there is shown in fig2 a simox wafer 20 comprising a monocrystalline silicon substrate 22 , a simox film 24 , and an oxygen impanted layer 26 of silicon dioxide . a layer 28 of silicon dioxide having a thickness of about 2000 angstroms is disposed on the surface of the simox film . this layer 28 is a protective layer which will prevent oxidation of the simox film 24 during subsequent heating of the wafer 20 and will act as a sink for contaminants . fig2 also shows a pair of polycrystalline silicon layers 30 and 32 each having a thickness of about 5000 angstroms . the layer 30 is deposited on the layer 28 of cvd oxide and the layer 32 is deposited on the surface of the substrate 22 opposite the simox film 24 . these poly layers are formed in any suitable manner that is well known in the art . while the invention may be practiced without the layers 30 and 32 , these layers of polycrystalline silicon enhance the migration of contaminants to the protective layer 28 as will be explained below . in practicing the method of the present invention , a layer 28 of silicon dioxide , having a thickness of about 2000 angstroms , is deposited on the surface of the simox film 24 . the layer 28 of sio 2 is deposited by chemical vapor deposition ( cvd ), a process that is well known in the industry . the wafer 20 and layer 28 are then heated in an atmosphere of nitrogen having some residual oxygen , to a temperature which is sufficiently high to disolve foreign precipitates such as oxygen , carbon , heavy metals , and others which are present in the simox film 24 . in the present case , a temperature slightly above 1375 ° c . was used . fig3 shows a graph 40 which represents the density of precipitates with respect to the annealing temperature . as is shown , for temperatures between about 1300 ° c . and 1350 ° c ., the density of precipitates remains quite high . it is emphasized that this density of precipitates is strongly correlated to concentration of contamination . at a temperature of about 1375 ° c ., however , there is a dramatic decrease in the density of precipitates . care must be taken , however , to avoid exceeding the melting point of the simox material which is about 1410 ° c . if the melting point is exceeded the simox film will sag and loose its shape . the temperature need only be high enough to disolve the precipitates and contaminant nuclie present in the simox layer . this temperature is maintained for a period of about 90 minutes so that all of the precipitates have an opportunity to disolve . the contaminants from disolved precipitates have a propensity to migrate from the simox film into the adjacent layers of sio 2 . the mechanism by which this occurs is thought to be related to the higher solubility limit of heavy metals and other such contaminants in sio 2 than in monocrystalline silicon at high temperature . in any case , at a temperature slightly above 1375 ° c . the disolved precipitates do migrate out of the simox film 24 and into the sio 2 layers 26 and 28 on either side of the simox film . the wafer 20 and protective layers 28 are then slowly cooled at a rate that will maintain the tendency of the disolved precipitates to migrate toward the sio 2 layers . if the wafer and protective layer are cooled too quickly , the degree of contaminant migration is substantialy reduced . a cooling rate of about 3 ° c . to about 5 ° c . per minute was found to be satisfactory . this procedure leaves the simox film 28 substantially free of foreign precipitates , which greatly enhances the operating characteristics of devices fabricated in this simox film . migration of some of the foreign precipitates into the layer 26 of implanted sio 2 , however , degrades somewhat the level of purity of this dielectric layer . this , of course , may adversely affect its usefulness in applications requiring high dielectric isolation . one way to reduce the undesirable migration of precipitates into the implanted layer 26 of sio 2 is to apply a slight stress to the surface of the substrate 22 opposite the layer 26 . this can be accomplished by depositing a layer of polycrystalline silicon onto both sides of the simox wafer . a layer 30 of polycrystalline silicon having a thickness of about 5000 angstroms is deposited on the surface of the protective layer 24 and a similar layer 32 of polycrystalline silicon is deposited on the bottom of the substrate 22 as shown in fig2 . the layers 30 and 32 cause a stress in the substrate 22 and simox layer 24 , however , the layer 30 causes a somewhat smaller stress due to the stress absorbing effect of the protective layer 28 of sio 2 . this results in a structure having a net stress in the simox film 24 favoring migration of foreign contaminants in a direction toward the protective layer 28 rather than toward the layer 26 of implanted sio 2 . additionally , the layer 30 provides additional protection against oxidation of the simox film 24 . in addition to the reduction in the density of precipitates , the concentration of heavy metals in the simox layer is substantially reduced after annealing as is shown in fig4 through 7 . fig4 through 7 include graphs 42 , 44 , 46 , and 48 which depict the concentration of contaminants of carbon , ni , fe , and cu respectively for various annealing temperatures . the concentrations shown were obtained by secondary ion mass spectroscopy ( sims ) measurements in the usual manner . the important advantage of the present invention is that the defect density of simox films may be significantly reduced , thereby permitting the fabrication of higher performance semiconductor devices in such films . an example of this is depicted in the graph 50 , shown in fig8 where improvement in gate oxide dielectric strength in the simox layer is ploted against annealing temperature . for the temperature required to disolve oxygen precipitates , in the present example about 1375 ° c ., the gate oxide integrity is dramatically improved .