Abstract:
A method and apparatus ( 1 ) for machining a semiconductor or ceramic workpiece ( 32 ) causes a portion of the workpiece to undergo high pressure phase transformation to form a high pressure phase transformation portion which has altered optical properties from the remainder of the workpiece. A laser is irradiated on the high pressure transformation portion to heat the high pressure transformation portion and cause it to soften and then the workpiece is plastically deformed, at smaller loads and forces, along the softened portion in order to accomplish the machining of the workpiece with a machining tool ( 21 ).

Description:
FIELD OF THE INVENTION 
     The present invention relates to a process for machining or deforming a workpiece in which a laser is used to soften the workpiece while the workpiece material is undergoing a high pressure phase transformation and the high pressure phase transformed portion of the workpiece then is physically softened and deformed. 
     The present invention also is directed to an apparatus for performing high pressure phase transformation machining of a workpiece which comprises a laser source for generating laser radiation, a machining tool holder, a machining tool capable of transmitting the laser radiation, a force generating means for bringing the machining tool into contact with the workpiece at a pressure of at least the hardness of the workpiece and means for effecting relative movement between the workpiece and the machining tool. 
     DESCRIPTION OF THE PRIOR ART 
     Laser-assisted machining of materials which are difficult to work with have recently become very popular. These conventional methods involve the application of heat to a workpiece to cause a softening of the workpiece material and the facilitation of the machining of the workpiece. With these conventional methods, it is necessary that only the portion of the material which is to be removed by the cutting tool be subjected to the heat so that permanent damage does not occur to the remaining material of the workpiece. Plasma tubes have been used to effect the heating of the workpiece material but this type of apparatus is so bulky it does not really permit the localized heating of smaller areas of the material of the workpiece in close proximity to the cutting tool. 
     Conventional laser-assisted machining also requires that the laser and the machining tool be decoupled, that is, that the laser radiation that heats and softens the workpiece be in advance of the machining tool. This makes the apparatus needed to perform conventional laser-assisted machining very complicated, expensive, and macroscopic in size. 
     U.S. Pat. No. 4,229,640 discloses a process for machining a workpiece through the use of a cutting tool having a cutting edge and an adjacent leading face which lifts shavings from the workpiece material in which localized heating of the workpiece material about to be removed is accomplished by a laser beam arranged to heat two distinct local areas of the workpiece adjacent to the cutting tool. However, this process and apparatus requires a great deal of energy since macroscopic deformation is achieved at very high temperatures of at least greater than the material&#39;s glass transition temperature or above the binder/sintering material&#39;s thermal softening point for polycrystalline materials and, as a result, a large laser source and considerable amount of energy (kW) are required. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method and apparatus for machining a workpiece which performs microscopic deformation of the workpiece by forming high pressure phase transformation of a portion of the workpiece while exposing this portion of the workpiece to irradiation by a laser. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an apparatus for performing micro-laser assisted high pressure phase transformation machining of a workpiece according to the present invention. 
         FIG. 2  illustrates a machining tool having a reflective coating provided thereon to focus the laser radiation. 
         FIG. 3  illustrates a machining tool which utilizes a lens to focus the laser radiation. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Under normal conditions, semiconductor and ceramic materials are hard and brittle and do not readily plastically deform. Macroscopic deformation of these materials can only be achieved at very high temperatures. Conventional laser-assisted machining allows macroscopic deformations by heating the workpiece to a temperature greater than the glass transition temperature of the material of the work, or above the thermal softening temperature of the binder (sintering agent) material. At size scales of micrometers to millimeters, brittle fracture of semiconductor and ceramic materials tends to dominate over a ductile or plastic response because it requires less energy to generate and propagate a crack than what it requires to plastically deform a volume of material. Ductile or plastic response of these nominally brittle materials is confined to small size scales at low temperatures due to the formation of a high pressure transformed phase. The present invention takes advantage of the small size scale ductile plastic response by using a laser to heat preferentially the high pressure phase transformed material of the workpiece. 
     The drawing shows an apparatus  1  of the present invention for performing micro laser assisted high pressure phase transformation machining of a workpiece. This apparatus comprises a source  2  for generating a laser radiation, a machining tool holder  11 , such as a ferrule, a machining tool  21  and a workpiece holder  31 . The workpiece  32  of the present invention is made of a material which is transmissible to the wavelength of the laser radiation used in the present invention but forms an opaque or reflective material phase during high pressure phase transformation. That is, in its normal state, the workpiece  32  transmits the laser radiation therethrough but either absorbs or reflects the laser radiation during high pressure phase transformation. The workpiece  32  is preferably made of a semiconductor material, such as silicon or germanium, or a ceramic material, such as silicon nitride or silicon carbon. The material of the workpiece  32  is not critical as long as it is transparent to the wavelength of the laser light used in its normal state and is capable of undergoing high pressure phase transformation to form an opaque or reflective phase. 
     The source  2  for generating laser light can be any conventional laser radiation generating device that is capable of generating laser radiation having a wavelength of from 1 to 5 microns. Particularly preferred is a laser radiation generating device that is capable of generating infrared laser radiation such as an IR diode. 
     The present invention is based on the discovery that under extremely high pressures, such as occur at the contact interface between the machining tool  21  and the workpiece  32 , semiconductor and ceramic materials transform from covalent and/or ionic bonded structures to high pressure phase transformed metallic structures. It was discovered that these high pressure phase transformed materials had a different crystal structure, such as a β-tin structure, as compared to the structure of the material under normal or low pressure conditions. The pressure needed to form the high pressure phase transformed material was found to be at least equal to the material&#39;s hardness. Since the high pressure transformed phase of the workpiece  32  reflects or absorbs the laser radiation while the remainder of the workpiece  32  transmits the laser radiation, the high pressure transformed phase selectively heats and softens while the remainder of the substrate is unaffected. The power range for the source for generating laser radiation is 8 mW-1 kW, with 8 mW-100 W being preferred. 
     As the machining tool  21 , any material which can transmit the desired wavelength of the laser light, is harder than and inert to the substrate and that can be shaped to the desired form can be used. Especially preferred as a material of the machining tool  21  in the present invention is diamond. Since pressure is defined a force per unit area, it is desirable that the tip radius of the machining tool  21  be as small as possible, with a tip radius of from 20 nanometers to 12 microns being preferred. If the tip radius of the machining tool  21  is small enough, the translation of the machining tool  21  along the surface of the workpiece  32  can be sufficient to form the high pressure transformation phase on the workpiece  32 . Alternatively, external weights, forces or loads  12 , can be applied to the machining tool holder to ensure that the tip of the machining tool  21  contacts the workpiece  32  surface at a sufficient pressure, to cause or generate the high pressure phase transformation, and this pressure can be monitored by a load sensor. 
     In order to help focus the laser radiation, indicated by. the arrows in  FIGS. 2 and 3 , through the machining tool  21 , as shown in  FIG. 3 , a focusing lens  22  can be provided between the source  2  for generating laser radiation and the machining tool  21  or, as shown in  FIG. 2 , the machining tool can be coated with a reflective metal  23 , such as gold or aluminum. 
     An adhesive can be used to join the machining tool  21  to the machining tool holder  11  as long as the adhesive is able to adhere to the machining tool  21  and to the machining tool holder  11  and is transparent to the wavelength of the laser light used. The adhesive can be provided around the periphery of the machining tool  11  or at the back end thereof to secure it to the to the machining tool holder  11 . An ultraviolet-curable epoxy is preferably used as the adhesive. 
     If desired, a fiber optic cable  13  can be used for increased flexibility in transmitting the laser radiation from the source  2  to the machining tool  21 . 
     Due to the sharpness of the tip of the machining tool  21 , by bringing the machining tool  21  into contact with the surface of the workpiece  32  and effecting relative movement therebetween, a high-pressure phase transformation occurs at the contact point between the machining tool  21  and the workpiece  32 . 
     Relative motion between the machining tool  21  and the workpiece  32  can be accomplished by moving the machining tool  21 , a workpiece holder  31 , or both relative to each other. 
     Since the laser light is transmitted through the machining tool  21  to the high pressure transformed phase, the high pressure transformed phase becomes preferentially heated and softens and allows plastic deformation therealong. This enables the apparatus  1  of the present invention to act like a machining tool, such as a lathe, to shape the workpiece  32  and is particularly useful in the optical, semiconductor, and structural ceramic industries. Additionally, since the laser radiation is transmitted through the machining tool  21  as opposed to ahead of it, the machining tool  21  and the laser can be coupled together, which reduces the complexity of the apparatus. Moreover, since only the high pressure transformed phase of the workpiece  32  is heated, a smaller laser source  2  and less energy is required, thereby making the machining procedure more economical.