Method for making uniform the thickness of a Si single crystal thin film

Abstract: A Si single crystal thin film is classified according to thickness into several areas such that the areas where the thin film is thicker is made oxide layer-free and the areas where the thin film is thinner is covered with oxide layer. Then, oxidation is conducted so that the thicker the thin film the lower the Si interface becomes, utilizing the different growth rates of the oxide layer in these areas. The thin film surface with a resulting staircase configuration is then leveled by the subsequent polishing treatment. In other method, oxide layer is formed in such way that the areas with a thicker thin film thickness will have a thinner oxide layer and the areas with a thinner thin film thickness will have a thicker oxide layer, and oxidation is conducted such that the thicker the thin film the lower the Si interface becomes.

Description: FIELD OF THE INVENTION This invention relates to a method for making uniform the thickness of a Si single crystal thin film, and in particular to a method for making uniform the thickness of a Si single crystal thin film with an SOI structure. BACKGROUND OF THE INVENTION One of the methods for fabricating semiconductor elements is to form a semiconductor layer on a semiconductor substrate with an insulation layer already formed directly on its surface and then fabricate semiconductor elements in the semiconductor layer. In particular, the method in which a so-called SOI (Silicon On Insulator) substrate is used is drawing attention as a method for fabricating semiconductor elements. The SOI substrate includes a Si single crystal layer on an insulation layer. The Si single crystal layer is a thin film with a thickness of several micrometers, and the accuracy of the thickness of the Si single crystal thin film depends on the mechanical accuracy of the surface grinder and the polishing accuracy. Usually, the thickness of the Si single crystal thin film is within ±0.5 micrometers of the target absolute thickness. However, in order to improve the performance of the semiconductor elements and mass-produce them with good yield, it is desirable to have the dispersion of the local thickness of the semiconductor layer such as a Si single crystal thin film within ± 0.1 micrometers of the target absolute thickness. It is impossible to sufficiently improve the performance of the semiconductor elements with the conventional dispersion of ± 0.5 micrometers. In "Proceedings of the IEEE 1987 Custom Integrated Circuits Conference, May 4 to 7, 1987. pp. 443 - 446 there is disclcsed a dielectrically isolated intelligent power switch and a manufacturing method therefor, where silicon and silicon dioxide etching is carried out from the wafer-side surface into the internal oxide or further into an adequate depth within another wafer. In "IEEE Photonics Technology Letters", Vol. 2, No. 10, Oct. 1990, pp. 721 - 723, there is disclosed a technique for the fabrication of opto-electronic integrated circuits wherein (a) a step-formed InP-substrate is fabricated by wet etching, (b) semi-conductor layers are grown on the step-formed substrate, (c) the depression in the epitaxial layer-surface is filled in with photoresist, (d) both the epitaxial layers and the photoresist are etched simultaneously with a non-selective, reactive ion beam etching (RIBE) technique in which both materials are etched at the same rate and (e) the remaining cap of photoresist is removed to complete the fabrication process. In the "Proceedings of the 4th international symposium on silicon-on-insulator technology and devices", Montreal, Canada, 8 bis 12 March 1990, pages 61 to 71, there are disclosed the surface conditions of a base wafer for void-free bonding with a SiO 2 layer in the fabrication of a bonded wafer with SCI structure for bipolar devices. There is also disclosed a method for thinning the bonded wafer by surface grinding and by conventional chemical-mechanical polishing. SUMMARY OF THE INVENTION Accordingly, it is an object of the invention to provide a method for reducing the dispersion of the thickness of a Si single crystal thin film with an SOI structure, thus making uniform the thickness of the Si single crystal thin film. According to the first feature of the invention, a method for making uniform the thickness of a Si single crystal thin film with an SOI structure includes the following steps of: (a) measuring the surface condition and the thickness unevenness of a Si single crystal thin film (13) by two-dimensional and three-dimensional images to identify the peak portions (A) having the thickest thickness from closed curve patterns of equal thickness contour curves on said two-dimensional image; (b) forming a primary Si oxide layer (14) with approximately uniform thickness on said Si single crystal thin film (13) by thermal oxidation; (c) masking the surface of said primary Si oxide layer (14) with a masking material (15) except for said peak portions (A); (d) removing said primary Si oxide layer (14) only on the surface of said peak portions (A) by an etching treatment using said masking material (15) as a mask; (e) removing said masking material (15); (f) performing on oxidation treatment to form an secondary Si oxide layer (14') on said peak portions (A) and on the remaining portions still having said primary Si oxide layer (14), whereby the downward displacement of the interface between said secondary Si oxide layer (14') and said Si single crystal thin film (13) is deeper for said peak portions (A) than for the remaining portions still having said primary Si oxide layer (14); (g) removing said primary and secondary Si oxide layers by an etching treatment to form a level gap between said peak portions (A) and the remaining portions on the surface of said Si single crystal thin film (13); and (h) mirror-polishing said Si single crystal thin film (13) to level its surface. According to the second feature of the invention, a method for making uniform the thickness of a Si single crystal thin film with an SOI structure includes the following steps of: (a) measuring the surface condition and the thickness unevenness of a Si single crystal thin film (13) by two-dimensional and three-dimensional images and classifying said surface according to the thickness of said Si single crystal thin film into n (n is an integer equal or greater than two) areas (A, B, C, D, E) in order of thickness from the first area (A) to the n-th area (E); (b) forming a primary Si oxide layer (14) with approximately uniform thickness on said Si single crystal thin film (13) by thermal oxidation; (c) masking the surface of said primary Si oxide layer (14) with a masking material (15) except for the first area (A) classified in said step (a) as having the thickest thickness of said Si single crystal thin film; (d) removing the surface of said primary Si oxide layer (14) only on said first area (A) down to a certain depth by an etching treatment using said masking material (15) as a mask creating a level gap between the etched surface of said primary Si oxide layer (14) on said first area (A) and the adjacent portion of said primary Si oxide layer (14) ; (e) repeating the steps (c) and (d) from the second area (B) to the n-th thickness area (E) whereby each time the surface of said primary Si oxide layer (14) is at first remasked with a masking material (15) except for said first area (A), the areas having a primary Si oxide layer (14) with a previously reduced thickness and the next thickest area and further removing the surface of said primary Si oxide layer (14) only on the unmasked areas down to a certain depth by an etching treatment so to create a primary Si oxide layer (14) with n level gaps between adjacent portions becoming thicker in n steps from the area (A) to the area (E); (f) removing the masking material (15); (g) performing a thermal oxidation to form a secondary Si oxide layer (14') on said first area (A) and said areas (B) to (E) still having said primary Si oxide layer (14), whereby the displacement of the interface between said secondary Si oxide layer (14') and said Si single crystal film (13) is deeper for the areas of said Si single crystal film (13) having a thinner remaining primary Si oxide layer (14) thereon; (h) removing said remaining primary (14) and secondary Si oxide layers (14') by an etching treatment to form n level gaps between the adjacent areas (A) to (E) on the surface of said Si single crystal thin film (13); and (i)mirror-polishing said Si single crystal thin film (13) to level its surface. The measuring step (a) in the first feature of the invention and the classifying step (a) in the second feature of the invention may be carried out by the multiple light interference method. The masking material used in the first and second features of the invention may be a synthetic resin film which is chemically stable in the etching solution used in the etching treatment, such as a one-sided adhesive synthetic resin sheet or a synthetic resin coating which has been cured after application. Next, the mechanism of the invention will be explained. The growth of Si thermal oxide layer, in terms of its relationship to the thickness Tox [micrometers] of the oxide layer and the oxidation time t [min] is expressed by the following equation (except when the oxidation temperature is low): T ox 2 =C 1 t (C 1 is a constant) In the case of dry oxidation, the following equation can be used for calculation when the oxidation temperature T is 1100 ° C or higher: T ox 2 =21.2 t · exp (-Ea/kt) where Ea is the activation energy, which is 1.33 eV in this case. In the case of steam oxidation, the following equation can be used for calculation when the oxidation temperature T is 1100 ° C or higher and the treatment time t > 5 minutes: T ox 2 =7.6 t · exp(-Ea/kT) In this case, Ea is 0.80 eV. Therefore, on the Si single crystal thin film of the SOI substrate, when portions with and without the Si oxide layer are formed in advance and then additional oxide layer is formed by oxidation treatment on the substrate, the resulting depth-wise relative position of the interface between the Si oxide layer and the Si single crystal thin film after the oxidation treatment, measured from the surface or interface of the thin film just before the oxidation treatment, is lower for the portions without the oxide layer than for the portions with the oxide layer. That is, since the growth rate of the oxide layer in the area with pre-existing oxide layer is different from the rate in the area without pre-existing oxide layer, the depth of the Si single crystal thin film layer consumed along with the growth of the oxide layer is also different between these areas. As a result, Si in the area without oxide layer is consumed more, and the interface between Si oxide layer and the Si single crystal thin film is lowered proportionally, so that the thickness of the Si single crystal thin film in the area becomes thinner. Therefore, if the area where the Si single crystal thin film is relatively thick is made to be an area with thinner oxide layer as residual and thin area with relatively thin of the film is etched to have thicker oxide layer, then oxidation treatment will consume more Si in the area with the thinner oxide layer, i.e. the thicker film area, and thus the area becomes thinner in the thickness of the film, as for the film. Therefore, by classifying the thin film into several areas and repeatedly conducting oxidation treatment on thicker areas which do not have oxide layer, as described above, it is possible to make the film thickness relatively thin and make uniform the thickness of the entire Si single crystal thin film. On the other hand, if Si oxide layers with different thicknesses are formed on different portions, and oxidation treatment is conducted on the substrate to form additional oxide layer, then the depth-wise downward displacement of the interface between the Si oxide layer and the Si single crystal thin film layer is smaller for portions with thicker oxide. That is, since the growth rate of the oxide layer in the area with thick pre-formed oxide layer is different from the rate in the area with thin pre-formed oxide layer, the depth of the single crystal thin film layer consumed along with the growth of the oxide layer is also different between these areas. As a result, Si in the area with thin pre-formed oxide layer is consumed more, and the interface between Si oxide layer and the Si single crystal thin film is lowered proportionally, so that the thickness of the Si single crystal thin film in the area becomes thinner. Therefore, if the area where the Si single crystal thin film is relatively thick is made to be an area with thin oxide layer and the area where the Si single crystal thin film is relatively thin is made to be an area with thick oxide layer, then subsequent oxidation treatment will consume more Si in the area with thin oxide layer, i.e. the thicker area, and thus the area becomes thinner. Therefore, by classifying the thin film into several areas, forming oxide layer in such way that the areas with thicker thin film thicknesses have thinner oxide layer, and conducting oxidation treatment in this condition, it is possible to make the thin film thickness relatively thin for originally thicker areas, and thus make uniform the thickness of the entire Si single crystal thin film. BRIEF DESCRIPTION OF THE DRAWINGS This invention will be explained in more detail in conjunction with appended drawings, wherein: Fig. 1 is a process diagram showing a method for making uniform the thickness of a Si single crystal thin film in a first preferred embodiment according to the invention; Figs. 2A and 2B are two-dimensional and three dimensional diagrams respectively showing by contours the surface condition of the SOI substrate before the thin film thickness making uniform treatment in the first preferred embodiment according to the invention; Figs. 3A and 3B are two-dimensional and three-dimensional diagrams respectively showing by contours the surface condition of the SOI substrate after the thin film thickness making uniform treatment in the first preferred embodiment according to the invention; Fig. 4 is a process diagram showing a method for making uniform the thickness of a Si single crystal thin film in a second preferred embodiment according to the invention; Figs. 5A and 5B are two-dimensional and three dimensional diagrams respectively showing by contours the surface condition of the SOI substrate before the thin film thickness making uniform treatment in the second preferred embodiment according to the invention; and Figs. 6A and 6B are two-dimensional and three-dimensional diagrams respectively showing by contours the surface condition of the SOI substrate after the thin film thickness making uniform treatment in the second preferred embodiment according to the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of this invention are described below by referring to the attached drawings. First, a method for making uniform a thickness a Si single crystal thin film in the first preferred embodiment will be explained. In Fig. 1(a), oxide layers 11 and 12 of the prescribed thickness are formed on a base wafer 10 by thermal oxidation. A bond wafer 13 is bonded onto the oxide layer 11. The thickness of the bond wafer 13 which is a Si single crystal thin film is not uniform with, for example, the left side being thicker than the right side as shown in the drawing. Such nonuniformity of the thin film thickness shows up as interference fringes when observing the wafer. This unevenness of the thin film thickness is measured with a thickness meter such as a Nano-spec, and the thin film thickness nonuniformity of the bond wafer 13 is depicted by contours every 0.06 micrometers, for example. In this embodiment, in order to simplify the description, let us assume that the Si single crystal thin film is divided in terms of thickness into three areas by two contours every 0.06 micrometers. In the method of this embodiment, an oxide layer 14 with a thickness of 0.4 micrometers is formed on the upper surface of the bond wafer 13 (Fig. 1(a)). Then, all areas except for the thickest area A of the bond wafer 13, based on the measurement results mentioned earlier, are masked by a masking material 15 (Fig. 1(b)). Then, the wafer is etched with the etching solution (HF aqueous solution) to remove the oxide layer on the unmasked area A (Fig. 1(c)). Next, after removing the masking material 15, an oxidation treatment is conducted under the conditions which will form oxide layer (14') with a thickness of 0.2 micrometers on the area A of the bond wafer 13 which is not covered with the oxide layer (Fig. 1(d)). While oxide layer with a thickness of 0.2 micrometers is formed on the area A, the thickness of the newly formed oxide layer on the other areas is less than 0.2 micrometers, because there is the oxide layer with a thickness of 0.4 micrometers already formed on these areas. Therefore, downward displacement of the interface between Si oxide and the Si single crystal thin film caused by oxidation is less for these areas than for the area A. As a result, an interface level gap is created between the area A and the other areas. Next, removing the entire oxide layer with etching results in a level gap between the area A and the other areas on the surface of the bond wafer 13 (Fig. 1(e)). The area A on the bond wafer 13 is now 0.27 micrometers thinner than its original thickness, while the other areas are 0.21 micrometers thinner than their original thicknesses, thus the level gap mentioned earlier is 0.06 micrometers. Therefore, the area A has become 0.06 micrometers relatively thinner at its boundary to the other areas, thus improving the nonuniformity of the thin film thickness. Next, an oxide layer 16 with a thickness of 0.4 micrometers is formed again on the upper surface of the bond wafer 13 (Fig. 1(f)). Then, the oxide layer 16 on the thinnest area C on the bond wafer 13 is masked with a masking material 17, leaving the area A and the next thinnest area B unmasked (Fig. 1(g)). Then, the wafer is etched with the etching solution (HF aqueous solution) to remove the oxide layer on the unmasked areas A and B (Fig. 1(h)). Next, after removing the masking material 17, an oxidation treatment is conducted under the conditions which will form oxide layer 14' with a thickness of 0.2 micrometers on the areas A and B on the bond wafer 13 which are not covered with the oxide layer (Fig. 1(i)). While oxide layer with a thickness of 0.2 micrometers is formed on the areas A and B, similar to those in Fig. 1(d), downward displacement of the interface between the Si oxide and the Si single crystal thin film caused by oxidation is less for the area C than for the areas A and B, because there is the oxide layer with a thickness of 0.4 micrometers already formed on the area C. As a result, an interface level gap is created between the areas B and C. Next, removing the entire oxide layer with etching results in two level gaps: one is the level gap between the areas B and C created by the previous process, and the other is the already existing level gap between the areas A and B (Fig. 1(j)). Each level gap is 0.06 micrometers. Therefore, the thickness of the bond wafer 13 is made uniform in a staircase fashion. The level gaps created on the surface can subsequently be leveled by mirror-polishing. Normally, polishing away ten times the thickness of the maximum single level gap is enough. For this embodiment, polishing away 0.6 6 micrometers or more would be sufficient. Figs. 2A and 2B show by contours the surface condition of the SOI substrate before the thin film thickness making uniform treatment, in which Fig. 2A is a two-dimensional diagram and Fig. 2B is a three-dimensional diagram. Figs. 3A and 3B show by contours the surface condition of the SOI substrate after the thin film thickness making uniform treatment, in which Fig. 3A is a two-dimensional diagram and Fig. 3B is a three-dimensional diagram. As shown in these diagrams, the wafer has narrowly spaced contours and widely dispersed thin film, thickness before the thin film, thickness making uniform treatment, but after the thin film thickness making uniform treatment the wafer has contours more widely spaced and the dispersion of the thin film thickness is smaller. It is confirmed that the dispersion of the thin film thickness of the wafer is improved from 0.77 micrometers to 0.38 micrometers with the method described above. Next, a method for making uniform a thickness of a Si single crystal thin film in the second preferred embodiment will be explained. In the second preferred embodiment, like elements are given like reference numerals in the first preferred embodiment. In Fig. 4(a), oxide layers 11 and 12 of the prescribed thickness are formed on a base wafer 10 by thermal oxidation. A bond wafer 13 is then bonded onto the oxide layer 11. The thickness of the bond wafer 13 is not uniform with the left side being thicker than the right side as shown in the drawing like in the first preferred embodiment. The thickness nonuniformity of the bond wafer 13 is depicted by contours every 0.06 micrometers, and in order to simplify the description, let us assume that the film is divided in terms of thickness into five areas by contours every 0.06 micrometers. In the method of this embodiment, an oxide layer 14 with a thickness of 1.0 micrometers is formed on the upper surface of the bond wafer 13 (Fig. 4(a)). Next, all areas except for the thickest area A of the bond wafer 13, based on the measurement results mentioned earlier, are masked with a masking material 15. This wafer is then etched with the etching solution (HF aqueous solution) to remove 0.17 micrometers thickness of the oxide layer on the unmasked area A (Fig. 4(b)). Next, all areas except for the area A and the next thickest area B of the bond wafer 13 are re-masked with the masking material 15. The wafer is then etched with the etching solution (HF aqueous solution) to remove 0.18 micrometers thickness of the oxide layer on the unmasked areas A and B (Fig. 4(c)). Next, in the same manner as described above, all areas except for the areas A, B and the next thickest area C of the bond wafer 13 are re-masked with the masking material 15. The wafer is then etched with the etching solution (HF aqueous solution) to remove 0.25 micrometers thickness of the oxide layer on the unmasked areas A, B and C (Fig. 4(d)). Next, in the same manner as described above, all areas except for the areas A, B, C and the next thickest area D of the bond wafer 13 are re-masked with the masking material 15. The wafer is then etched with the etching solution (HF aqueous solution) to remove 0.40 micrometers thickness of the oxide layer on the unmasked areas A, B, C and D. Then the masking material is removed (Fig. 4(e)). The resulting condition is such that the oxide layer becomes thicker from the area A to the area E in steps, and, in particular, the area A has almost no oxide layer. Oxidation treatment is conducted on the wafer under conditions which form oxide layer 14' which a thickness of 1.0 micrometers on the area A, which does not have oxide layer (Fig. 4(f)). Next, the entire area of the oxide layer is etched with the etching solution (HF aqueous solution) (Fig. 4(g)). While oxide layer with a thickness of 1.0 micrometers is formed on the area A, other areas, having pre-formed oxide layer with prescribed thicknesses, have newly formed oxide layer thinner than 1.0 micrometers. That is, areas with thicker oxide layer, i.e. areas where the bond wafer 13 is thinner, have less amount of oxide growth. Therefore. the interface between the Si oxide and the Si single crystal thin film sinks lower in areas where the bond wafer 13 was originally thicker, thus making uniform the thickness of the entire bond wafer 13 in a staircase fashion. The level gaps created on the surface can subsequently be leveled by mirror-polishing. Normally, polishing away ten times the thickness of the maximum single level gap is enough. For this embodiment, polishing away 0.6 micrometers or more would be sufficient. Figs. 5A and 5B show by contours the surface condition of the SOI substrate before the thin film thickness making uniform treatment, in which Fig. 5A is a two-dimensional diagram and Fig. 5B is a three-dimensional diagram. Figs. 6A and 6B show by contours the surface condition of the SOI substrate after the thin film thickness making uniform treatment, in which Fig. 6A is a two-dimensional diagram and Fig. 6B is a three-dimensional diagram. As shown in these diagrams, the wafer has narrowly spaced contours and widely dispersed thin film thickness before the thin film thickness making uniform treatment, but after the thin film thickness making uniform treatment the wafer has contours more widely spaced and the dispersion of the thin film thickness is smaller. It is confirmed that the dispersion of the thin film thickness of the wafer is improved from 0.45 micrometers to 0.22 micrometers with the method described above. In these embodiments. it is not necessary to limit the thickness difference between the contours to 0.06 micrometers. This difference should be determined based on the required accuracy. By subdividing the areas even more as necessary and repeating the process describes above, it is possible to improve the accuracy of the film thickness uniformization of the bond wafer 13. As described thus far, this invention makes it possible to reduce the dispersion of the thickness of a Si single crystal thin film with an SOI structure and thereby make uniform the thickness of the Si single crystal thin film. As an example, the method according to the invention may be practiced as follows: After having measured the surface condition and the thickness unevenness of a Si single crystal thin film (Step a) the primary Si oxide layer (thickness 0.4 µm) may be produced by thermal oxidation in an N 2 :O 2 = 5:3 atmosphere at a temperature of 1050°C over a period of 50 minutes (Step b). A polyvinyl chloride or polyethylene type resin may be used as masking material in Step b. The following steps may be carried out in order to have the masking material cover only the "valleys" and not the "peaks": (1) First, the entire surface area is covered with the masking material which is an adhesive resin; (2) Closed curve patterns are identified which indicate the thickest peak portion from equal thickness contour curves; (3) The masking material is cut along the equal thickness contour curve which surrounds the thickest peak portion; and (4) The portion of the masking material which covers the thickest peak portion of the Si single crystal thin film is peeled off. The primary Si oxide layer on the surface of the peak portions may be selectively removed with a HF solution (concentration 10 %) at room temperature for 6 minutes. Thereafter, the masking material remaining in the "valleys" may be peeled off by hand or machine (Step c). The secondary Si oxide layer (and the following Si oxide layers, if necessary) may be formed and treated in the same manner as the first Si oxide layer. The polishing (Step e) may be carried out with a suspension of colloidal silicate particles (neutral pH).

Claims: Method for making uniform the thickness of a Si single crystal thin film (13), comprising the steps of: (a) measuring the surface condition and the thickness unevenness of a Si single crystal thin film (13) by two-dimensional and three-dimensional images to identify the peak portions (A) having the thickest thickness from closed curve patterns of equal thickness contour curves on said two-dimensional image; (b) forming a primary Si oxide layer (14) with approximately uniform thickness on said Si single crystal thin film (13) by thermal oxidation; (c) masking the surface of said primary Si oxide layer (14) with a masking material (15) except for said peak portions (A); (d) removing said primary Si oxide layer (14) only on the surface of said peak portions (A) by an etching treatment using said masking material (15) as a mask; (e) removing said masking material (15); (f) performing on oxidation treatment to form an secondary Si oxide layer (14') on said peak portions (A) and on the remaining portions still having said primary Si oxide layer (14), whereby the downward displacement of the interface between said secondary Si oxide layer (14') and said Si single crystal thin film (13) is deeper for said peak portions (A) than for the remaining portions still having said primary Si oxide layer (14); (g) removing said primary and secondary Si oxide layers by an etching treatment to form a level gap between said peak portions (A) and the remaining portions on the surface of said Si single crystal thin film (13); and (h) mirror-polishing said Si single crystal thin film (13) to level its surface. Method for making uniform the thickness of a Si single crystal thin film (13), comprising the steps of: (a) measuring the surface condition and the thickness unevenness of a Si single crystal thin film (13) by two-dimensional and three-dimensional images and classifying said surface according to the thickness of said Si single crystal thin film into n (n is an integer equal or greater than two) areas (A, B, C, D, E) in order of thickness from the first area (A) to the n-th area (E); (b) forming a primary Si oxide layer (14) with approximately uniform thickness on said Si single crystal thin film (13) by thermal oxidation; (c) masking the surface of said primary Si oxide layer (14) with a masking material (15) except for the first area (A) classified in said step (a) as having the thickest thickness of said Si single crystal thin film; (d) removing the surface of said primary Si oxide layer (14) only on said first area (A) down to a certain depth by an etching treatment using said masking material (15) as a mask creating a level gap between the etched surface of said primary Si oxide layer (14) on said first area (A) and the adjacent portion of said primary Si oxide layer (14) ; (e) repeating the steps (c) and (d) from the second area (B) to the n-th thickness area (E) whereby each time the surface of said primary Si oxide layer (14) is at first remasked with a masking material (15) except for said first area (A), the areas having a primary Si oxide layer (14) with a previously reduced thickness and the next thickest area and further removing the surface of said primary Si oxide layer (14) only on the unmasked areas down to a certain depth by an etching treatment so to create a primary Si oxide layer (14) with n level gaps between adjacent portions becoming thicker in n steps from the area (A) to the area (E); (f) removing the masking material (15); (g) performing a thermal oxidation to form a secondary Si oxide layer (14') on said first area (A) and said areas (B) to (E) still having said primary Si oxide layer (14), whereby the displacement of the interface between said secondary Si oxide layer (14') and said Si single crystal film (13) is deeper for the areas of said Si single crystal film (13) having a thinner remaining primary Si oxide layer (14) thereon; (h) removing said remaining primary (14) and secondary Si oxide layers (14') by an etching treatment to form n level gaps between the adjacent areas (A) to (E) on the surface of said Si single crystal thin film (13); and (i)mirror-polishing said Si single crystal thin film (13) to level its surface. Method according to claims 1 or 2, wherein said steps a) to g) and a) to h), respectively, are repeated as necessary prior to said polishing step. A method according to Claim 1, wherein said measuring step (a) is carried out by the multiple light interference method. A method according to claim 2, wherein said measuring and classifying step (a) is carried out by the multiple light interference method with two-dimensional and three-dimensional images. A method according to Claims 1 or 2, wherein said masking material (15) is a synthetic resin film which is chemically stable in an etching solution used in said etching treatment. A method according to Claim 6, wherein said masking material (15) is a one-sided adhesive synthetic resin sheet. A method according to Claim 6, wherein said masking material (15) is a synthetic resin coating which has been cured after application.

Inventors: ABE TAKAO, NAKAZATO YASUAKI, UCHIYAMA ATSUO, ABE, TAKAO, NAKAZATO, YASUAKI, UCHIYAMA, ATSUO, ABE, TAKAO, C/O HANDOTAI ISOBE KENKYUSHO, Nakazato, Yasuaki, c/o Nagano Denshi Kogyo K.K., Uchiyama, Atsuo, c/o Nagano Denshi Kogyo K.K.
Applicants: NAGANO DENSHI KOGYO KABUSHIKI, SHINETSU HANDOTAI KK, NAGANO DENSHI KOGYO KABUSHIKI KAISHA, SHIN-ETSU HANDOTAI KABUSHIKI KAISHA
Application year: 1992
Main classification: H01L21
IPC codes: H01L21