Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2002-08-27
2004-12-07
Leung, Quyen (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
Reexamination Certificate
active
06829270
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a nitride III-V compound semiconductor substrate, its manufacturing method, a method of manufacturing a semiconductor light emitting device and a method of manufacturing a semiconductor device, which are especially suitable for use in manufacturing semiconductor lasers and light emitting diodes, or electron transporting devices.
2. Description of the Related Art
In recent years, semiconductor lasers using nitride III-V compound semiconductors such as AlGaInN (hereinbelow called GaN compound semiconductor lasers) have been under active research and development in the hope of making semiconductor lasers capable of emitting light over the range from the blue region to the ultraviolet region necessary for enhancing the density of optical discs. Lately, efforts are being expended to farther improve their lifetimes and properties toward their practical use.
When manufacturing such a GaN compound semiconductor laser, a laser structure is most typically formed by crystal growth of a GaN compound semiconductor layer on a sapphire substrate. For example, using a sapphire substrate sized 50 mm (2 inches) in diameter and 430 &mgr;m in thickness, a GaN compound semiconductor layer is grown thereon up to a thickness around 7 &mgr;m in total.
However, if a GaN compound semiconductor has a thickness around 7 &mgr;m on a sapphire substrate as mentioned above, the sapphire substrate warps due to a difference in thermal expansion coefficient between the sapphire and the nitride III-V compound semiconductor such as GaN. This warpage measures as large as 80 &mgr;m.
This large warpage of the sapphire substrate adversely works against exposure in a manufacturing process of a GaN compound semiconductor laser and polishing of the bottom surface of the sapphire substrate.
More specifically, in the exposure process, the sapphire substrate, having GaN compound semiconductor layers grown thereon and a resist coated on its surface, undergoes exposure through a photo mask. If the sapphire substrate largely warps as mentioned above, distance between the photo mask and the resist may become uneven within the area of the substrate, or a dimensional deviation may be produced between the photo mask and the substrate within the area of the substrate. Thus the mask cannot accurately fit the entire surface of the substrate. As a result, especially when a base GaN layer is laterally grown on the sapphire substrate by ELO (epitaxial lateral overgrowth) and GaN compound semiconductor layers forming a laser structure are grown thereon by crystal growth, it is difficult to form a ridge in a less-defective region (wing portion) between a seed crystal and a coalescing portion of the lateral growth, and the ridge often deviates from the wing portion. Therefore, this problem adversely affects the laser properties, and in particular, its lifetime, and also degrades the production yield.
For making cavity edges, it is the most usual way to cleave a sapphire substrate having GaN compound semiconductor layers grown thereon. For easier cleavage, it is necessary to thin the sapphire substrate by partly removing it from the bottom by polishing. However, if the sapphire substrate largely warps as mentioned above, it often cracks during polishing.
Furthermore, if the warpage of the sapphire substrate is large during crystal growth of the GaN compound semiconductor layers, because of uneven temperature distribution along the plane, the resulting GaN compound semiconductor layers become uneven in composition and thickness.
Under the circumstances, there is a demand for a technique capable of sufficiently reducing warpage of substrates, namely, not to exceed 70 &mgr;m, to remove the above-mentioned problems caused by warpage of sapphire substrates.
OBJECTS AND SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a nitride III-V compound semiconductor substrate and its manufacturing method capable of limiting warpage of substrates not to exceed 70 &mgr;m.
A further object of the invention is to provide a method of manufacturing a semiconductor light emitting device that can be used when manufacturing the semiconductor light emitting device by using a nitride III-V compound semiconductor substrate made by forming a nitride III-V compound semiconductor layer on a substrate of a material different from the nitride III-V compound semiconductor layer, and can limit warpage of the substrate not to exceed 70 &mgr;m, thereby to successfully carry out exposure in the lithographic process and polishing of the bottom surface of the substrate.
A still further object of the invention is to provide a method of manufacturing a semiconductor device that can be used when manufacturing the semiconductor device by using a nitride III-V compound semiconductor substrate made by forming a nitride III-V compound semiconductor layer on a substrate of a material different from the nitride III-V compound semiconductor layer, and can limit warpage of the substrate not to exceed 70 &mgr;m, thereby to successfully carry out exposure in the lithographic process and polishing of the bottom surface of the substrate.
The Inventor conducted vigorous research to solve the above-indicated problems. An outline of the research is shown below.
GaN layers were grown on sapphire substrates sized 430 &mgr;m and 700 &mgr;m in thickness, and 50 mm in diameter.
FIG. 1
shows measured values of thickness of GaN layers and measured values of warpage (H) of the sapphire substrates. For growth of the GaN layers, metal organic chemical vapor deposition (MOCVD) was used. It is appreciated from
FIG. 1
that the warpage of the sapphire substrate increases proportionally to the thickness of the GaN layer.
In case a laser structure of GaN compound semiconductor layers is formed on the sapphire substrate, for the purpose of forming a less-defective layer by using a lateral growth technique such as ELO or preventing the operation voltage from increasing when both the n-side electrode and the p-side electrode are formed on a common plane, thickness of the n-type GaN layer grown as the base layer of the laser structure on the sapphire substrate is preferably not smaller than 3 to 5 &mgr;m. However, if the GaN layer is grown by 5 &mgr;m on the sapphire substrate sized 430 &mgr;m in thickness and 50 mm in diameter, the warpage exceeds 70 &mgr;m as shown in FIG.
1
. In contrast, if the GaN layer is grown by 5 &mgr;m on the sapphire substrate sized 700 &mgr;m in thickness and 50 mm in diameter, the warpage largely decreases to around 30 &mgr;m. As such, when GaN compound semiconductor layers forming a laser structure (cladding layer, waveguide layer, active layer, and so on, which are approximately 2 &mgr;m thick in total) are formed on a 5 &mgr;m thick base GaN layer, the increase of warpage of the substrate is small. That is, warpage largely depends on the thickness of the base GaN layer.
FIG. 2
shows measured values of warpage of diametrically 50 mm long and x(&mgr;m) thick sapphire substrates having y(&mgr;m) thick GaN layers thereon, putting Z=y/x on the abscissa and warpage H(&mgr;m) on the ordinate. It is appreciated from
FIG. 2
that the warpage reaches and surpasses 80 &mgr;m under Z in excess of 0.013, and it adversely affects the mask-fitting in the photolithographic process and polishing of the substrate bottom surface. If y/x is adjusted not to exceed 0.011, the warpage can be limited not to exceed 70 &mgr;m. To ensure that the warpage does not exceed 40 &mgr;m to enhance the production yield, Z must be 0.006 or less.
Basic data of
FIGS. 2 and 3
is collectively shown in FIG.
3
.
Also from the viewpoint of improving evenness of the temperature distribution along the substrate plane during crystal growth, warpage of the substrate must be minimized. Evenness of the temperature distribution is required for ensuring evenness of the Al composition distribution of the cladding layer of the GaN compound semiconductor laser, distribution of the thickness of each layer and di
Asano Takeharu
Goto Osamu
Ikeda Shinro
Shibuya Katsuyoshi
Suzuki Yasuhiko
Kananen Ronald P.
Leung Quyen
Rader & Fishman & Grauer, PLLC
Sony Corporation
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