Method of manufacturing thin quartz crystal wafer

A method of manufacturing a thin quartz crystal wafer from a quartz crystal block which is cut from a crystal body of synthetic quartz crystal and has a flat principal surface, comprises the steps of (a) converging a laser beam at a region in said quartz crystal block at a predetermined depth from the principal surface thereof to cause multiphoton phenomenon state, thereby breaking Si—O—Si bonds of quartz crystal in said region to form voids in said region, and (b) peeling said thin quartz crystal wafer from a body of said quartz crystal block along said voids. The above process is repeatedly performed on one quartz crystal block to peel off a plurality of thin quartz crystal wafers successively from the principal surface of the quartz crystal block. Each of the thin quartz crystal wafers is divided into individual quartz crystal blanks for making quartz crystal units.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of manufacturing a thin quartz crystal wafer from a crystal body of synthetic quartz crystal, and more particularly to a method of manufacturing a thin quartz crystal wafer using a laser beam.

2. Description of the Related Art

Synthetic quartz crystal that is produced by growing quartz crystal according to hydrothermal synthesis or the like is known as a major material of electronic components typified by quartz crystal units. A quartz crystal unit comprising a quartz crystal blank cut from synthetic quartz crystal and hermetically sealed in a casing is used as a frequency control element in an oscillator or a filter. An AT-cut quartz crystal blank whose resonant frequency is inversely proportional to its thickness is widely used in such a crystal unit. A crystal blank is generally manufactured by cutting a thin quartz crystal wafer having a desired thickness. In recent years, as the communication frequency is as high as 100 MHz or higher, for example, a crystal blank used as a quartz unit has a thickness of about 18 μm or less. Efforts have been made to develop a process of manufacturing such a crystal blank.

FIGS. 1A to 1Cshow successive steps of a conventional process of manufacturing a thin quartz crystal wafer. Thin quartz crystal wafer1is cut from quartz crystal block2in the form of a rectangular parallelepiped having flat surfaces. As shown inFIGS. 1A to 1C, if an AT-cut crystal blank is to be finally cut out, then quartz crystal block2is cut from a crystal block of synthetic quartz crystal along predetermined orientations (X-, Y′-, and Z′-axes) of quartz crystal. The X-, Y′-, and Z′-axes refer to crystalline axes that are crystallographically determined for quartz crystal. Quartz crystal block2is cut by a wire saw or a blade saw along line A—A inFIG. 1Ato produce relatively thick quartz crystal wafer3having a thickness along the Y′-axis. The thickness of thick quartz crystal wafer3is of about 350 μm. Thereafter, thick quartz crystal wafer3is polished or ground into thin quartz crystal wafer1having a prescribed thickness. If a crystal blank for use in a 100 MHz crystal unit is to be produced from thin quartz crystal wafer1, thin quartz crystal wafer1has a thickness of about 18 μm. Then, thin quartz crystal wafer1is cut into individual crystal blanks along line B—B and line C—C inFIG. 1Cby photolithographic etching.

Finally, as shown inFIG. 2, exciting electrodes5and extension electrodes6are formed on respective principal surfaces of crystal blank4, extension electrodes6extending from respective exciting electrodes5to an end of crystal blank4and having portions folded back onto the other principal surfaces across the end of crystal blank4. Crystal blank4with exciting electrodes5and extension electrodes6mounted thereon is hermetically sealed in a casing, and predetermined electric connections are made to extension electrodes6, thus completing a crystal unit.

According to the above manufacturing process, however, thin quartz crystal wafer1is obtained from a thick quartz crystal wafer having a thickness of several hundreds μm by polishing or grinding in the unit of μm. Therefore, the manufacturing process produces material wastes and is low in productivity. Since a wafer cut by the machining process using a wire saw or a blade saw has a thickness ranging from 200 to 400 μm as a lower limit, it is necessary to polish or grind thick quartz crystal wafer3in order to produce thin quartz crystal wafer1therefrom.

A technique known as “stealth dicing” has been proposed for producing a thin silicon semiconductor wafer having a thickness of about 30 μm without polishing or grinding. This technique employs a laser beam having a wavelength that is transmissive with respect to a semiconductor wafer to be processed thereby. The laser beam is converged inside the semiconductor wafer to cause multiphoton absorption in the converged area, thereby forming an internally modified region from which the semiconductor wafer starts to be divided. Details of stealth dicing are disclosed in Takaoka Hidetsugu, “Principles and features of stealth dicing technique optimum for dicing ultrathin semiconductor wafers”, Electronic materials (Denshi Zairyou in Japanese) (ISSN 0387-0774), Vol. 41, No. 9, pp. 17–21, September 2002, and Japanese laid-open patent publication No. 2002-205181 (JP, P2002-205181A).

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method of manufacturing a thin quartz crystal wafer with increased productivity, with reduced quartz crystal wastes which is caused by polishing and grinding.

Another object of the present invention is to provide a method of manufacturing a crystal unit inexpensively using a method of manufacturing a thin quartz crystal wafer with increased productivity.

The objects of the present invention can be achieved by a method of manufacturing a thin quartz crystal wafer from a quartz crystal block which is cut from a crystal body of synthetic quartz crystal and has a flat principal surface, the method comprising the steps of (a) converging a laser beam at a region in the quartz crystal block at a predetermined depth from the principal surface thereof to cause multiphoton phenomenon state, thereby breaking Si—O—Si bonds of quartz crystal in the region to form voids in the region, and (b) peeling the thin quartz crystal wafer from a body of the quartz crystal block along the voids.

According to the present invention, the stealth dicing technique is applied to a quartz crystal block for manufacturing thin quartz crystal wafers with high productivity. The steps (a) and (b) may be repeatedly carried out on the quartz crystal block from which the thin quartz crystal wafer has been peeled, for thereby peeling a plurality of thin quartz crystal wafers successively from the principal surface of the quartz crystal block. According to this process, quartz crystal wastes may be reduced, and the productivity may further be increased. The principal surface of the quartz crystal block may be polished after the thin quartz crystal wafer has been peeled therefrom, and the step (a) may be carried out on the quartz crystal block. The laser beam may thus be well transmitted into the quartz crystal block, allowing the process of peeling off thin quartz crystal wafers successively from the quartz crystal block to be carried out better.

Each of the thin quartz crystal wafers thus obtained may be divided into individual crystal blanks for use in crystal units. Using such crystal blanks, crystal units can be produced inexpensively.

DETAILED DESCRIPTION OF THE INVENTION

A method of manufacturing a thin quartz crystal wafer according to a preferred embodiment of the present invention will be described below.

According to the embodiment, as shown inFIG. 3A, thin quartz crystal wafer1is cut from quartz crystal block2in the form of a rectangular parallelepiped having flat surfaces. AT-cut quartz crystal blanks are produced from thin quartz crystal wafer1. As shown inFIG. 3A, quartz crystal block2is cut from a crystal body (not shown) of synthetic quartz crystal along X-, Y′-, and Z′-axes of quartz crystal. Specifically, quartz crystal block2has six surfaces including a pair of XZ′ surfaces, a pair of XY′ surfaces, and a pair of Y′Z′ surfaces. If the XZ′ surfaces of quart crystal block2are regarded as principal surfaces, then these principal surfaces are first polished to a mirror finish.

Then, while quartz crystal block2is moving in the direction of the Z′-axis, one of the principal surfaces of quartz crystal block2is continuously irradiated with laser beam P applied in the direction of the Y′-axis. When one cycle of scanning quartz crystal block2with laser beam P in the direction of the Z′-axis is completed, quartz crystal block2is slightly moved in the direction of the X-axis. Then, while quartz crystal block2is moving in the direction of the —Z′-axis, quartz crystal block2is continuously irradiated with laser beam P. Line D—D inFIG. 3Brepresents the path of the beam spot of laser beam P on the irradiated principal surface of quartz crystal block2in one cycle of scanning quartz crystal block2with laser beam P in the direction of the Z′-axis. Laser beam P is focused or converged by a lens system (not shown) at a position within quartz crystal block2which is about 25 μm deep from the principal surface of quartz crystal block2. Laser beam P is of a wavelength that is transmissive with respect to quartz crystal and is capable of breaking Si—O—Si (silicon-oxygen-silicon) interatomic bonds in quartz crystal by way of multiphoton absorption.

As a result, multiphoton absorption occurs due to the convergence of laser beam P in a region within quartz crystal block2which is about 25 μm deep from the principal surface of quartz crystal block2, locally breaking Si—O—Si interatomic bonds of in quartz crystal. The broken interatomic bonds produce an optically damaged state, forming voids along the path of laser beam P in quartz crystal block2. Since quartz crystal block2is moving along the Z′-axis and the X-axis, a number of voids are clustered in quartz crystal block2along a plane that is about 25 μm deep from the principal surface of quartz crystal block2.

Thereafter, the principal surface of quartz crystal block2is applied to a fixture base such as a glass plate or the like by optical bonding or the like, and then heated to expand, activate, and explode the voids formed in quartz crystal block2. The clustered voids are joined together along the plane, fully destroying interatomic bonds between a main body of quartz crystal block2and a surface layer (which will become thin quartz crystal wafer1). As a result, the surface layer is peeled off the main body of quartz crystal block2, producing thin quartz crystal wafer1having a thickness of about 25 μm.

After thin quartz crystal wafer1has been obtained, the principal surface of quartz crystal block2from which thin quartz crystal wafer1has been peeled is polished. Then, while quartz crystal block2is being scanned in the directions of the Z′-axis and the X-axis, laser beam P is converged at a position that is about 25 μm deep from the principal surface of quartz crystal block2to cause multiphoton phenomenon state. Voids are now formed in quartz crystal block2by multiphoton absorption, and then quartz crystal block2is heated to peel off next thin quartz crystal wafer1. The above process is repeated to obtain a number of thin quartz crystal wafers1successively from quartz crystal block2.

Then, opposite principal surfaces of each of thin quartz crystal wafers1are polished until thin quartz crystal wafer1has a desired thickness. Thereafter, exciting electrodes5and extension electrodes6are integrally formed on both the principal surfaces of each of regions of thin quartz crystal wafers1which is to serve as a crystal blank. As shown inFIG. 4, on the end of the region which corresponds to each crystal blank and to which extension electrodes6extend, electrode layers serving as part of extension electrodes6are disposed on both principal surfaces. These electrode layers on both principal surfaces are electrically connected to each other via through-holes7defined in thin quartz crystal wafer1. Thereafter, thin quartz crystal wafers1is divided into individual crystal blanks by a machining process using a wire saw or a blade saw. In this manner, a number of crystal blanks4as shown inFIG. 4are obtained from each of thin quartz crystal wafers1.

According to the manufacturing method described above, since thin quartz crystal wafer1is produced by using stealth dicing technology and applying a laser beam to the principal surface of quartz crystal block2, thin quartz crystal wafer1can directly be obtained from quartz crystal block2, rather than from a thick quartz crystal wafer which would otherwise need to be produced from quartz crystal block2. Accordingly, the amount of quartz crystal that is wastefully ground off is highly reduced, and hence any quartz crystal wastes are minimized. For example, if a thin quartz crystal wafer having a thickness of 18 μm (corresponding to a resonant frequency of 100 MHz in case of an AT-cut crystal blank) is obtained from a conventional thick quartz crystal wafer having a thickness of 350 μm, then an amount of quartz crystal which corresponds to a thickness of 332 μm is wasted. According to the present embodiment, however, because a laser beam is converged at a depth of 25 μm from the principal surface of a quartz crystal block to peel a thin quartz crystal wafer from the quartz crystal block and the thin quartz crystal wafer is polished to a thickness of 18 μm, only an amount of quartz crystal which corresponds to a thickness of 7 μm is wasted. Consequently, the manufacturing method according to the present invention is 47 times more efficient than the conventional manufacturing process, and hence is highly productive.

According to the present embodiment, after one thin quartz crystal wafer1is peeled off quartz crystal block2, the principal surface of quartz crystal block2is polished again, and the laser beam is applied to quart crystal block2. Therefore, the laser beam can reliably be transmitted into quartz crystal block2, and thin quartz crystal wafers1can successively be obtained from quartz crystal block2. Each of thin quartz crystal wafers1is then divided into individual crystal blanks4. Consequently, crystal units can be produced inexpensively. Extension electrodes6are formed on both principal surfaces of regions of thin quartz crystal wafer1which correspond to respective crystal blanks, and are electrically connected to each other via through holes7. As a result, extension electrodes6can extend from one to the other of the principal surfaces of regions of thin quartz crystal wafer1before they are divided. According to the present embodiment, therefore, crystal units can be assembled immediately after thin quartz crystal wafer1is divided into crystal blanks.

The present invention is not limited to the preferred embodiment which has been described above, but various changes or modifications may be made therein.

For example, when laser beam P is applied to quartz crystal block2it may be intermittently applied not only in the direction of the X-axis, but also in the direction of the Z′-axis. In the above embodiment, after an optically damaged state is produced in quartz crystal block2by making the multiphoton phenomenon state, quartz crystal block2is heated to peel thin quartz crystal wafer1therefrom. However, rather than heating quartz crystal block2, quartz crystal block2may be immersed or dipped in an etching solution to chemically peel thin quartz crystal wafer1therefrom.

Furthermore, after each thin quartz crystal wafer is divided into individual crystal blanks, excitation electrodes and extension electrodes may be formed on each of the crystal blanks.