Lithography with infrared illumination alignment for advanced BiCMOS backside processing

Driven by new applications such as BiCMOS embedded RF-MEMS, high-Q passives, Si-based microfluidics for bio sensing and InP-Si BiCMOS heterointegration [1-4], accurate alignment between back and front side is highly desired. In this paper, we present an advanced back to front side alignment technique and implementation of it into the back side processing module of IHP's 0.25/0.13 μm high performance SiGe:C BiCMOS technology. Using the Nikon i-line Stepper NSR-SF150, a new infrared alignment system has been introduced. The developed technique enables a high resolution and accurate lithography on the back side of the BiCMOS-processed Si wafers for additional backside processing, such as backside routing metallization. In comparison to previous work [5] with overlay values of 500 nm and the requirement of two-step lithography, the new approach provides significant improvement in the overlay accuracy with overlay values of 200 nm and a significant increase of the fabrication throughput by eliminating the need of the two-step lithography. The new non-contact alignment procedure allows a direct back to front side alignment using any front side alignment mark (Fig. 2), which generated a signal by reflecting the IR light beam. Followed by a measurement of the misalignment between both front to back side overlay marks (Fig. 3) using EVG®NT40 automated measurement system, a final lithography process with wafer interfield corrections is applied to obtain a minimum overlay of 200 nm. For the specific application of deep Si etching using Bosch process, the etch profile angle deviation across the wafer (tilting) has to be considered as well. From experimental data, an etch profile angle deviation of 8 μm across the wafer has been measured (Fig. 7). The overlay error caused by tilting was corrected by optimization and adjustment of the stepper offset parameters. All measurements of back to front side misalignment were performed with the EVG®40NT automated measurement system whereas the deep etch tilting errors were measured with an optical microscope using special vernier scales embedded in the backend-of-line metallization layer (Fig 4 and Fig. 5) of the IHP's 0.25/0.13 μm SiGe:C BiCMOS technology. By applying the proposed method of back to front side alignment using infrared illumination alignment, the accuracy of backside fabrication processes like deep Si etching can be significantly improved. The developed technique is very promising to shrink the dimensions by minimizing the back to front side misalignment to improve the device performance of backside integrated components and technologies.