Solid State Scientific Corporation (SSSC) in cooperation with its partners is currently developing an imaging LADAR receiver using a novel epitaxial layer transfer (ELT) integration technique developed by AFRL/SNHC. ELTis an alternative to indium bump bonding for integrating hybrid components to electronic circuitry.

ELT allows the integration of an entire, unprocessed, epitaxial structured optoelectronic wafer (LED, LASER, Photodiode, etc.) with any application specific circuit or array of circuits. This process is virtually alignment-free. Subsequently, devices or circuits are fabricated directly on the host wafer using standard photolithography. The transferred epilayer is thin and semi-transparent, allowing precise alignment of arbitrarily small optical devices to the host circuit, limited only by lithographic constraints. The formed structures (devices) are individually isolated thereby reducing all resistive, capacitive and inductive parasitics between the devices (passive or active) and the circuit. The resulting integrated subsystem is robust, and any potential strain due to differences in thermal coefficients is minimized. Figure 1 illustrates the basic concept.

 

 
Fig.1: Basic concept of the epitaxial transfer process.
 
 
 
 
  1. A III-V epi-wafer and a circuit wafer are placed in contact and integrated

  2. The InP substrate handle wafer is removed leaving only the epilayer on
    the circuit wafer.

  3. The epilayer is patterned using conventional photolithography.

  4. Optically isolated photodetectors fully integrated into the ASIC.
 

A first-generation LADAR receiver is shown in Fig. 2. This is an array of VSD detectors integrated to a 256 x 256 LADAR read-out integrated circuit (ROIC) using the ELT technique. A portion of the integrated array is shown in Fig. 2(a).

Fig. 2(b) illustrates the cross-sectional view of the ELT bond and Fig. 2(c) illustrates an electrically active pixel.

 
   
 
Fig. 2: Secondary electron microscopy image of a) LADAR ROIC
and detector epilayers after integration, b) four pixels after integration
and c) one integrated pixel with top metal for electrical contact.
 
 

To complement ELT integration, a suite of InGaAs-based photodetectors has been developed that includes PINs, APDs, and MSMs capable of operation beyond 10 GHz. This photodetector technology is compatible with ELT. No significant differences are observed when devices of the same geometries and design characteristics are processed by both conventional methods and by the ELT method.

 
   
     
 
Program Partners: