from stages in manufacturing a gan substrate to grinding and etching in the present invention will be explained in more detail . gan freestanding layers are created according to the method set forth in japanese unexamined pat . app . pub . nos . 2000 - 12900 and 2000 - 22212 . an epitaxial lateral overgrowth ( elo ) mask is laid onto a ( 111 ) gaas wafer , and the gan is grown by a vapor - phase epitaxy technique such as hydride or metalorganic - chloride vapor phase epitaxy ( hvpe or mo - chloride vpe ). the gan is grown on the elo mask to reduce stress in the crystal , and moreover is subjected to facet growth to reduce dislocations . the deposition yields gan of 100 μm to several mm in thickness , and the gaas substrate is removed to give an independent gan substrate . techniques for removing the gaas base substrate include dissolving with aqua regia , shaving off by polishing , and delaminating by a lift - off process . gan films grown thin render single , freestanding gan wafers ; when thick they are cut with a wafer saw to yield a plurality of wafers . as - grown gan crystal after the gaas has been removed is often convex along the back side , and the bow amplitude h is often ± 40 μm to as much as ± 100 μm . the roughness ( r max ) along the back side can be 10 μm or more . such serious bow occurs owing to the large difference in thermal expansion coefficient between the base substrate and the gan , and to the massive dislocations produced by their mismatching lattices . occurrences of such bow are inevitable despite a mask - utilizing lateral overgrowth technique as just discussed being carried out . to have the gan substrates be as they should for manufacturing semiconductor devices onto them , the bow must be decreased , and the front and back sides planarized ( lowering the degree of surface roughness ). henceforth a discussion of the present invention will develop . the post - grinding damaged layer on the substrates was evaluated by cross - sectional observation using scanning electron microscopy ( sem ) and cathode luminescence ( cl ). from the observation results , it was evident that on a substrate in which the gan crystal face was ground employing # 325 diamond grit , the depth of the damaged layer was approximately 4 . 8 μm . the mesh ( size ) of the diamond grit correlates with the surface roughness . the rougher the grit is the rougher the surface ground with the grit will be . with finer grit texture , the face ground with the grit will turn out planar . in turn , since the damaged layer arises from grinding , the damaged layer ought to bear a relationship to the roughness of the grit . this means that by way of the roughness of the grit texture , there ought to be a correlation between the thickness of the damaged layer and the surface roughness . given these considerations , the relation between the depth of the damaged layer and the surface roughness was investigated . the results are shown in the fig2 graph . the horizontal axis is the mesh (#). the larger the number , the finer the grit is . plotted in the graph are damaged layers on gan crystal planed with # 80 , # 325 and # 1000 grit , versus roughness . the vertical axis on the left indicates damaged layer depth ( thickness in μm ), while the vertical axis on the right indicates surface roughness ra ( μm ). from the graph it will be understood that the lower the surface roughness , the thinner will be the damaged layer . the depth of the damaged layer is dependent on the grain size of the diamond grit employed . the significance of this is that the depth of the damaged layer can be controlled . using a fine - textured grit diminishes the damaged layer and makes it smooth . by the same token , using a coarse - textured grit allows a thick damaged layer to be created deliberately . grinding with a grit of a suitable texture smoothes , and produces a damaged layer on , the gan substrate face . the damaged layer acts to stretch the surface on which it is formed . if the action is excessive , the crystal will end up buckling oppositely . in order to rectify this , the damaged layer should be partially removed , and to do so etching was carried out . for the etch , both wet etching using chemical agents and dry etching using plasma were tried . after being processed , the surface of a gan substrate underwent wet etching . koh ( aqueous solution , 8 n concentration ) was heated to 80 ° c ., and the gan substrate was wet - etched by immersing it into the solution . the bow was not , however , altered . this means that a gan crystal face on which a damaged layer has been produced by polishing is not wet - etched by koh . the ( 0001 ) faces of gan have polarity . one face ( the ga face ) is terminated with gallium atoms , and the other face ( the n face ) is terminated with nitrogen atoms . the ga face is hard and unyielding , and is chemically stable . no chemical agent that can effectively etch a ga face exists . since the front side was the ga face and the back side was the n face , when the substrate was dipped into the koa solution the back - side n face was slightly etched but the front - side ga face was not etched at all . because the front side , being polished , had the damaged layer , koh did not remove the front - side damage layer . wet - etching gan with a strong alkali like heated koh , or a strong acid such as h 3 po 4 has been documented . but such instances have amounted only to erosion of the n face . the gan that the present invention inventors manufacture possesses a composite front side in which the n face and the ga face appear in alternation . since wet - etching the gan in an etchant such as koh or h 3 po 4 etches only the n face , creating pits , the front side ends up being ragged . despite the pains taken to polish the front side , it ends up ruined , not amounting to anything . ultimately , therefore , wet - etching of the front side ( ga face ) proves to be impossible . the back side ( n face ) of gan substrates is ground . a damaged layer is created on the back side by polishing , and the substrates buckle convexly along the back side ( bow : negative ). it was discovered that when substrates having a negative bow are wet - etched with an 8 n koh solution at 80 ° c . or with h 3 po 4 phosphoric acid , with elapsed etch time the absolute value of the bow decreases . that is , the back side — being the n face — is etched by a strong alkali and a strong acid , and by the very diminishment of the diminishing damaged layer , the bow is curtailed . this means that back - side polishing and wet etching form a method that can be utilized to curtail bow . results of thus utilizing the method are shown in fig3 . under conditions for back - side wet etching identical to those just noted , the back side of a gan substrate was wet - etched . the horizontal axis in the graph represents the wet - etching depth ( μm ), and the vertical axis , bow ( μm ). from the graph it is evident that wet - etching a concave gan substrate whose front side possessed an initial − 33 μm concave bow curtailed the bow . when some 5 μm had been etched , the bow went to around − 10 μm ; etching beyond that did not lead to diminishment of the − 10 μm bow . in addition , variation in the thickness was under several μm , which was at the non - problematic level . wet - etching the back side of the substrate gave the gan crystal — whose front side , being globally mirror - finished , was transparent — a clouded appearance like frosted glass . this was because the back side had been surface - roughened . since the bow was reduced , in situations in which it is acceptable for the back side to be glazy , the substrate can be used in that state . there are situations , however , in which the back side being glazy would create problems — in which the back side has to be a mirrorlike surface . in such cases , arrangements have to be made to remove the damaged layer by dry - etching the back side . when removal is by dry etching , the back side does not become frosted - glasslike . the fact that wet - etching the ga face is impossible , while wet - etching the n face is possible has been noted . the n face ( back side ) can be rid of a damaged layer by either wet etching or dry etching . for the front side , removal is only by means of dry etching . inasmuch as wet etching is ineffectual , the only option for etching the front side ( ga face ) is by dry etching . provided that dry etching is feasible , by that means removing a damaged layer along the front side of a gan substrate ought to be possible . performing dry etching of gan under the following conditions makes it possible to etch the front side . plotted in fig4 is the relationship between front - side etching depth and bow when the front side ( ga face ) of a gan substrate was dry - etched at : chlorine flow rate = 10 sccm ; pressure = 1 pa ; antenna power 300 w ; bias 10 w . the horizontal axis is the etching depth ( μm ); the vertical axis is the bow ( μm ). although the bow was initially 40 μm , the etching carried out proceeded to curtail the bow : when the etching depth was 0 . 8 μm , the bow had decreased to + 30 μm ; at 1 . 3 μm etching depth the bow had decreased to + 22 μm ; at 2 μm etching depth , the bow had subsided to + 16 μm ; at 3 . 6 μm etching depth , the bow had subsided to + 13 μm ; at 5 . 5 μm etching depth , the bow had curtailed to + 10 μm ; and when the etching depth had gone to over 6 μm , the bow no longer subsided , staying at the + 10 μm level . it was realized that although with the front side being the ga face , the front side could not be etched by wet etching techniques , with a dry etching technique — reactive ion etching ( rie )— the ga face too could be etched . then it was also realized that by means of the etching , positive bow ( convexity in the front side ) decreases . this was a crucial discovery . with the damaged layer being on the front side , the layer brought about positive bow ( convexity along the front side ). since what gave rise to the positive bow was curtailed because the front side was reduced , the bow proceeded to decrease . such is the plausible interpretation . under the same conditions as with the front side , dry etching was possible on the back side ( n face ) of a gan substrate . by means of dry etching using chlorine plasma , removal of a damaged layer from the back side was also possible . removing the damaged layer from the back side altered the bow from being concave with respect to the front side to being convex with respect to the front side . ( the bow changed heading from negative - ward to positive - ward .) and removing the damaged layer on the substrate back side was possible without spoiling the surface smoothness of the back side . herein it will become clear that bow can be controlled by combining grinding or a like mechanical process , and dry etching . a damaged layer forms when either the front side ( ga face ) or the back side ( n face ) is ground . the damaged layer produces compressive force on the ground face , tending to stretch it . the front side therefore deflects convexly when a damaged layer is made on the front side . and the back side deflects convexly when a damaged layer is made on both sides . the bow rate can be modulated by the thickness d of the damaged layer , and the damaged layer can be removed by dry etching . if thus the thickness of the damaged layer is decreased , the bow will change from being convex to being concave . these are the reasons why bow can be controlled by the formation of a damaged layer . such instances are illustrated in fig5 . the plural vertical lines drawn within the wafers represent dislocations . further , fine stipples are drawn by the front / back side of the wafers ; these are the damaged layer produced by grinding . fig5 a illustrates a technique for a wafer whose front side is convex ( h & gt ; 0 ), in which grinding the concave back side creates a damaged layer on the back side to curtail the bow . fig5 b illustrates a technique for a wafer whose front side is concave ( h & lt ; 0 ), in which grinding the concave front side forms a damaged layer on the front side to curtail the bow . fig5 c illustrates a technique of back - side dry - etching in which the back side of a wafer whose front side is concave ( h & lt ; 0 ) is ground to create on the back side a damaged layer , and the damaged layer on the back side is reduced and thinned down . the bow in a gan substrate deposited by a vapor - phase deposition onto a heterosubstrate , from which the base substrate is removed , is ± 40 to as much as ± 100 μm . if thus the bow is large , the error in the optical exposure pattern during device fabrication by photolithography will be too great . when contact exposing a substrate it is pressed upon , and if there is bow , the substrate can crack . therefore , bow in the gan substrate has to be + 30 μm to − 20 μm . more desirably , the bow is + 20 μm to − 10 μm , and optimally it is + 10 μm to − 5 μm . gan substrates are transparent . forming thin films onto the gan wafers by metalorganic chemical vapor deposition ( mocvd ) or molecular - beam epitaxy ( mbe ), or vapor - depositing electrodes on the wafers means that they are placed on a susceptor with a built - in heater and heated ; but because the wafers are transparent , they do not sufficiently absorb the radiant heat from the heater . rather than the radiant heat , a wafer absorbs heat from the susceptor due to thermal conduction . because the absorption route is by thermal conduction , it is vulnerable to how the wafer and susceptor are in contact . to make the heating uniform , the state of contact between the wafer and susceptor must be made uniform . if there is bow in the wafer , thermal conduction will be restricted to the central portion ( concave bow ) or to the peripheral portion ( convex bow ). with uniform heating being impossible on account of such bow , a strong , diametrically oriented temperature distribution is set up in the wafer . consequently , the characteristics of the fabricated devices end up being inconsistent . in this respect gan substrates differ vastly from si and gaas substrates . thus , as far as bow is concerned , more severe conditions are imposed on gan substrates than on si or gaas substrates . since in order to make thermal conduction uniform , globally even contact with the susceptor is sought , zero bow is ideal . the spread in which bow is tolerated is not identical above and below zero : a tolerance range in which above , where bow is convex , is up to 30 μm , and below , where bow is concave , is as far as 20 μm . if with bow being large semiconductor devices are fabricated by photolithography onto gan crystal wafer obtained by using vapor - phase deposition to grow gan onto a heterosubstrate and stripping off the heterosubstrate , error in the transfer pattern will be significant . and there will be instances of cracking in the wafer when it is vacuum - chucked . inasmuch as the present invention brings the wafer bow to within + 30 μm to − 20 μm , even vacuum - chucked the wafer will not crack . wafers according to the present invention do not fracture even when masks for contact exposure are set onto the wafers . since there is no bow , the mask pattern is accurately transferred onto the resist , and errors do not appear in the optical exposure pattern . these features improve device - fabrication yields . inasmuch as a damaged layer is exploited to eliminate bow , the damaged layer of the present invention remains behind to some extent . a maximum of 50 μm of the damaged layer along the back side , and a maximum of 10 μm of the layer along the front side will in some cases be present . the damaged layer along the front side is so thin as not to be a hindrance when fabricating devices . even along the back side , since the damaged layer is 50 μm or less , disruptions , such as growth of cracks or incidents of fracturing , following from wafer - processing based operations do not arise . what the present inventors discovered is that grinding a nitride substrate surface with grit having a coarse mesh produces a damaged layer and the damaged layer has a stretching effect on the surface , and that by means of etching to diminish the damaged layer this action that tends to stretch the surface is curtailed . accordingly , a novel technique by the present invention is the production of a planar substrate with minimal bow by introducing a ( grinding ) damaged layer onto the front side / back side of a nitride substrate , and removing the layer in part . when the bow h is taken into consideration including its sign , front - side damaged layer introduction s and back - side etching t increase the bow h , while front - side etching u and back - side damaged layer introduction w decrease the bow h . this means : h graduated increase processes — front - side damaged layer introduction s , back - side etching t ; h graduated decrease processes — front - side etching u , back - side process - transformed layer introduction w . with front - side damaged layer introduction s and front - side etching u being stand - alone processes they do not necessarily have to form a pair . likewise , with back - side etching t and back - side damaged layer introduction w being stand - alone processes they do not necessarily have to form a pair . but because the etching process has to be for removing a damaged layer , front - side damaged layer introduction s has to go ahead of front - side etching u . likewise , back - side damaged layer introduction w has to precede back - side etching t . going a step further , the sign of these processes is taken to express increase / decrease in bow . thus , s and t take positive values ; u and w take negative values . since the absolute value of the change in bow due to etching is smaller than that of change in bow due to a damaged layer , s + t is positive ; u + w is negative . that is : letting the initial bow be h i and the final bow be h o , then fundamentally ideally the final bow h o is 0 , but there is an optimal range about 0 , and it is satisfactory to have the range be given the significance of equation ( 5 ), what this means is that increasing the bow through front - side grinding ( since s is positive ), decreasing the bow by front - side grinding ( since u is negative ), decreasing the bow by back - side grinding ( since w is negative ), and increasing the bow by back - side grinding ( since t is positive ) produces an appropriate ( from − 20 μm to + 30 μm ) final bow h o . for the sake of simplicity , the final bow h o may be conceived of as being 0 . given the parameters in equations ( 1 ) through ( 4 ), no matter what the initial bow h i , it should be possible to bring the final bow to 0 , or else to within the appropriate range ( 6 ). nevertheless , the fact that the final thickness of the damaged layer along the front side is 10 μm or less imposes a restriction on s + u ( positive value ). in turn , the fact that the thickness of the damaged layer along the back side is 50 μm or less imposes a restriction on w + t ( negative value ). because on w + t can be a negative number whose absolute value is considerably large , implementations in which the initial bow h i is positive mean for the present invention that with the degree of freedom being especially large , the invention is more easily embodied . when the initial bow h , is positive — i . e ., when there is a convexity along the front side ( ga face )— then steps s and u can be omitted , and the bow can be curtailed simply according to in other words , this means that back - side grinding w and back - side etching t alone are sufficient . moreover , if it is the case that change in bow can be accurately controlled by back - side grinding , then the back - side etching t may be omitted . that is , such cases make it that this maintains that bow can be eliminated by back - side grinding w alone ( embodiment 3 ). in instances in which the initial bow h , is negative — i . e ., when there is a concavity along the front side ( ga face )— then since h has to be increased , s and t ( s , t both positive ) are required . but given this , because t necessarily entails w , what can be omitted is only front - side etching u . then what is possible in such instances is this states that bow can be curtailed by means of front - side grinding s , back - side grinding w , and back - side etching t alone ( embodiment 2 ). nonetheless , in some cases in which the initial bow h , is negative , using all four steps will be advisable : this states that bow can be curtailed by means of front - side grinding s , front - side etching u , back - side grinding w , and back - side etching t alone ( embodiment 1 ). techniques ( 9 ) and ( 10 ) can be utilized even when the initial bow h , is positive . accordingly , noting down altogether techniques possible by the present invention would be as follows . gan was grown by hvpe onto a gaas base substrate as described earlier . the gaas base substrate was removed to render freestanding , independent gan crystals . the as - grown gan crystal substrates thus obtained were 50 . 8 mm in diameter ( 2 - inch ) and 500 μm in thickness . the substrates had a concavity along the front side ( ga face ), with the absolute value of the bow being 40 μm or more ( h & lt ;− 40 μm ). the surface roughness of the front side was r max 10 μm or more . the surface roughness and bow were measured employing a stylus surface profilometer (“ surfcom ,” manufactured by tokyo seimitsu co .). the gan crystals were affixed by means of wax to a platen made of alumina ceramic , and were then ground under the conditions tabulated below . the planarity ( bow ) of the gan crystal substrate still affixed to the polishing platen immediately after grinding was ± 2 μm , and the surface roughness r max was 0 . 5 μm . because the polishing platen is perfectly flat , it stands to reason that bow in a substrate bound fast to the platen will be slight . the polishing platen was heated to 100 ° c . to peel the gan crystal substrate off the platen . the gan crystal substrate broken away from the polishing platen was ultrasonically cleansed in isopropyl alcohol . bow in the gan substrate in respective stages was then measured . grinding as just described was carried out on both the front side ( ga face ) and back side ( n face ). the grinding produced damaged layers . arrangements were made to etch the substrate so as to remove the damaged layer at once following grinding . although the n face ( back side ) could be wet - etched using koh , on the ga face ( front side ), inasmuch as wet etching is ineffectual , dry etching using a chlorine plasma was performed . of course , dry etching the back side also is possible . the etching conditions were : either the front side or the back side may be ground first . for procedure a and procedure b below , the respective sequences are indicated . it is not necessary to set the procedure so that an etching operation always follows on a grinding operation ; both substrate sides may be ground , following which both sides may then be etched ( procedure c and procedure d ). inasmuch as cleaning and drying are performed following the respective stages , such as when the substrate is broken away from the polishing platen , and after etching , herein they have been omitted . front - side grinding front - side dry etch ( chlorine plasma ) back - side grinding back - side wet etch ( koh ), or dry etch ( chlorine plasma ) the procedural order written out in slightly more detail would be as follows . grow substrate → affix to platen → grind front side → break away from ( lift off of ) platen → dry - etch front side → affix to platen → grind back side → break away from ( lift off of ) platen → wet - etch or dry - etch back side . back - side grinding back - side wet etch ( koh ), or dry etch ( chlorine plasma ) front - side grinding front - side dry etch ( chlorine plasma ) the procedural order written out in slightly more detail would be as follows . grow substrate → affix to platen → grind back side → break away from ( lift off of ) platen → wet - etch or dry - etch back side → affix to platen → grind front side → break away from ( lift off of ) platen → dry - etch front side . front - side grinding back - side grinding front - side dry etch ( chlorine plasma ) back - side wet etch ( koh ), or dry etch ( chlorine plasma ) back - side grinding front - side grinding back - side wet etch ( koh ), or dry etch ( chlorine plasma ) front - side dry etch ( chlorine plasma ) in embodiment 1 set forth below , procedure a is adopted , with the substrate bow being measured in the post - grown free state , in the post - grinding bound state as adhered to the platen , in the free state after being broken away from the platen , in the free state following front - side etching , in the bound state as adhered to the platen following back - side grinding , and in the free state following back - side . the bow in the free state of a ( 2 - inch φ , 500 - μm thickness ) gan crystal from which the gaas base substrate had been removed was h =− 50 μm ( front - side concavity ). the back side was affixed to the polishing platen and the front side was ground . the grinding conditions were as described earlier . the absolute value of the post - grinding front - side bow in the gan crystal as affixed in the bound state was no more than 1 μm . the bow in the gan crystal in the free state as having been lifted off the platen was h =+ 30 μm . this means that along the front side the crystal had gone convex . the reason for this is because a thick damaged layer had been introduced into the front side by the grinding , and the damaged layer generated stress that tended to stretch the front side . because the presence of a damaged layer on the front side is not acceptable , the front side was given a dry etch ( de ) with a chlorine plasma . thereafter the bow proved to be h =+ 10 μm . although the condition of convexity along the front side did not itself change , the amount of bow was reduced . in addition , the front side was affixed to the platen and the back side was ground . the grinding conditions were as described earlier . the post - grinding back - side bow in the gan crystal as adhered fast to the platen was no more than 1 μm . the bow in the gan crystal in the free state as having been undone from the platen was − 20 μm . the reason for this is because a damaged layer had been produced along the back side by the grinding , and the damaged layer acted to stretch that surface . the bow in the free state after the back side next had been dry - etched was h =− 5 μm . this means that the bow had for the most part disappeared . this bow sufficiently satisfies according to the present invention the condition : + 30 μm ≧ h ≧− 20 μm ; it satisfies the more preferable condition : + 20 μm ≧ h ≧− 10 μm ; and in fact it satisfies the optimal condition : + 10 μm ≧ h ≧− 5 μm . grinding gives rise to a damaged layer and since the layer pressingly stretches the ground surface , the bow changes to the opposite side . and the further significance is that when the damaged layer is removed by etching , the bow is curtailed in correspondence with the amount removed . in sum , what this means is that by combining grinding and etching , the bow can be reduced or eliminated . embodiment 2 is one in which the front - side dry etch ( de ) of embodiment 1 was omitted . the bow in the free state of a ( 2 - inch φ , 500 - μm thickness ) gan crystal from which the gaas base substrate had been removed was h =− 50 μm ( front - side concavity ). the back side was affixed to the polishing platen and the front side was ground . the grinding conditions were as described earlier . the absolute value of the post - grinding front - side bow in the gan crystal as affixed in the bound state was no more than 1 μm . the bow in the gan crystal in the free state as having been lifted off the platen was h =+ 30 μm . this means that along the front side the crystal had gone convex . the reason for this is because a thick damaged layer had been introduced into the front side by the grinding , and the damaged layer generated stress that tended to stretch the front side . no dry etch was performed on the front side , but the front side was affixed to the platen and the back side was ground . the grinding conditions were as described earlier . in the back - side grinding there were instance in which local cracking occurred . the post - grinding back - side bow in the gan crystal as adhered fast to the platen was no more than 1 μm . the bow in the gan crystal in the free state as having been undone from the platen was − 30 μm . the reason for this is because a damaged layer had been produced along the back side by the grinding , and the damaged layer acted to stretch that surface . the back side was next dry - etched . thereafter the bow in the free state was h =− 20 μm . this bow satisfies according to the present invention the condition : + 30 μm ≧ h ≧− 20 μm . this is a bow range within which photolithography is possible . of particular significance here is that because front - side etching was not carried out , a factor that makes h positive was diminished . the bow in the free state of a ( 2 - inch φ , 500 - μm thickness ) gan crystal from which the gaas base substrate had been removed was h =+ 30 μm ( front - side convexity ). the crystal was affixed to a ceramic platen , and both sides were ground so as to lessen the damaged layer . that meant a fine - mesh grit was employed . the ra was not more than 5 nm . in this embodiment , bow could be eliminated without creating a front - side - ground damaged layer and without etching , which was simpler . the front side was affixed to the polishing platen and the back side was ground . the grinding conditions were as described earlier . the post - grinding back - side bow in the gan crystal as adhered fast to the platen was no more than 1 μm . the bow in the gan crystal in the free state as having been lifted off the platen was + 10 μm . because the bow was “+,” back - side etching was not performed . significant in this embodiment — an instance in which the bow was convex — is that the bow could be curtailed simply by introducing a damaged layer into the back side . herein , should the bow be negative after the back side is ground ( convexity along back side ), etching the back side to take away part of the damaged layer will bring the surface closer to planar ( h → 0 ). only selected embodiments have been chosen to illustrate the present invention . to those skilled in the art , however , it will be apparent from the foregoing disclosure that various changes and modifications can be made herein without departing from the scope of the invention as defined in the appended claims . furthermore , the foregoing description of the embodiments according to the present invention is provided for illustration only , and not for limiting the invention as defined by the appended claims and their equivalents .