Abstract:
During the polishing of a semiconductor substrate, the semiconductor wafer that has been reduced in thickness, and hence in strength, by polishing, suffers outer-surface damage (or cracking) due to the initial damage caused by the use of polishing quartz. In order to solve these problems, the present invention applies a semiconductor substrate fixing jig formed with, on the face for fixing the semiconductor substrate, a groove(s) of almost the same diameter as that of the semiconductor substrate. Semiconductor substrate damage and cracking can be suppressed by applying this jig.

Description:
CLAIM OF PRIORITY 
     The present application claims priority from Japanese application serial no. 2004-137067, filed on May 6, 2004, the content of which is hereby incorporated by reference into this application. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a method of polishing semiconductor substrates in such a manner as to prevent the semiconductor substrates from cracking, and to a polishing jig to be used in the polishing method. 
     In the field of semiconductor manufacture, during manufacturing processes, thick substrates typically undergo processing in order to prevent semiconductor substrates from cracking. After the formation of patterns, the reverse side of each substrate is polished for the thickness of the substrate to match specifications. During this polishing process, semiconductor substrates are fixed to circular quartz plates using wax. Each of the quartz plates is further mounted in the polishing holder used for applying a load to the semiconductor substrate. An alkali polishing liquid containing a polishing agent is supplied to the quartz-made polishing surface of a polishing apparatus, and the semiconductor substrate that has been pressed firmly against the polishing surface is polished. 
     The above process is described below using  FIGS. 5A and 5B .  FIGS. 5A ,  5 B are views that explain semiconductor substrate polishing based on a related technology.  FIG. 5A  is a plan view from a polishing surface, showing a quartz disc  120  to which the patterned face of a semiconductor substrate  103  is fixed using a wax  104 , and a polishing holder  105  in which the quartz disc  120  is mounted.  FIG. 5B  is a sectional view that explains section IV-IV′ of the polishing holder assembly  100  and quartz surface plate  107  when viewed during polishing. In  FIG. 5A , notches  105   a  are provided in angle steps of 90 degrees in the polishing holder  105 . In  FIG. 5B , a polishing liquid  106  containing a polishing agent is supplied to the surface of the surface plate  107 . The surface plate  107  is rotating around its rotation axis not shown, and the polishing holder  105  itself is also revolving on its axis by means of a rotating mechanism not shown, so the semiconductor substrate  103  is polished while revolving on its axis. Such polishing is a combination of chemical polishing with a polishing liquid, and mechanical polishing with a polishing agent, and this polishing scheme is called CMP (Chemical-Mechanical Polishing). 
     Japanese Patent Laid-Open No. 2004-71667 describes solutions to the following problems associated with the related technology, in perspectives different from those of the present invention: 
     A space exists at section A in  FIG. 5B  mentioned above in “Background of the Invention”. During the initial phase of polishing, this space has the same height as the thickness of the substrate supplied to the wafer process. The polishing liquid  106 , when supplied to section A, repeatedly melts the wax  104  between the semiconductor substrate  103  and the quartz disc  120 , and as the wax  104  is consumed, it is thinned down and reduced in strength by polishing. As a result, the wax loses the fixing force and may permit the semiconductor substrate  103  to move slightly upward, thus damaging the outer surface of the substrate. The substrate is also prone to cracking, since it is in mechanical contact with the surface plate  107 . During the processes that follow the polishing process, such cracking causes further damage to the semiconductor substrate or results in semiconductor chip damage. 
     SUMMARY OF THE INVENTION 
     In order to solve the above problem, the present invention provides a jig for fixing a semiconductor substrate, with a circular groove slightly larger than a diameter of the semiconductor substrate. The semiconductor substrate is fixed to this groove by means of wax. For the grooveless jig used in the related technology, the wax between the semiconductor substrate and the jig begins to melt and flow out with the start of polishing. In contrast to the related technology, the present invention provides a circular groove slightly larger than the diameter of the semiconductor substrate, the side of the semiconductor substrate that is fitted in the groove, therefore, is covered with the wax used for fixing the semiconductor substrate and the jig, and thus prevents the wax from melting and flowing out. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the present invention will now be described in conjunction with the accompanying drawings, in which: 
         FIGS. 1A ,  1 B are views explaining an embodiment of a polishing jig according to the present invention; 
         FIGS. 2A ,  2 B are views explaining an embodiment of a polishing process according to the present invention; 
         FIGS. 3A ,  3 B are views explaining another embodiment of a polishing jig according to the present invention; 
         FIG. 4  is a schematic view of an optical transmission module embodying the present invention; and 
         FIGS. 5A ,  5 B are views explaining the polishing process according to the related technology. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the embodiments below, gallium arsenide, a compound semiconductor, is described as an example of a semiconductor substrate material. Compared with silicon, the compound semiconductors used for photosemiconductor devices, such as gallium arsenide (GaAs), indium phosphor (InP), and gallium nitride (GaN), have the properties of low hardness and brittleness. Also, the process of polishing to satisfy thickness specifications is performed nearly at the end of wafer processing, so the polishing process enhances an added value of the wafer. For an optical module that uses photosemiconductors, since the other components assembled during subsequent processes are high in price ratio, great damages result if cracks become conspicuous during subsequent processes. 
     Embodiments of the present invention are described hereunder with reference to the accompanying drawings. 
       FIGS. 1A and 1B  are views explaining an embodiment of a polishing jig according to the present invention.  FIGS. 2A and 2B  are views explaining an embodiment of a polishing process according to the present invention. A polishing jig formed of a quartz disc and having a groove with a depth of 100 micrometers (μm) and a diameter of 52.0 mm, on one face (face D) of the disc, is shown in  FIGS. 1A ,  1 B. In this case, face B for attaching an object to be polished, and face C for applying a load while in contact with a polishing holder require highly accurate processing since flatness levels of faces B and C affect in-plane thickness nonuniformity of the object to be polished. The groove depth of 100 μm is given as a reference for compliance with after-polishing thickness specifications of 100±10 μm of the object to be polished. Therefore, since parallelism between faces D and B directly affects uniformity of thickness of the object to be polished, initial accuracy of the quartz material itself is also required. 
       FIG. 2A  is a plan view from a polishing surface, showing a quartz jig  101  with face B to which a patterned face of a gallium arsenide wafer  103  with a thickness of 350 μm and a diameter of 50.8 mm is fixed using a wax  104 , and a polishing holder  105  in which the quartz jig  101  is mounted.  FIG. 2B  is a sectional view that explains section III-III′ of a polishing holder assembly  200  and quartz surface plate  107  when viewed during polishing. The polishing holder  105 , although shown as an integrated unit in  FIG. 5B , is split into a polishing ring section  105   b  and a loading section  105   c , in the present embodiment. In  FIG. 2B , a polishing liquid  106  containing a polishing agent is supplied to the surface of the surface plate  107 . The surface plate  107  is rotating around its rotation axis not shown, and the polishing holder  105  itself is also revolving on its axis by means of a rotating mechanism, so a reverse face (nonpatterned face) of the gallium arsenide wafer  103  is polished when the wafer is revolving on its axis. Also, a clearance between the quartz jig  101  and the loading section  105   c  is fixed by a suction pressure applied from a vacuum source not shown. The loading section  105   c  has a mass of 10 to 15 kg. 
     In the present embodiment, the diameter of the gallium arsenide wafer  103  is 50.8 mm and the diameter of the groove in the quartz jig is 52.0 mm, so that there is only a difference of 0.6 mm between both dimensions at one side. The wax  104  is liquefied by heat and then uniformly applied to a groove interior of the quartz jig so as not to generate bubbles. Next after the gallium arsenide wafer vacuum-chucked by vacuum tweezers has been mounted, the wafer is fixed by pressurizing and cooling the wax. An excess of the wax fills in an entire space equivalent to the differential diameter of 0.6 mm at one side. This prevents the the problem encountered in the related technology, namely, the melting and outflow of the wax between the semiconductor substrate and the quartz jig. Hence, it is possible to suppress the damage to the semiconductor substrate being polished, and its cracking likely to occur during polishing and to cause damage during subsequent processes. 
     The damaging and cracking are also suppressed since sidewalls of the groove work as walls in such a manner that they will prevent cracking of the semiconductor substrate thinned down by polishing. 
     Additionally, in the present embodiment, the polishing liquid selected polishes only gallium arsenide and does not polish quartz. The thickness of the semiconductor substrate can therefore be easily controlled by matching the groove depth of the quartz jig to thickness specifications of the substrate after being polished. More specifically, whether the semiconductor substrate has been polished to completion can be judged by confirming that the difference in diameter (in other words, a difference in level) between the gallium arsenide substrate and the polishing jig has disappeared. In the above-described embodiment, a thickness of the wax is ignored for simplicity of description. In actuality, however, the thickness cannot be ignored and the depth of the groove needs to equal the thickness specifications of the substrate plus the thickness of the wax. 
     Furthermore, since the quartz jig for fixing is formed with accurate flatness, it is possible to obtain semiconductor substrates substantially free from in-plane thickness nonuniformity and required to have highly accurate flatness. Substrate thickness specifications are determined by particular characteristics of optical elements and a layout design for element mounting in subsequent processes. 
     Experiments indicate that even if difference in the groove diameter of the quartz jig is about 5 mm for a maximum diameter tolerance of the semiconductor substrate, it is possible to fill in the groove section with wax (for a groove depth of 100 μm). Preferable difference, however, is 2 mm or less. 
     The wax here does not refer only to beeswax, and the wax can be any kind of wax, only if it is solid at room temperature and can be changed into a liquid of a low viscosity by applying heat. 
     Although a gallium arsenide wafer is exemplified as the semiconductor substrate in the above embodiment, the substrate may be any other different type of compound semiconductor substrate or may be a silicon wafer. Although a surface plate made of quartz is exemplified as the surface plate, this may be a polishing cloth. Although a quartz jig is exemplified as the jig for attaching the semiconductor substrate, the kind of material is of no matter, only if the material is corrosion-resistant against the polishing liquid used (i.e., only if the material is resistant to corrosion/polishing). For example, the material may be glass or a ceramic material. 
       FIGS. 3A and 3B  are views explaining yet another embodiment of a polishing jig according to the present invention.  FIG. 3A  shows a polishing jig having four grooves each with a depth of 100 μm and a diameter of 26.6 mm, on one face of a quartz disc. Four semiconductor substrates each with a diameter of 25.4 mm can be polished at a time using the quartz jig  102  in  FIGS. 3A ,  3 B. Furthermore, it is possible to suppress the damage to the semiconductor substrate being polished, and its cracking likely to occur during polishing and to cause damage during subsequent processes. 
       FIG. 4  is a schematic view of an optical module  300  on which is mounted a semiconductor device that applies the present invention. In  FIG. 4 , a gallium arsenide wafer with its reverse side polished using a manufacturing method according to the present invention is chipped into a laser diode  301  by undergoing reverse-side metalizing, cleaving, and/or the like. The laser diode  301  is then connected to a stem  303  via solder  304 . A light-emitting position of the laser diode is present on the side of its patterned face, and light is conducted into optical fibers  302  through a lens not shown. A central position error of the optical fibers  302  needs to stay within ±3 μm of an oscillating position thereof, and a thickness tolerance of the laser diode  301  is set to ±10 μm to satisfy restrictions on a thickness of the solder (not shown) for fixing the optical fibers  302 . 
     According to the present invention, since the wax for fixing the semiconductor substrate and the jig can be prevented from melting and flowing out, the cracks in the semiconductor substrate can also be prevented without damaging its outer surface.