micromechanical structures , including plates , beams and the like , are typically created using a high concentration boron diffusion on a surface of a silicon wafer . the boron diffusion acts as an etch stop for anisotropic etching which yields the structures by etching away exposed undoped and lightly doped silicon material . boron diffusion on a surface , to an appropriate depth , results in a concentration gradient that causes a stress gradient in the wafer which is proportional to the concentration . the stress gradient causes geometric distortions , such as curling , when the structure is freed up by etching . such curling limits travel of the device and therefore limits the size of device that can be fabricated . a flexibly supported plate having undesirable geometric distortion according to the prior art , is illustrated in fig1 . referring to fig2 a , an illustrative embodiment of a micromechanical structure constructed according to the invention to minimize geometric distortion , is fabricated using a mass of silicon 10 . in this embodiment of a micromechanical plate such as for an accelerometer , the n type silicon mass 10 has a large area tub 12 etched therein , outlining the structural feature , in this case a substantially symmetrical plate , to be created . an etch stop diffusion 14 , such as with a boron dopant , is made onto all exposed surfaces of the tub 12 , effectively establishing a doped bottom side 30 and doped side walls 32 , 34 . the tub 12 is then backfilled , preferrably with single crystal silicon 16 , although polysilicon may also be used to backfill the tub 12 . a top surface 18 of the backfilled mass 10 is then back lapped to a mirror smoothness . the filled tub 12 is then sealed with a high concentration boron diffusion 20 on a top side of the filled surface , as illustrated in fig2 b . a four - sided doped envelope results which has a relatively lightly doped interior bulk . in this illustrative case of an accelerometric plate , relatively shallow diffusions are provided to define flexible linkages 22 , 24 for attaching the plate 26 to a surrounding silicon frame 25 , as illustrated in a plan view of a fully etched plate 26 in fig2 c . the plate 26 is etched substantially free of the mass 10 , in this case by anisotropic etching . a void 28 is created by the etching which leaves the plate 26 free to rotate about an axis formed by the flexible linkages 22 , 24 . as illustrated in fig2 d , high concentration etch stop diffusions are present on all sides of the freed plate 26 , including the bottom side 30 and the side walls 32 , 34 created when the unfilled tub 12 was diffused , and the top side 36 created when the filled tub was sealed . the resulting structure is a substantially symmetric balanced stress structure , relatively free of the geometric distortions present in the prior art . alternatively , a low net strain structure , such as the accelerometric plate of fig2 c , can be fabricated as a sequence of layers as illustrated in fig3 a . a mass of n type silicon may provide a base or substrate 40 upon which a series of epitaxial layers is grown . to fabricate a plate according to this alternative embodiment , a first or lower epitaxial layer 42 is grown over the substrate 40 , which optionally may have integral electrodes effected therein as a highly doped region 44 . the lower epitaxial layer 42 is grown to a height which will provide adequate clearance for a freed plate of the desired plate thickness . the desired plate thickness in this illustrative embodiment is approximately 8 microns . the thickness of the first epi layer 42 is about 15 microns . a first patterned diffusion 46 , comprising a p + boron diffusion at etch stop concentration , is driven into the first epi layer 42 to a depth of about 3 microns . the diffusion is typically performed through openings in a thermally grown oxide layer which has been patterned in the shape of the desired structure by photolithography , in this case a substantially rectangular plate . a second or upper epitaxial layer 48 is grown to a thickness of about 5 microns . a second or upper patterned p + diffusion 50 is driven into the second epi layer 48 , at etch stop concentration , to a depth of about 3 microns . in the illustrative acclerometric plate , substantially shallower p + diffusions 52 , 54 are patterned on the upper epi layer 48 to provide flexible linkages between the plate and the remaining silicon mass subsequent to etching . anisotropic etching yields a plate structure 56 having a cross - section such as illustrated in fig3 b . the etching produces a void 58 which leaves the plate substantially free to rotate about an axis formed by the diffused flexible linkages 52 , 54 . the plate 56 comprises the first diffusion 46 and the second diffusion 50 with epitaxially - grown silicon layer 48 sandwiched therebetween . the resultant doping distribution is symmetrical about the center of the plate 56 . the net strain on the structure is therefore balanced such that the freed up plate 56 is substantially flat with respect to the substrate 40 . because of a redistribution of the diffusion within the silicon epitaxy , the sandwiched epitaxial layer 48 enjoys substantial etch resistance . while the structure illustrated comprises only two epitaxial layers and two diffusions resulting in the sandwiched epitaxial layer 50 , a sequence of epitaxial layers and diffusions can be fabricated to a desired depth providing considerable flexibility in the verticle dimensions of a structure fabricated according to the invention . it should be appreciated that numerous layers can be sequentially applied to construct the desired low net strain structure . fig3 a and 3b illustrate that intermediate layers , such as intermediate epi layer 47 with intermediate etch resistant doped pattern 49 can be layered to a desired structures thickness . referring now to fig4 a and 4b , geometric distortion in a planar micromechanical structure can be substantially eliminated by providing such a structure with a series of grooves or corrugations . grooves provide rigidity in a direction transverse to the length of the groove . various processes known in the semiconductor processing art can be used to create grooves in silicon . a planar structure such as discussed hereinbefore is typically in tension along various axes coaxial with an axis formed by flexible linkages or flexures , as a result of shrinkage of doped material comprising the plate and flexures . a result of the same shrinkage is that the planar structure curls at the ends which are not held in tension by the flexures , such as is illustrated in fig1 . a series of grooves disposed in orthogonal relation with the axis formed by the flexures , provides rigidity which is effective in substantially eliminating the curling . such a structure as illustrated in fig4 a and 4b is fabricated from a mass of silicon 70 , in this illustrative embodiment an n type silicon substrate . the substrate 70 is etched with a series of troughs 72 by an etch process as known in the art . in this case a plasma etch process is used to create troughs 72 having a u - shaped cross - section . other etch processes can be used , such as anisotropic etching which creates a substantially v - shaped cross - sectioned trough . the etched surface having troughs 72 is diffused with a dopant , in this case boron , at etch stop concentration to create an etch resistant corrugated surface 74 . the troughs or corrugations are subsequently filled with mono or polysilicon to produce a desired planar surface 76 . it is conceivable that filling the troughs could be dispensed with , depending on the ultimate structure desired . the planar surface 76 is diffused in a pattern to render the desired shape of the ultimate micromechanical structure , in this case a substantially rectangular planar member 80 attached to the silicon mass by flexible linkages . anisotropic etching frees the planar member 80 and creates a void 82 beneath the planar member 80 . the planar member 80 having the corrugated surface 74 as a bottom side thereof remains subsequent to etching , free to rotate in the void 82 about an axis created by the flexible linkages . the planar structure has relatively balanced stresses and is substantially distortion free along various axes orthogonal to the axis created by the flexible linkages axis . although boron diffusion and anisotropic etching are discussed herein to fabricate micromechanical structures having balanced stresses , one of ordinary skill in the art will appreciate that other dopants can be used to effect etch stopping and other etch processes can be used to create different effects . other etch stop mechanisms such as electrochemical may also be employed . while the invention is discussed in the context of a plate for a micromechanical accelerometer , other micromechanical structures , including planar structures such as cantilever beams and non - planar structures , can be fabricated according to the invention . although the invention has been shown and described with respect to an illustrative embodiment thereof , it should be understood by those skilled in the art that the foregoing and various other changes , omissions and additions in the form and detail thereof may be made without departing from the spirit and scope of the invention as delineated in the claims .