Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal
Reexamination Certificate
2001-03-28
2004-01-13
Mulpuri, Savitri (Department: 2812)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
C438S033000, C438S798000, C438S463000
Reexamination Certificate
active
06677173
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to group III nitride semiconductor devices (hereinbelow, also expressed simply as “devices”), and more particularly to a method of fabricating a semiconductor laser devices which employs a group III nitride material system.
2. Description of the Related Art
A laser device requires a pair of reflectors or reflecting mirrors for forming an optical resonator to operate. In the case of fabricating semiconductor laser devices (of Fabry-Perot type) using the semiconductor materials such as GaAs etc., the reflecting mirrors are mostly formed by the cleavage of GaAs crystal substrates.
The crystal system of group III nitride semiconductors is one similar to a hexagonal system, called “wurtzite type”, unlike the sphalerite type of group III-V semiconductors, but it also has a definite cleavage plane. It is accordingly the best to form a laser device structure on, for example, the GaN bulk crystal substrate.
However, in the case of fabricating a semiconductor laser device by the use of the group III nitride materials, a nitride bulk crystal to be employed as the substrate has not been produced yet. Therefore, the device is inevitably fabricated by expitaxially growing a nitride crystal film as an underlayer on a different kind of substrate of sapphire, SiC or the like.
Heretofore, as methods of fabricating the reflector surfaces of nitride lasers on substrates, the following four 1)-4) have been known:
1) A laser structure of grown nitride films is fabricated on a substrate, and it is shaved by dry etching such as reactive ion etching (RIE), thereby to obtain reflector surfaces. Fabricating a laser structure by growing nitride films on a substrate, then forming reflector mirrors by dry etching such as reactive ion etching (RIE).
2) Growing nitride films the C-plane, namely, (0001) plane or the A-plane, namely, (11{overscore (20)}) plane (hereafter referred to as (11-20) plane)of a sapphire substrate, and splitting the wafer along the (1{overscore (100)}) plane (hereafter referred to as (1-100) plane) or (1{overscore (102)}) plane (hereafter referred to as (1-102) plane) of sapphire substrate, thereby obtain reflector mirrors.
3) Growing a laser structure on a SiC substrate, and thinning the back surface thereof, and cleaving the resultant structure along with the substrate, thereby obtaining reflector mirrors.
4) After growing a thick, for example 100 &mgr;m-thick GaN film on a sapphire substrate, removing the sapphire substrate by grinding or lapping, then using the remaining GaN film a substrate crystal on which a laser structure is formed.
Favorable single-crystal films have ever been obtained on the C-plane and A-plane of sapphire. The sapphire substrate is very difficult to be split as compared with a GaAs substrate etc. which have hitherto been employed for a semiconductor laser etc. It has therefore been common practice to avoid the method based on the cleavage, and to obtain the reflective surfaces by the etching (RIE). The sapphire does not have a clear cleavage plane like those of Si, GaAs etc. Regarding the C-plane, however, the sapphire can be tentatively split along the (1-100) plane. Also, regarding the A-plane, it can be split along the (1-102) plane, namely, a so-called “R-plane” favorably in a state considerably close to the cleavage of the ordinary crystal.
Nevertheless, the respective methods 1)-4) have disadvantages as stated below.
Regarding the forming method 1) which employs the RIE, it is difficult to obtain reflective surfaces perpendicular to the waveguide, and also hard to obtain smooth surfaces favorable for the reflector mirrors. Accordingly, the method 1) has the problem that the far field image of emitted light forms multiple spots. In particular, the formation of the multiple spots of the emitted light is ascribable to the fact that the sapphire cannot be effectively etched even by the dry etching such as RIE. As shown in
FIG. 1
, in a laser device with the reflector surfaces
2
formed by etching, portion of the emitted beam is reflected by the part of a sapphire substrate
3
indicated by (S) in the figure (the part left without being etched), and the reflected light interferes with the main beam, so that the far field image forms the multiple spots. The formation of the multiple spots in the far field image is fatal to a light source for an optical disk system, and hence, a laser device thus fabricated cannot be put into practical use at all.
In the case of forming method 2), the growth on the sapphire C-plane has the troublesomeness that the structure cannot be split unless the sapphire substrate is thinned by polishing the back surface thereof, and has the problem of low reproducibility in the splitting process. These problems are ascribable to the fact that the sapphire (1-100) plane is not the genuine cleavage plane. Since the sapphire is a very hard crystal, it cannot be split along scribing lines without being thinned. More specifically, when it is intended to obtain split surfaces which are practical for a laser device, the sapphire substrate needs to be thinned down to about 100 &mgr;m. In the case of polishing the back surface of the substrate on which a laser structure has already been formed the wafer is warped and distorted by the difference between the thermal expansion coefficients of the sapphire and nitrides, or by a residual stress attendant upon the polishing. On account of the warp and the distortion, wafer breakage is very prone to occur during the polishing process. This is very disadvantageous for mass production.
The crystal orientation of the GaN grown on the sapphire C-plane rotates by 30 degrees relative to that of the substrate. Accordingly, when the sapphire substrate is split along the (1-100) plane, the overlying GaN is to be split along the (11-20) plane. Since the cleavage plane of a GaN crystal is the (1-100) plane, the GaN is somewhat forced to be split along the crystal plane not being the cleavage plane, in this case. Owing to the symmetry of the GaN crystal, however, a very good fissured surface is obtained when the splitting is in a direction precisely along the (11-20) plane.
Meanwhile, since the (1-100) plane is not the cleavage plane, the sapphire can also be split even when a scribing line is drawn with a deviation. In this case, the GaN is to be split in a direction deviating from the (11-20) plane.
Therefore, low reflectivity and irregularity in the wave front of emitted light are incurred to deteriorate the quality of the mirror facet for a laser.
Further, the growth on the sapphire A-plane in the forming method 2) has the problem that the quality of the fissured surface of the GaN is unsatisfactory.
Since the R-plane being the (1-102) plane which is the parting plane of the sapphire, the A-plane sapphire can be easily parted even with a thickness of 250-350 &mgr;m ordinarily applied to a substrate. However, in the case where, as shown in
FIG. 2
, a laser structure is formed on the A-plane of the sapphire substrate and is parted from the direction indicated by an arrow, a plurality of fine striations appear on the side surface of the GaN. The appearance of the plurality of striations is ascribable to the fact that the sapphire substrate constitutes most of the thickness of the wafer and the cracks therefore propagates along the R-plane of sapphire crystal. Although the sapphire substrate cracks along its R-plane, the (1-100) plane of the GaN grown on the sapphire A-plane deviates by 2.4 degrees from the sapphire R-plane. Therefore, even after the crack has reached a sapphire/GaN interface, it propagates into the GaN crystal along the R-plane of the underlying sapphire to a slight. However, the GaN tends to split along the (1-100) plane being its cleavage plane, such a plurality of (1-100) planes form a stepped fissured surface. Therefore, the second method of fabricating the reflector surfaces in accordance with the growth on the sapphire A-plane is also disadvantageous in that the quality of the fissured surface of the Ga
Mulpuri Savitri
Pioneer Corporation
Sughrue & Mion, PLLC
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