Off-axis epitaxial lift off process
Embodiments described herein provide processes for forming and removing epitaxial films and materials from growth wafers by epitaxial lift off (ELO) processes. In some embodiments, the growth wafer has edge surfaces with an off-axis orientation which is utilized during the ELO process. The off-axis orientation of the edge surface provides an additional variable for controlling the etch rate during the ELO process- and therefore the etch front may be modulated to prevent the formation of high stress points which reduces or prevents stressing and cracking the epitaxial film stack. In one embodiment, the growth wafer is rectangular and has an edge surface with an off-axis orientation rotated by an angle greater than 0° and up to 90° relative to an edge orientation of <110> at 0°.
1. A method for forming an epitaxial film stack during an epitaxial lift off process, the method comprising:
providing a growth wafer made of gallium arsenide and diced from a round wafer at a predetermined angle to produce a plurality of edge surfaces on the growth wafer, each edge surface being oriented in a direction other than that of a cleavage plane of the round wafer that is indicative of a crystallographic orientation of the round wafer, wherein none of the plurality of edge surfaces has an orientation that is perpendicular to the crystallographic orientation of the round wafer indicated by the cleavage plane, and wherein none of the plurality of edge surfaces has an orientation that is parallel to the crystallographic orientation of the round wafer indicated by the cleavage plane;
growing a sacrificial layer over the growth wafer;
forming the epitaxial film stack over the sacrificial layer;
removing the sacrificial layer by a lateral etching process, the lateral etching process including a first etch rate of the sacrificial layer near corners of the plurality of edge surfaces that is faster than a second etch rate of the sacrificial layer near sides of the plurality of edge surfaces; and
separating the epitaxial film stack from the growth wafer after complete removal of the sacrificial layer.
2. The method of claim 1 , wherein the orientation of each of the edge surfaces includes an off-axis orientation that is at an angle greater than 0° and less than 90° with respect to the crystallographic orientation of the round wafer, the crystallographic orientation being a <110> crystal lattice orientation.
3. The method of claim 1 , wherein the lateral etching process produces an etch front of a wet etch solution that encompasses the sacrificial layer at the interface of the wet etch solution and the sacrificial layer.
4. The method of claim 3 , wherein the etch front forms substantially octagonal geometry comprising alternating sides of a plurality of side edge fronts and corner edge fronts.
5. The method of claim 4 , wherein the corner edge fronts advance at the first etch rate of the sacrificial layer near the corners of the plurality of edge surfaces and the side edge fronts advance at the second etch rate of the sacrificial layer near the sides of the plurality of edge surfaces.
6. The method of claim 4 , wherein the plurality of side edge fronts and corner edge fronts converge at a center of the sacrificial layer at a same time to complete the removal of the sacrificial layer.
7. The method of claim 3 , wherein the geometry of the etch front of the sacrificial layer transitions during the epitaxial lift off process from a substantially rectangular geometry, to a substantially octagonal geometry, and then to a substantially rounded geometry.
8. The method of claim 3 , wherein the sacrificial layer has a rectangular geometry and comprises side edges and corners prior to being exposed to the wet etch solution.
9. The method of claim 1 , wherein the predetermined angle is 45° and corresponds to an off-axis orientation of the growth wafer relative to the round wafer.
10. The method of claim 1 , wherein the first etch rate is 1.4 times the second etch rate.