IN MAKING A TARGET FOR A TELEVISION CAMERA TUBE, IT IS NECESSARY TO THIN THE BACKSIDE OF A SEMICONDUCTIVE SUBSTRATE, WHICH IS OPPOSITE A FACE HAVING A DIODE ARRAY THEREON. TO THIN THE SUBSTRATE, IT IS POSITIONED FACE DOWN ON A FLLUID NONDELETERIOUS TO THE SUBSTRATE AND DIOD ARRAY. THE FLUID IS CONTAINED IN A CAVITY OF A HOLDING DEVICE. THE POSITIONING IS SUCH THAT THERE IS NO SPACE BETWEEN THE FLUID AND THE SUBSTRATE. AN APERTURED TOP MEMBER IS THEN MOUNTED ON THE SUBSTRATE AND THE HOLDING DEVICE TO RETAIN THE SUBSTRATE ON THE FLUID. THE ASSEMBLY OF THE HOLDING DEVICE AND TOP MEMBER WITH THE SUBSTRATE THEREBETWEEN IS IMMERSED AND ROATED IN AN ETCHANT TO THIN THE BBACKSIDE OF THE SUBSTRATE THROUGH THE APERTURE OF THE TOP MEMBER. THE FLUID PROVIDES A TIGHT SEAL ON THE FACE OF THE SUBSTRATE TO PREVENT THE ETCHANT FROM DAMAGING SUCH FACE AND THE DIODE ARRAYL THEREON A VENT EXTENDING FROM THE CAVITY OF THE HOLDING DEVICE TO ITS OUTER EDGE ASSIST IN THE SUBSEQUENT REMOVAL OF THE SUBSTRATE FROM THE CAVITY.