The vertical cavity surface emitting laser (VCSEL) element propagates in a direction perpendicular to a substrate. In comparison with an ordinary edge emitting type semiconductor laser, the vertical cavity surface emitting laser element can be achieved at a low cost, low power consumption and in a compact device, suitable for a two-dimensional device. For these reasons, the vertical cavity surface emitting laser elements attract the attention of many people.
As disclosed in, for example, U.S. Pat. No. 5,493,577, the vertical cavity surface emitting laser element has a current blocking structure and employs a selective oxidation and confined structure made of AiAs or the like.
The current blocking structure of the vertical cavity surface emitting laser element is formed by arranging a semiconductor substrate or a semiconductor member having a mesa structure of a circular shape or a rectangular or quadrangular shape in a high-temperature steam atmosphere, oxidizing it from an outer peripheral portion on a side surface of the mesa structure, made of a p-AlAs or AlGaAs layer, to the center of the mesa structure, and forming a selective oxidation layer made of a region of the oxidized AlO and an unoxidized region. The refractive index of the oxidized region of the selective oxidation layer is about 1.6, which is lower than those of the other semiconductor layers. Therefore, emitted beams are confined in the unoxidized region surrounded by the oxidized region to thereby lower a threshold value.
Meanwhile, when the size of the unoxidized region in the selective oxidation layer deviates from an optimum value, an oscillation characteristic of light output or the like is also deviated. Thus, there may occur a problem of drop in yield.
In order to avoid such a deviation of the unoxidized region of the selective oxidation layer from the predetermined size, there have been proposed several methods.
A method of indirectly monitoring the degree of oxidation using a monitored oxidation pattern is disclosed in Japanese Unexamined Patent Application Publication No. 2004-95934. With this method, a pattern of stripes is provided to monitor an oxidation rate in addition to a shape of an ordinary resonator, and reflectance of the pattern is measured inside an oxidation chamber to obtain the degree of oxidation.
A method of monitoring the degree of oxidation in real time is disclosed in Japanese Unexamined Patent Application Publication No. 2003-179309. With this method, a semiconductor member under an oxidation process is observed via an observation port with a microscope, and the degree of oxidation is estimated using contrast observed with the microscope between an oxidized region and an unoxidized region, thereby controlling the oxidation to follow.
In the method of indirectly monitoring the degree of oxidation using the monitored oxidation pattern disclosed in Japanese Unexamined Patent Application Publication No. 2004-95934, there are problems including that an area for mounting the vertical cavity surface emitting laser element is limited and cost becomes high. This is because it is necessary to make patterns around the monitored oxidation pattern blank to enable monitoring the oxidation rate with high accuracy, and an ordinary resonator cannot be mounted by this blank pattern.
Further, in the method of observing the degree of oxidation with the microscope disclosed in Japanese Unexamined Patent Application Publication No. 2003-179309, it is necessary to enhance the degree of oxidation and also shorten the distance between a semiconductor member and the microscope to focus the microscope on a portion where the mesa structure is formed. However, when the distance between the observation port and the semiconductor member is shortened, moisture vapor concentration distribution on the semiconductor member is scattered, and therefore the amount of oxidation becomes not uniform. Thus, the yield is lowered. Further, when the distance between the observation port and the semiconductor member is shortened, the refractive index of the observation port may be changed by heat emitted from a heater, and optical elements such as a lens installed in the microscope may be deformed. In these cases, the focus is deviated to degrade measurement accuracy. Thus, the yield may be lowered. Further, because it is impossible to reuse the microscope in this method, the cost may become higher.