We have fabricated MIM capacitors for MMIC applications, with capacitances as high as 600pF/$ extrm{mm}^2$ and excellent electrical properties of the insulator layer. Silicon nitride thin film is the desirable material for MMIC capacitor fabrication. Standard MIM capacitance in MMIC is 300pF/$ extrm{mm}^2$ with an insulator layer thickness of more than 2000$AA$. However, capacitors with thin insulator layers have breakdown voltages as low as 20V. We have deposited insulator layers by PECVD in our MIM structure with an air bridge between the top metal and the contact pad. The PECVD process was optimized for fabricating the desired capacitors to be used in MMIC. Silicon nitride(Si$_{x}$N$_{y}$) thin films of about 1000$AA$ thick show capacitances of about 600pF/$ extrm{mm}^2$, and breakdown voltages above 70V at 100nA.A.A.