Semiconductor device manufacturing: process – Chemical etching – Liquid phase etching
Reexamination Certificate
1999-06-07
2001-01-23
Powell, William (Department: 1765)
Semiconductor device manufacturing: process
Chemical etching
Liquid phase etching
C438S458000, C438S460000
Reexamination Certificate
active
06177359
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to the fabrication of semiconductor materials, and, more particularly, to a method for detaching and transferring an epitaxially grown layer from one substrate to another substrate.
BACKGROUND OF THE INVENTION
A building block of many electronic devices such as diodes, transistors, and lasers is semiconductor material that can be grown over a substrate. The semiconductor material is fabricated by growing an epitaxial layer of the semiconductor material upon a substrate. The substrate material may be, and frequently is, of a different composition and lattice parameter than the material used to grow the epitaxial layer. For example, an epitaxial layer of gallium nitride (GaN) may be grown upon a sapphire substrate.
Various light emitting structures may be fabricated from the semiconductor material. To complete these structures, it is often desirable to detach and transfer the epitaxial layer from the original substrate to another substrate. For example, when fabricating an edge-emitting laser, it is necessary to cleave the epitaxial layer to obtain a reflecting surface. Cleaving is a process by which an edge of a wafer, including a substrate and an epitaxial layer, is nicked, or scored, and the wafer snapped to expose a clean, smooth cross-section of the epitaxial material along the lattice facet. A parallel pair of facets are used as reflective mirrors in an edge-emitting semiconductor laser.
Cleaving, however, is difficult to perform while the GaN epitaxial layer is attached to a sapphire (Al
2
O
3
) substrate. Therefore, when a GaN epitaxial layer is grown upon sapphire, it is desirable to separate the GaN from the sapphire substrate. This is so because the sapphire substrate does not cleave easily, and also because the sapphire and the GaN epitaxial material have different lattice constants, resulting in lattice planes that do not accurately align. This prevents obtaining a clean, parallel facet of the epitaxial layer by cleaving if the epitaxial layer and the sapphire substrate are attached to each other. Therefore, for edge-emitting lasers it is desirable to detach and transfer the epitaxial layer from the sapphire substrate to a different substrate, such as silicon (Si), that is more easily cleaved.
A vertical cavity surface emitting laser (VCSEL) generally includes a region of multiple quantum well layers composed of very thin alternating layers of GaN and AlGaN around which are formed distributed Bragg reflectors.
In conventional VCSELs, at least the DBR between the substrate and the quantum-well layers is formed from alternating GaN and AlGaN layers so that the quantum well layers can be grown on crystalline material with about the correct lattice parameters. However, crystalline GaN/AlGaN DBRs are difficult to make with sufficient reflectivity. Dielectric DBRs composed of alternating layers of two dielectric materials such as silicon dioxide (SiO2) and hafnium oxide (HfOx) are easier to make with the required reflectivity, but quantum well layers cannot be grown on top of these dielectric layers.
It would be desirable to be able to grow a partial VCSEL structure, including a first cladding layer, a set of quantum well layers, a second cladding layer and a first dielectric DBR, on one substrate and transfer it to another substrate. This places the second cladding layer and the first DBR in contact with the new substrate and exposes the first cladding layer, over which could then be grown a second dielectric DBR.
In the past, epitaxial layers have been separated from sapphire substrates by using a laser to melt the epitaxial layer at its interface with the substrate on which it is grown. In the case of a GaN epitaxial layer grown over a sapphire substrate, the output of an ultra-violet laser is directed through the sapphire substrate and melts a thin layer of the GaN near the GaN/sapphire interface, thus enabling the GaN epitaxial layer to be separated from the sapphire substrate.
A drawback with using a laser to separate an epitaxial layer from the substrate is that due to the limitation of the laser spot size, only a small portion of the epitaxial layer may be separated at a time.
Another drawback with using a laser to separate the epitaxial layer from the substrate is that it is difficult to control the penetration depth of the laser light. This may result in a portion of the GaN epitaxial layer being rendered potentially unusable due to surface roughening and thermal shock within the material. The penetration depth is difficult to control because the heat dissipated at the GaN/sapphire interface cannot be precisely controlled.
Thus, an unaddressed need exists in the industry for a more controllable, practical method for detaching an epitaxial layer from a substrate and transferring the epitaxial layer to another substrate.
SUMMARY OF THE INVENTION
The invention provides a method for detaching an epitaxial layer from one substrate and transferring it to another substrate. Although not limited to these particular applications, the method for detaching an epitaxial layer from one substrate and transferring it to another is particularly suited for detaching an epitaxial layer of a GaN material from a sapphire substrate and transferring it to a substrate of another material such as silicon (Si). The GaN material can include members of the Group III-V family including, but not limited to, gallium nitride (GaN), indium gallium nitride (InGaN), indium nitride (InN), aluminum gallium nitride (AlGaN), aluminum nitride (AlN), aluminum indium gallium nitride (AlInGaN), gallium arsenide nitride (GaAsN), indium gallium arsenide nitride (InGaAsN), aluminum gallium arsenide nitride (AlGaAsN), gallium phosphide nitride (GaPN), indium gallium phosphide nitride (InGaPN), aluminum gallium phosphide nitride (AlGaPN), etc.
The invention can be conceptualized as a method for detaching an epitaxial layer from one substrate and transferring the epitaxial layer to another substrate, the method comprising the steps of: growing a first epitaxial layer over a first substrate; depositing a mask over a portion of a surface of the first epitaxial layer; growing a second epitaxial layer over the first epitaxial layer; forming a trench in the second epitaxial layer to expose the mask; bonding a second substrate to the second epitaxial layer; and introducing an etchant through the trench to etch the mask to detach the second epitaxial layer from the first epitaxial layer.
When employed in connection with the fabrication of an epitaxial layer over a sapphire substrate, the above-mentioned method for detaching an epitaxial layer from a substrate and transferring it to another substrate results in a stable, uniform epitaxial layer, suitable for fabricating high quality optical devices.
The invention has numerous advantages, a few which are delineated below merely as examples.
An advantage of the invention is that it allows the formation of high quality facets, suitable for the production of high quality optical devices.
Another advantage of the invention is that it allows VCSELs to be fabricated that incorporate high-reflectivity dielectric DBRs grown on a detached, exposed surface of an epitaxial layer.
Another advantage of the invention is that it improves the current distribution within an optical device.
Another advantage of the invention is that it is simple and easily implemented on a mass scale for commercial production.
REFERENCES:
patent: 5073230 (1991-12-01), Maracas et al.
patent: 5328549 (1994-07-01), Bozler et al.
patent: 5376229 (1994-12-01), Miller et al.
patent: 5683596 (1997-11-01), Kaneko
patent: 5710057 (1998-01-01), Kenney
Beaumont, et al., “Lateral Overgrowth of GaN on Patterned GaN/Sapphire Substrate via Selective Metal Organic Vapour Phase Epitaxy: A Route to Produce Self Supported GaN Substrates”, Journal of Crystal Growth, 189/190, 1998, pp. 97-102.
Shuj Makamura, “InGaN/GaN/AlGaN-based Laser Diodes Grown on Free-Standing GaN Substrates”, Material Science and Engineering, B59, 1999, pp. 370-375.
Chen Yong
Wang Shih-Yuan
Agilent Technologie,s Inc.
Powell William
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