with reference now to fig1 there is illustrated , in cross - section , a thin film photovoltaic cell structure according to the present invention . while the illustrated structure is very similar to that taught in the above referenced tyan , et al , patent , the materials of the preferred embodiment are preferably similar to those taught in u . s . pat . no . 4 , 388 , 482 issued to hamakawa , et al , on june 14 , 1983 . the basic supporting structure of the cell array is a glass substrate 10 , typically two millimeters thick . a transparent conductive oxide , tco , layer 12 has been deposited and patterned on the upper surface of substrate 10 to form front electrodes . tco layer 12 is typically two thousand angstroms thick . a semiconductor layer 14 containing an active junction is deposited over tco layer 12 and again appropriately patterned . thus , at 16 , a portion of the semiconductor layer 14 has been removed to allow a back electrode to contact one edge of tco layer 12 . in the preferred embodiment , layer 14 is an amorphous silicone p - i - n structure similar to that taught by hamakawa and having a total thickness of about four thousand angstroms . a final back contact conductor layer 18 is deposited as a continuous sheet over layers 12 and 14 . as illustrated , layer 18 fills the grooves 16 formed in the semiconductor layer 14 and thereby contacts the tco layer 12 . in the preferred embodiment , layer 18 is formed by sputtering or evaporating aluminum to a thickness from about one thousand to about five thousand angstroms . after layer 18 is deposited it must be patterned to form separate back electrodes for each of the separate cells forming the solar array . on the left side of fig1 layer 18 is shown prior to the isolation step and the location of a desired groove is illustrated by the dotted lines 20 . the groove must remove all of the back contact metal 18 and desirably removes all of the underlying semiconductor material . however , to avoid damage to the tco layer 12 , the groove should be no deeper than illustrated at 20 and also at 24 . depending upon the conductivity of various portions of semiconductor layer 14 , it is allowable in some cases , as taught by tyan , et al , to remove only a portion of the semiconductor material 14 . thus , as illustrated by groove 22 , it is often allowable to leave a portion of the semiconductor 14 in place . in the preferred form , as illustrated by grooves 24 , all of the semiconductor layer 14 underlying the desired scribed lines in back electrode layer 18 is removed . with reference now to fig2 there is illustrated a manual scribing table 30 modified for performing the process of the present invention . table 30 includes a base plate 32 adapted for holding a glass plate 34 on which a photovoltaic cell array is being produced . a slide 36 is supported on a bar 38 mounted on one edge of base plate 32 . a bar 40 extends from slide 36 at right angles to the slide bar 38 . a scribe holding block 42 is slidably carried on bar 40 and carries an ultrasonic scribing head 44 , the details of which are described with reference to fig3 below . a power supply 45 is connected to the ultrasonic scribing head 44 to provide the required driving signal . with reference now to fig3 the pivoting support structure for ultrasonic head 44 is illustrated in more detail . a u - shaped bracket 46 , normally attached to block 42 , has two pivot points 48 including miniature ballbearings . a collar 50 is sized to fit within bracket 46 and has pivot arms 52 for engaging the pivot points 48 . a circular aperture 54 in collar 50 is sized to receive the transducer portion 56 of the ultrasonic scribing head . a set screw 60 in one edge of collar 50 is provided for fixing the position of transducer 56 within aperture 54 . ultrasonic head 44 includes beam 62 through which ultrasonic vibrations are coupled to a scribe stylus 64 . a set screw 66 allows adjustment and replacement of stylus 64 . in the preferred embodiment the ultrasonic scribing head 44 and power supply 45 were parts of an ultrasonic cutter sold under the part number auc - 101 by alessi industries inc . of costa mesa , calif . the power supply 45 provides an electrical driving signal at 60 khz at power levels adjustable between 0 and 1 watt . in using the ultrasonic scribe it is first necessary to establish an appropriate force to be applied to the scribe tip or stylus 64 . this force is adjustable up to about twenty grams simply by adjusting the position of scribing head 44 within collar 50 so that a desired portion of its weight is supported on tip 64 . to achieve tip force greater than twenty grams , a spring has been added between bracket 46 and transducer head 56 . when a desired tip pressure has been set , power supply 45 is activated and adjusted to provide a preselected level of driving power to the ceramic magnetostrictive transducer within scribing head 44 . scribing table 30 is then operated in a basically conventional manner . a substrate 34 , with appropriate films deposited thereon , is positioned on base plate 32 as shown in fig2 . slide 36 is moved along bar 38 until stylus 64 is positioned over substrate 34 at a desired scribe line location on one edge of the substrate . the stylus is then lowered to contact substrate 34 and block 42 is manually drawn along bar 40 to cut a straight scribe line across substrate 34 . in commercial production it is expected that scribe table 30 will be replaced with an automatic x - y positioning system which will automatically cut the large number of scribe lines needed , preferrably under computer control . various combinations of scribe tip type , tip force , and ultrasonic energy level were used in experiments to generate grooves 22 or 24 in the structure illustrated in fig1 having an aluminum back contact 18 with a thickness of about 5 , 000 angstroms . a tungsten carbide scribe tip supplied with the equipment having a rounded end with a 0 . 002 inch radius was used at tip forces ranging up to 200 grams and over the entire range of power levels up to 1 watt . while some reasonable scribing was achieved at the high force and power levels the results were not repeatable and therefore not useful for production work . much better results were achieved by using a tungsten carbide stylus having a radius of 0 . 00025 inch which had been broken off to provide an essentially round flat tip , or truncated cylinder , rather than the normal smooth hemispherical end . good repeatable scribe lines were cut at a tip force of about 100 grams and an ultrasonic power level of about 0 . 55 watt . the scribe lines generated were about 0 . 002 inches wide and provided sharp cutting of the aluminum metal and removal of the underlining semiconductor material , without significant damage to the tco layer . the best results were achieved using a diamond tipped stylus having a tip radius of 2 . 5 micron . using a tip force of 5 to 7 grams and a power level of 0 . 45 watt , clean scribed lines were cut at a width of about 15 microns through the back aluminum metal and the underlying silicon film , without significant damage to the tco layer . these results were found to be quite repeatable and thus appear suitable for production use . on microscopic examination of the diamond tip used in this experiment it was discovered that it was partly broken and therefore included sharp edges which apparently aided in generation of clean scribed lines . visual examination of the scribed lines indicate that there is no smearing of the soft aluminum metal and no shunting across the scribed groove was detectable . it is believed that the clean removal of the aluminum layer is a result of rapid removable of the brittle silicon layer in the form of small chips caused by the ultrasonic vibration of the scribe tip . in summary , it is possible to achieve highly repeatable , reliable and very narrow scribe grooves in the back contact metal with very low tip force and ultrasonic power level if very small diameter scribe tips are used . while the present invention has been illustrated and described with reference to specific apparatus and methods of use it is apparent that various modifications and changes can be made within the scope of the present invention as defined by the appended claims .