The present invention relates to a method of etching a semiconductor substrate and, more particularly, it relates to a process of anisotropic etching.
In a conventional process for fabricating a single-crystal semiconductor substrate, there has widely been used a so-called anisotropic etching technique. To etch the single-crystal substrate non-isotropically, this technique uses a solution with which individual surface regions of the single-crystal substrate are etched at different rates depending on their respective directions, based on the fact that each surface plane has different activation energy in accordance with its crystallographic direction.
FIGS. 4(a) to 4(g) illustrate an example of the conventional process for anisotropically etching the semiconductor substrate.
As shown in FIG. 4(a), a bare silicon substrate 1 having surface plane (100) is prepared. First, the surface of the silicon substrate 1 composed of bare silicon is thermally oxidized so as to form a SiO.sub.2 film 2 (see FIG. 4(b)). Next, a first photoresist 3 is applied onto the SiO.sub.2 film 2 (see FIG. 4(c)). The resulting first photoresist 3 is then patterned so that a specified opening A is formed in a part of the photoresist 3 (see FIG. 4(d)). After etching away the pare of the SiO.sub.2 film 2 which underlies the opening A (FIG. 4(e)), the first photoresist 3 is removed (FIG. 4(f)).
Subsequently, the entire substrate is subjected to first-time anisotropic etching so that the silicon substrate 1 underlying the opening A of the SiO.sub.2 film 2 is grooved, thereby forming a first etched region 4 which is hollow with a specified difference in level (see FIG. 4(g)).
As the solution for anisotropically etching the silicon substrate 1, ethylene diamine, potassium hydroxide, pyrocatechol, and the like are known. An etching mask which is not dissolved in these solutions is usually composed of a SiO.sub.2 film.
Thus, the conventional process comprises the steps of thermally oxidizing the silicon substrate 1 so as to form the SiO.sub.2 film 2 composed of a thermal oxidation film, patterning the resulting SiO.sub.2 film 2 by using a normal photolithographic technique, and finally etching the substrate with the patterned SiO.sub.2 film serving as an etching mask by immersing it in the anisotropic etchant solution. If the patterned SiO.sub.2 film serving as an etching mask is formed with a window region having as its side the (110) direction of the silicon substrate, the resulting etched region 4 is in the form of a truncated pyramid or a pyramid having the (111) plane (forming a 54.degree. angle with respect to the main face of the substrate) as its slant faces, for the (100)-Si substrate is used.
A first problem of the prior art mentioned above arises when the anisotropic etching step is repeated more than once.
That is, after performing the first anisotropic etching, the surface of the silicon substrate 1 becomes rugged as shown in FIG. 5, so that, if a second photoresist 8 is to be applied by spin coating, a discontinuity is easily generated in the photoresist 8 at the edge portion 6 of the first etched region 4. The discontinuity results from the increasing thinning of the photoresist at the edge portion 6, which is caused while the second photoresist 8 is applied by spin coating, for the second photoresist 8 is spread by utilizing a centrifugal force.
The generation of the discontinuity can be prevented to some extent by adjusting the rotational speed for spin coating. However, if the depth of the groove formed by anisotropic etching reaches several 10 .mu.m, the number of revolutions should be reduced to the order of 1000 rpm. If the number of revolutions is thus reduced, the discontinuity is not generated at the edge portion 6, whereas other problems such as the unsatisfactory spreading of the second photoresist 8, non-uniform thickness of the photoresist, and inability to perform fine photolithography may occur, so that was difficult to eliminate the discontinuity.
On the other hand, second-time anisotropic etching is performed according to the steps illustrated in FIGS. 6(a) to 6(f), which will be described below:
When the first-time anisotropic etching is completed, the SiO.sub.2 film 2, which served as the first etching mask, is removed from the surface of the silicon substrate 1 (see FIG. 6(a)), followed by the thermal oxidation of the entire silicon substrate 1, so as to form a SiO.sub.2 film 15, composed of a thermal oxidation film, which will serve as a second etching mask (see FIG. 6(b)). The second photoresist 8 is applied onto the surface of the SiO.sub.2 film 15 and then formed into a pattern having a second opening B (see FIG. 6(c)). At this stage, however, the discontinuity mentioned above was already generated at the edge portion 6 of the first etched region 4.
If the second-time anisotropic etching is performed with the discontinuity remaining at the edge portion 6, after performing the etching process for patterning the SiO.sub.2 film 15 (see FIG. 6(d)), the second photoresist 8 is removed (see FIG. 6(e)) and then the silicon substrate 1 underlying the opening B is grooved by anisotropic etching, thus forming a second etched region 10 (see FIG. 6(f)).
In this process, however, the silicon substrate 1 at the edge portion 6 of the first etched region 1 is also anisotropically etched, as shown in FIG. 6(f), so that the first etched region 4 is disadvantageously deformed.
Thus, if the SiO.sub.2 film 15 is patterned by etching so as to form a second etching pattern with the discontinuity of the second photoresist 8 remaining at the edge portion 6 of the first etched region 4, the SiO.sub.2 film at the edge portion 6, which should serve as the mask, is etched away, so that the edge portion 6 undergoes anisotropic etching in the subsequent anisotropic etching process, resulting in the deformation of the first etched region 4.
In the case where the second etched region is formed inside the first etched region 4 so as to obtain a silicon substrate of cross-sectional structure shown in FIG. 3, the first etched region 4 is also deformed through the same process as mentioned above, resulting in a second problem.
As shown in FIGS. 7(a) to 7(d), to realize the structure shown in FIG. 3, the silicon substrate 1 (see FIG. 7(a)) which has gone through the first-time etching (see FIGS. 4(a) to 4(g)) is thermally oxidized so as to form a SiO.sub.2 film 20 (FIG. 7(b)). Then, the second photoresist (not shown) having the second opening B which extends from the left-hand edge 6a of the first etched region 4 is formed. The resulting second photoresist is used as a photomask in patterning the SiO.sub.2 film 20 by etching, thereby forming the second etching mask (see FIG. 7(c)).
Even when the second photoresist is applied without generating a discontinuity at the edge portion 6, the edge portion 6a does not completely match the end portion of the mask, because, in most cases, there is the displacement of the mask .DELTA.d due to the imperfect precision with which the mask is aligned with respect to the edge portion 6a. If anisotropic etching is performed with such a mask displacement, the vicinity of the edge 6a, which corresponds to the edge portion 6 in above FIG. 4(f), is also etched, so that the first etched region is disadvantageously deformed (see FIG. 6(d)).
In this case, even if the width .DELTA.d of the mask displacement can be reduced to, e.g., 0.5 .mu.m, the mask displacement can not be ignored because a flat plane is required on the level of the wavelength of light in the case of using the slant face of the etched region as, e.g., a reflecting mirror for a near infrared ray. Moreover, a real photoresist film has large waviness, so that it is difficult in practice to reduce the width .DELTA.d of the mask displacement to 0.5 .mu.m or less.