Diamond Membranes

One challenge that diamond continues to face is the unavailability of affordable, large area (inch-size) wafers that are suitable for applications that require high crystalline quality, such as optics, electronics and quantum. Fraunhofer USA Center Midwest (CMW), while continuously working on further lateral size expansion of SCD wafers, has developed diamond membranes that can be used to sidestep existing limitations with SCD wafers.

Semiconductor membranes are freestanding sheets of a semiconducting material that are only a few microns (µm) thick. These membranes possess the same properties as the corresponding bulk material (wafer), but the thinness of the membrane makes it flexible. This allows for seamless transfer and integration of these membranes with other semiconductor materials. Successful membrane transfer requires very smooth membrane and wafer surfaces, which can be achieved via the Fraunhofer USA CMW CMP process that was specifically developed for these types of technology. This time of assembling devices of different semiconductor materials is called heterointegration and allows for (i) upscaling of materials to wafer dimensions of mature materials, and (ii) being able to leverage intrinsic preferred properties of materials.

In case of diamond, it is possible to transfer active p-type diamond device areas onto pre-processed gallium nitride (GaN) and aluminum nitride (AlN) wafers. That way, the device will leverage diamond being a natural p-type material, while GaN and AlN are natural n-type materials. Further, it represents a size upscaling since all the diamond components can be synthesized and processed on a SCD wafer node that is currently available, and then upcycle to a multi-inch GaN or AlN wafer that is compatible with commercial microfabrication equipment.

Fraunhofer USA CMW developed a smart-cut method for creation of SCD membranes. These membranes can be tailor made in lateral dimensions and shape, thickness and doping.

Successful transfer and bonding of these SCD membranes to semiconductor materials (Si, SiC, GaN, Ga₂O₃, AlN), glass and polymers has been successfully demonstrated.

Fraunhofer USA CMW has demonstrated diode performance of p-type SCD membranes that were transferred to GaN and Ga₂O₃.

Work on SCD membranes has led Fraunhofer USA CMW to develop an alternate membrane technology using polycrystalline material. The motivation behind that was that fabrication SCD membranes is a somewhat technically challenging and costly process. That made its potential use as heat spreader fairly uneconomical.

These PCD membranes can be synthesized and processed across 4- to 8-inch wafer scale. Membrane release can be achieved through a standard and affordable wet chemical release process. These polycrystalline diamond membranes can be further processed, such as 3D structuring into heat sinks, or thinning and removal of the nucleation layer, to achieve thermal conductivity on the order of 1200 W/mK, which is still sufficient for many thermal management applications.