Analog integrated circuit of a low level high voltage current source with capacitive load compensation

Abstract

Atomic Force Microscopes (AFM) are used to sense the deflections on crystalline surfaces and convert the information gathered to an image of surface map. In a newly proposed interferometric sensor, cantilevers are micromachined into the shape of interdigitated fingers to form a diffraction grating and the tip displacement is determined by measuring the intensity of diffracted modes. Detection of modes is achieved by means of a photodetector (PD) array mounted just above the cantilevers. For this purpose, a PD array of 2x8 composed of MSM-PDs is designed and manufactured. A (p superscript plus)/n PD is also designed for testing. In addition, a high voltage low level current source is required to drive the tips of the cantilevers for the purpose of lithography. The current source designed for this purpose can supply 10nA current through the resistive cantilever in spite of an existing capacitance at load. Current source includes an internal capacitive current compensation circuit to fix the current of resistive load at 10nA. Test of design circuit is done and prototypes of chip are fabricated using Alcatel Mietec 12T CMOS process. PSD array and current source are supposed to be prototypes of the related blocks in AFM.