Abstract:
An aqueous grinding fluid has compositions mainly consisting of 40.0-95.0 wt. % a polyalcohol and/or a polyalcohol derivative, 5.0-50.0 wt. % water, 0.1-3.5 wt. % bentonite, 0.1-5.0 wt. % cellulose and weight %, 0.5-10.0 wt. % mica and optionally a small amount of a surfactant or polar solvent. Bentonite, cellulose and mica are preferably added with such combination to adjust a viscosity of a slurry to 150-300 mPa.second as a value measured by a rotating cylinder type viscometer. The aqueous suspension is nonflammable and stabilized in dispersiveness and settleability-proof of abrasive grits as well as a viscosity for a long time, and facilitates washing and cleaning of sliced wafers and a grinding machine without radiation of stinks.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to an aqueous grinding fluid useful for wire-sawing or band-sawing such a bulk as an silicon ingot, a compound semiconductor block or a quartz block.  
           [0002]    A silicon ingot prepared by a pulling method is sequentially processed by steps of separating its top and tail, grinding its periphery and marking an orientation flat. Thereafter, the silicon ingot is sliced to wafers of predetermined thickness. Although a slicing machine provided with an inner blade has been used so far for slicing the silicon ingot, a wire-sawing machine using a piano wire or the like has been recently developed in order to enable slicing a big ingot to large-diameter wafers.  
           [0003]    A wire-sawing method uses a wire which runs unidirectionally or reciprocatively in contact with a surface of an ingot, together with a slurry in which abrasive grits are suspended. The slurry is fed into the ingot by running motion of the wire, and the abrasive grits in the slurry dynamically rub a surface of the ingot. In a band-sawing method, the similar slurry is fed together with a band into an ingot. The ingot is sliced to a plurality of wafers by rubbing action of the abrasive grits.  
           [0004]    A conventional slurry is an oily slurry prepared by suspending abrasive grits in an oily coolant, i.e. a dispersion medium mainly composed of a mineral oil. Silicon carbide particles of approximately 20 μm in size are representative abrasive grits. The oily coolant is a grinding fluid which contains a hydrocarbon such as mineral oil or vegetable oil, an aliphatic acid, an aliphatic ester, etc.  
           [0005]    The oily slurry has the advantage that abrasive grits are kept in a stable suspended state, and is offered at a relatively low cost. However, the oily slurry is not suitable for unmanned-operation of a sawing machine for a long time due to its inflammability, and a working environment becomes worse due to radiation of stinks. In addition, an organic solvent such as trichloroethane or dichloromethylene, which puts harmful influences on the environment, is necessarily used for washing wafers sliced off an ingot. Use of such an oily slurry also makes it difficult to clean the sawing machine. Besides, the environment would be damaged by incineration of a waste slurry.  
           [0006]    Accounting the disadvantages of such an oily slurry, there is a demand for provision of an aqueous slurry which is not inflammable without radiation of stinks for improvement of a working environment and safety and also unneccesitates use of an organic solvent for washing.  
           [0007]    Abrasive grits suspended in an oily or aqueous slurry has a tendency to precipitate and accumulate in the slurry with the lapse of time. Due to sedimentation of abrasive grits, a weight of accumulated abrasive grits is charged to abrasive grits at a lower layer, resulting in coagulation and caking of the abrasive grits. Such coagulation and caking of abrasive grits in an aqueous slurry is accelerated as compared with abrasive grits in an oily slurry, since an aqueous coolant is inferior of keeping abrasive grits in a suspended state.  
           [0008]    A slurry shall be continuously stirred in order to inhibit coagulation and caking of abrasive grits. Long-time stirring is necessary for cracking coagulated cakes in prior to grinding workpieces. However, it is difficult to completely crack coagulated cakes by such stirring. As a result, a ratio of abrasive grits suspended in the slurry is reduced, so as to deteriorate an a grinding function of the slurry. The coagulation and caking of abrasive grits also causes such troubles as plugging of pipings.  
           [0009]    Dispersiveness and settleability-proof of abrasive grits are important properties especially for an aqueous slurry, wherein sedimentation of abrasive grits more frequently occurs compared with an oily slurry. The dispersiveness and settleability-proof of abrasive grits shall be stable at a higher level for a long time, accounting the tendency that a grinding step becomes longer and longer in time as enlargement of a silicon ingot, a polycrystalline bulk or the like to be sliced. For instance, if dispersiveness and settleability-proof of abrasive grits changes during a long-time grinding step, a slurry consequently changes its viscosity. Change of the viscosity varies a grinding function of the slurry between initial and final stages of slicing, so as to cause such defects as thickness deviation, warps, chippings, poor flatness or ruggedness on wafers sliced off an ingot.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention aims at provision of a novel aqueous grinding fluid which overcomes the above-mentioned problems. Properties such as dispersiveness and settleability-proof of abrasive grits are stabilized at higher levels for a long time by combinative addition of bentonite, cellulose and mica to an aqueous dispersion medium mainly composed of a polyalcohol or its derivative, so as to keep a viscosity of the slurry at a constant value. The proposed aqueous liquid is nonflammable without radiation of stinks, and makes it easy to wash and clean sliced wafers as well as a sawing machine.  
           [0011]    The aqueous grinding fluid according to the present invention has compositions mainly consisting of 40.0-95.0 wt. % a polyalcohol or its derivative, 5.0-50.0 wt. % water, 0.1-3.5 wt. % bentonite, 0.1-5.0 wt. % cellulose and 0.5-10.0 wt. % mica.  
           [0012]    Bentonite, cellulose and mica are preferably added to an aqueous dispersion medium with such the combination that a viscosity of an aqueous grinding fluid measured by a rotating cylinder-type viscometer is kept within a range of 150-300 mPa.second.  
           [0013]    The aqueous grinding fluid is preferably adjusted to pH 5.0-12.0 by addition of a carboxylic acid or carboxylate. Lubricity of the fluid may be improved by addition of a surfactant or polar solvent.  
         DETAILED DESCRIPTION OF THE INVENTION  
         [0014]    The aqueous grinding fluid according to the present invention contains a polyalcohol or its derivative as a main component. Inflammability of the polyalcohol or its derivative is diminished by addition of crystal water. Dispersiveness and settleability-proof of abrasive grits are stabilized at higher levels by combinative addition of bentonite, cellulose and mica.  
           [0015]    A main component may be one or more of polyalcohols and polyalcohol derivatives which preferably include 2-4 carbon atoms per one molecular. The polyalcohol derivative may be an alkylene glycol, alkylene glycol derivative, glycol ester, glycol ether, etc.. The polyalcohol and/or its derivative effectively improves lubricity of the aqueous fluid to a workpiece during grinding, due to an alkyl group superior of affinity with an oily component. A mixing ratio of the polyalcohol and/or its derivative is adjusted within a range of 40.0-95.0 wt. % (preferably 60.0-90.0 wt. %) in order to ensure predetermined lubricity.  
           [0016]    The polyalcohol and/or its derivative as a main component is modified to nonflammable by addition of water. Although water content necessary for modification of the polyalcohol and/or its derivative to nonflammable is varied in response to a kind and concentration of the polyalcohol and/or its derivative, the inventors have discovered from various experiments that a nonflammable grinding fluid is obtained at water content within a range of 5.0-50.0 wt. % (preferably 10.0 wt. % or more). Addition of water is also effective for keeping bentonite, cellulose and mica in a stable swelling and dispersed state. Such the effect is clearly realized by addition of 5.0 wt. % or more of water.  
           [0017]    Since the water is incorporated as crystal water in bentonite, cellulose and mica, evaporation of water during storage or grinding work is suppressed. Consequently, the grinding fluid can be circulated during grinding work without necessity of additional water supply, so as to facilitate maintenance of the grinding fluid at proper water content during grinding work. Water in the grinding fluid also cools a grinding tool and a workpiece such as an silicon ingot, so that the life of the grinding tool is prolonged and that sliced products such as wafers are improved in dimensional accuracy.  
           [0018]    Bentonite thickens a slurry which is prepared by addition of abrasive grits to an aqueous grinding fluid and inhibits sedimentation of abrasive grits. Such a bentonite is preferably a hydrophilic bentonite, e.g. montmorillonite and beidellites, nontronite, saponites, hectorites, stevensite and Vermiculite. The bentonite is mixed within a range of 0.1-3.0 wt. % (preferably 0.5-1.5 wt. % ) in order to capture abrasive grits without sedimentation.  
           [0019]    The bentonite exhibits such the thixotropy that the grinding fluid decreases its viscosity during grinding work with the lapse of time. Decrease of the viscosity caused by the bentonite is compensated by combinative addition of cellulose which exhibits such the rheopexy that the grinding fluid reversally increases its viscosity with the lapse of time. If a bentonite solely is added to an aqueous slurry, a network structure of the bentonite is destroyed by affections such as reaction with grinding chips, friction heats and shear stresses during grinding work. Collapse of the network means change of properties such as dispersiveness and settleability-proof of abrasive grits as well as a viscosity of the grinding fluid. On the other hand, in the case of an aqueous slurry which contains cellulose as colloidal particles dispersed therein, the cellulose particles are charged with electricity derived from hydration reaction and reformed to a firmer network structure when cracking occurs by a high shearing stress. Consequently, the slurry increases its viscosity. These effects of cellulose recover the viscosity which was decreased with collapse of the network structure of the bentonite, and properties such as dispersiveness and settleability-proof of abrasive grits and a viscosity of the slurry are kept at predetermined values.  
           [0020]    Cellulose is preferably a natural cellulose which is a main component of cell membrane of plants such as wood, cotton or hemp. The natural cellulose comprises a crystalline region where molecules are parallelly arranged with relatively high regularity and an amorphous region where molecular sequence is disordered. A cellulose crystallized by chemical treatment to remove the amorphous region is also useful. The cellulose is added to the grinding fluid at a ratio of 0.1-5.0 wt. % (preferably 0.1-1.5 wt. %) in order to stabilize a viscosity of the slurry.  
           [0021]    Adhesiveness of the slurry to a sawing wire or band is improved by addition of mica to the aqueous grinding fluid. Colloidal mica particles swollen and dispersed in an aqueous phase continue Brownian motion while repelling each other by surface charge derived from hydration reaction, so as to form a network structure which improves settleability-proof of abrasive grits. The mica particles exhibit constant thickening function regardless the lapse of time, and keep a viscosity of the slurry at a higher value in combination with bentonite and cellulose. In addition, since the mica includes water in its network structure and swells, evaporation of water is suppressed, and the slurry exhibits proper water retentiveness. These effects are apparently recognized by addition of 0.5-10.0 wt. % mica.  
           [0022]    Mica may be natural mica such as muscovite, phengite, sericite, illite, glauconite, celadonite, tobelite, phlogopite or biotite. Synthetic mica such as fluorine phlogopite which replaced an-OH radical of the phlogopite with F can be also used. Since the synthetic mica has a thermal decomposition temperature higher than natural mica, the synthetic mica is hardly degraded by a friction heat during grinding work.  
           [0023]    Addition of mica together with bentonite and cellulose effectively suppresses change of properties caused by collapse or decomposition of network structures, so as to ensure a slurry having dispersiveness and settleability-proof of abrasive grits as well as a viscosity stabilized at higher levels for a long time. On the other hand, an aqueous slurry which does not contain bentonite shows the tendency that its viscosity increases with the lapse of time, an aqueous slurry which does not contain cellulose shows the tendency that its viscosity descends with the lapse of time, and an aqueous slurry which does not contain mica has an insufficient viscosity  
           [0024]    Bentonite, cellulose and mica swell in the contact with the water, and change to gelling states including water. The bentonite, cellulose and mica gels are converted to sols or dispersoids by dynamic frictions during stirring or the like. Hereon, bentonite and mica, which are lamellar silicate minerals composed of a plurality of structure units piled up together, incorporate water between 2-4 layers of the lamellar structure and expand to a volume twice bigger than a value in a dry state. Since the swelling action is repeatedly continued during stirring, an amount of water incorporated in bentonite or mica reaches several times a volume of the mineral at an initial stage. Moreover, the water incorporated between the layers is converted to crystal water such as H 2 O or OH and formed to water molecule layers on surfaces and between layers of the mineral particles. Crystal water in the present specification means water incorporated as water molecular layers in the mineral particles. The same swelling reaction occurs in cellulose, and water is incorporated between cellulose molecules. The water incorporated as crystal water is not evaporated from a slurry during storage or grinding work, so as to maintain the slurry at a constant viscosity value.  
           [0025]    The aqueous grinding fluid is preferably adjusted at pH 5.0-12.0 in order to keep a proper antirust effect without generation of hydrogen gas. Although generation of hydrogen gas is accelerated as increase of a pH value, hydrogen gas which puts harmful influence is not substantially generated at a pH value of 12.0 or less. The antirust effect is more vivid as increase of a pH value over 5.0 (preferably 9.0).  
           [0026]    The aqueous grinding fluid is preferably adjusted to a proper pH value by addition of an carboxylic acid or a carboxylate. The carboxylate may be an alkali metal salt, an alkaline earth metal salt, etc.. A surfactant or polar solvent may be added to the aqueous grinding fluid in order to improve lubricity. The surfactant may be nonionic or anionic. If concentration of the surfactant is excessive, biodegradability of the nonionic surfactant is deteriorated, or the anionic surfactant accelerates its foaming reaction. In this sense, the concentration of the surfactant is preferably controlled at 5.0 wt. % or less. When a heterocyclic nitrogen compound is used as a polar solvent s, its concentration is controlled at 10.0 wt. % or less. 
       
    
    
     EXAMPLE  
       [0027]    A bentonite dispersion A was prepared by addition of a hydrophilic bentonite (hectorite) to purified water at a ratio of 2.0 wt. %. A cellulose dispersion B was prepared by addition of cellulose to purified water at a ratio of 10.0 wt. %. Γ-butyl lactone as a polar solvent was added to the cellulose dispersion B at a ratio of 20 wt. % to prepare an aqueous suspension C.  
         [0028]    The aqueous suspension C was added to a propylene glycol liquid D and mixed in a colloidal mill. The bentonite dispersion A was then added to the mixed liquid, and further stirred. Thereafter, mica was added to the mixed liquid and stirred by the same way. Thus, an aqueous grinding fluid was prepared.  
         [0029]    The aqueous grinding fluid (undiluted solution) obtained in this way was testified for measurement of properties just after preparation and after 24 hours. A slurry prepared by addition of 50 wt. % silicon carbide as abrasive grits to the aqueous grinding fluid was also subjected to the same test. Dispersiveness and settleability-proof of abrasive grits were judged by stirring a sample suspension for 15 minutes in a homogenizer, letting the sample suspension stand in a measuring cylinder, and measuring an amount of sediments after 24 hours. A viscosity of the suspension or slurry was measured by a rotating cylinder type viscometer.  
         [0030]    Test results are shown in Table 1. It is recognized from Table 1 that the aqueous grinding fluids (Examples 1-3) which contained all of bentonite, cellulose and mica were superior of dispersiveness and settleability-proof of abrasive grits as well as high viscosity. These superior properties were not substantially changed, ever when the sample slurry was let stand 24 hours after preparation.  
         [0031]    On the other hand, the aqueous grinding fluid (Comparative Example 1) which did not contain bentonite remarkably increased its viscosity after 24 hours in comparison with a value just after preparation. The aqueous grinding fluid (Comparative Example 2) which did not contain cellulose remarkably decreased its viscosity after 24 hours in comparison with a value just after preparation. Dispersiveness and settleability-proof of abrasive grits were deteriorated with such the viscosity change. The aqueous grinding fluid (Comparative Example 3) which did not contain mica was poor of viscosity just after preparation and also after 24 hours.  
         [0032]    It is recognized from this comparison that the aqueous grinding fluid according to the present invention is suitable for slicing an ingot or the like under stable conditions, since it has properties which hardly change with the lapse of time. In actual, when a silicon ingot of 400 mm in diameter was sliced by a steel wire of 0.18 mm in diameter using an aqueous slurry prepared by addition of 50 wt. % silicon carbide to the aqueous grinding fluid (Example 1), wafers superior of dimensional accuracy and plane property were obtained. It took 24 hours to slice the silicon ingot to wafers, but the properties of the slurry at a time just after initiation of slicing were not substantially varied at a time just before finish of slicing. Due to the stable properties of the slurry, no difference in the plane property of the wafers was detected between initial ground surface and final ground surface.  
                                                                                                                                                                             TABLE 1                           EFFECTS OF COMPOSITIONS OF AQUEOUS GRINDING FLUIDS ON PROPERTIES                THE PRESENT INVENTION   COMPARATIVE EXAMPLES            NOTE   1   2   3   1   2   3                    Components (wt. %)                               Propylene glycol   72.9   73.4   78.9   81.7   80.0   81.7       Water   18.0   18.0   18.0   18.0   18.0   18.0       Bentonite   0.3   0.3   0.3    0.3   —   —       Cellulose   0.3   0.3   0.3   —   —    0.3       Mica   2.0   2.0   2.0   —    2.0   —       Carboxylate   2.0   2.0   0.5   —   —   —       Polar solvent   4.0   4.0   —   —   —   —       Surfactant   0.5   —   —   —   —   —            Settleability proof of   A   A   A   A   A   A   A   C   B   B   C   B       abrasive grits       Viscosity                                                       As undiluted   120    120   120    120   110    110   160   240    70    70    60    60       (by type B viscometer)       Slurry (by   170    320   170    280   170    300   170    70   110   110   100   160       type VT04       viscometer)       Slurry (B type) 220   3400   220   1200   220   3800   220    65   110   110    75   260            pH   9.50   9.50   9.40   9.60   7.20   7.00       Surface tension   35.0   40.0   40.0   40.0   40.0   40.0       (mN/m)       A boiling temp. (° C.)   110   110   112   110   110   110       Friction coefficient   0.130   0.130   0.120   0.130   0.130   0.130                                                          
 
         [0033]    An aqueous grinding fluid according to the present invention as above-mentioned contains water, so that it is nonflammable without radiation of stinks which would occur from an oily slurry. Moreover, evaporation of water is suppressed during storage or grinding work, since such water is incorporated as crystal water in combinatively added bentonite, cellulose and mica. Consequently, the aqueous grinding fluid is used under stable conditions without change of such properties as dispersiveness and settleability-proof of abrasive grits as well as a viscosity of the slurry. A slurry prepared by addition of abrasive grits to the aqueous grinding fluid exhibits grinding performance similar to that of a conventional oily slurry. Besides, worked products and a grinding machine can be washed and cleaned by water without harmful influences on the environment.