filling-in so as to adjust a height dimension of the auxiliary film protruding above an upper end position of the vibrating plate. In the method for fabricating a vibrator, the adjusting the height dimension protrudes the upper end portion of the auxiliary film above the vibrating plate by a film thickness dimension to be etched at the upper end portion of the auxiliary film in the etching process such that the top surface of the vibrating plate and the upper end position of the auxiliary film inside the through hole are aligned with each other after the etching process.
In the method for fabricating a vibrator, the first the sacrifice film, the second sacrifice film, and the third sacrifice film may each include at least one of boron, phosphorus, sodium oxide, potassium oxide and calcium oxide in addition to silicon and oxygen, and the auxiliary film may be a silicon oxide film made up of silicon and oxygen. In the method for fabricating a vibrator, at least one of a composition of the etching fluid and a composition of the first sacrifice film may be set such that an etching speed of the first sacrifice film at a time of usage of the etching fluid becomes 50 times or more of an etching speed of the auxiliary film.
The disclosure forms a vibrating plate, which is made up of a conductive film including silicon, and an electrode for vibrating this vibrating plate using a sacrifice film including silicon and oxygen. In this process, a through hole that passes through in the vertical direction is formed, and an auxiliary film constituted of silicon and oxygen is filled in the through hole. The auxiliary film is made to protrude upward and downward in advance from the top and bottom surfaces of the vibrating plate, assuming that a thickness of the auxiliary film will be reduced when the sacrifice films are etched. This can leave the auxiliary film inside the through hole even after the etching process of the sacrifice films has been completed. This makes it possible to form the auxiliary film in the course of the process of fabricating the vibrator, thus eliminating the need for interposing the auxiliary film between the vibrating plate and the electrode. As a result, a vibrator having a satisfactory frequency versus temperature characteristic can be obtained by combining the auxiliary film, which is made up of silicon oxide film, with the vibrating plate, which is made up of silicon, while bringing the vibrating plate and the electrode close to each other.
The principles, preferred embodiment and mode of operation of the present invention have been described in the foregoing specification. However, the invention which is intended to be protected is not to be construed as limited to the particular embodiments disclosed. Further, the embodiments described herein are to be regarded as illustrative rather than restrictive. Variations and changes may be made by others, and equivalents employed, without departing from the spirit of the present invention. Accordingly, it is expressly intended that all such variations, changes and equivalents which fall within the spirit and scope of the present invention as defined in the claims, be embraced thereby.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2013-069428 | Mar 2013 | JP | national |
