This invent ion relates to a bonded substrate with a silicon-on-insulator (SOI) structure for the fabrication of semiconductor devices and a method of producing the bonded substrate.
In the semiconductor industry the use of bonded SOI substrates having a buried insulator layer such as a silicon oxide layer is expanding. Particularly in the field of power ICs, the use of bonded substrates having a local SOI structure has been developed to meet the requirements of enlarging the scale of integration and enhancing the withstand voltage by fabricating a high voltage MOSFET of the vertical type and low voltage control circuit components in a single chip. The low voltage components, i.e., devices, are fabricated in a region above a buried insulator layer and the high voltage device in another region wherein no insulator layer is buried.
For example, JP-A 4-29353 shows the use of a bonded substrate with a local SOI stricture in a power IC. As shown in FIG. 4 of the accompanying drawings, a first single-crystal silicon wafer 30 is bonded to a second single-crystal silicon wafer 30. Before bonding the two wafers 30 and 50, a silicon oxide film 32 is formed in a principal surface of the first wafer 30. In the bonded substrate the oxide film 32 is buried adjacent to the bonding plane 52. The thickness of the first wafer 30 in the bonded substrate is reduced by mechanical grinding and subsequent chemical-mechanical polishing or chemical etching in order to form an active layer 30a having a suitable thickness above the bonding plane 52. The thickness of the active layer 30a should be accurate and uniform over the whole area since the thickness of the active layer seriously affects the characteristics of semiconductor devices fabricated in and on the active layer. In general, power ICs need an active layer of a large thickness which is tens of micrometers and in some cases reaches 50-70 .mu.m. In the active layer 30a, trenches 34 having V-shaped cross-sections are formed to define the periphery of a selected region 40 wherein low voltage components of a control circuit are to be fabricated. The trenches 34 reach the buried oxide film 32. A silicon oxide film 36 is formed on the inner walls of the trenches 34, and the trenches 34 are filled with a filler 38 such as polysilicon (polycrystalline silicon). By the oxide films 32 and 36, the region 40 is isolated from another region 42 wherein a high voltage MOSFET is to be fabricated. That is, the low voltage region 40 becomes an island-like region provided with a dielectric isolation structure.
The thickness of a single-crystal silicon layer above a silicon oxide layer can be measured by an optical method. For example, when the thickness of the silicon layer is smaller than about 10 .mu.m a useful optical method is interference color photometry. In this method, visible light is vertically incident on the silicon surface, and interference of light reflected from the silicon surface, and from the interface between the silicon layer, and the oxide layer is analyzed to determine the thickness of the silicon layer. However, as the thickniess of the silicon layer becomes larger the intensity of reflected light diminishes so that the thickness measurement becomes difficult, since there is a limit to the depth of penetration of visible light into a single-crystal silicon layer. Therefore, when the thickness of the silicon layer is larger than about 10 .mu.m it is usual to use infrared rays to which single-crystal silicon is transparent. More particularly, Fourier transform infrared spectroscopy (FT-IR) is used. FT-IR can be used whether the thickness of the silicon layer is larger than 10 .mu.m or smaller than 10 .mu.m.
In producing a bonded SOI substrate in which an oxide film is buried in the whole area of the substrate, the thickness of the active layer above the oxide film is examined by an optical measurement method to determine the extent of the final polishing of the wafer which provides the active layer. That is, the polishing operation and the thickness measurement are alternately repeated until the thickness of the active reaches a predetermined value.
However, in the ease of producing a bonded SOI substrate in which an oxide film is buried in a plurality of small areas, it is not easy to measure the thickness of the active layer because it is difficult to accurately detect the location of a small area wherein the oxide film is buried by observation from the substrate surface. In using FT-IR to measure the thickness of the active layer, it is necessary to irradiate the active layer and the underlying oxide film with infrared rays over an area not smaller than about 30 mm.sup.2. However, when the size of the individual chips to be fabricated in the bonded substrate is smaller than this minimum area, the thickness of the active layer cannot be measured since the oxide film does not have an area necessary for the measurement.
To realize a desired thickness of an active layer in a bonded SOI substrate without need of measuring the thickness, there are some proposals of burying relatively hard structures having a predetermined thickness or depth, such as trenches in which an oxide film is deposited, in the wafer which provides the active layer in order to use the buried structures as polishing stoppers. For example, JP-A 2-219265 shows a method illustrated in FIGS. 5(A) to 5(C) of the accompanying drawings. As shown in FIG. 5(A), in a surface of a first silicon wafer 60 a group of trenches 62 of uniform depth are formed in a lattice-like pattern. A silicon oxide film 64 is deposited on the wafer surfaces including the bottom and wall faces in the trenches 62. Then, as shown in FIG. 5(B), the first wafer 60 is bonded to a second silicon wafer 70 such that the openings of the trenches 62 are closed by the second wafer 70. Next, the thickness of the first wafer 60 is reduced by grinding so as to leave a small thickness above the bottom of the trenches 62. The silicon layer remaining above the bottom of the trenches 62 is removed by chemical-mechanical polishing. When the oxide film 64 at the bottom of the trenches 62 is exposed as shown in FIG. 5(C), the polishing automatically stops since the oxide film 64 is far stronger in resistance to chemical-mechanical polishing than silicon. As a result, an active layer 60a uniformly having a predetermined thickness is formed above the oxide film 64 adjacent to the bonding plane 72. The active layer 60a is divided into a plurality of regions by the oxide film 64 on the inner wall faces of the trenches 62. In this method it is optional to fill the void space in each trench 62 with polysilicon or a different filler.
The method illustrated in FIGS. 5(A) to 5(C) can be applied to the production of a bonded SOI substrate having island-like regions provided with a dielectric isolation structure. For example, JP-A 3-142952 shows a bonded substrate which is shown in FIG. 6 of the accompanying drawings. First and second silicon wafers 80 and 90 are bonded together. Prior to the bonding, trenches 82 having V-shaped cross-sections and a depression 84 are formed in the first wafer 80 to define a region 100 for low voltage devices and another region 102 for high voltage devices. A silicon oxide film 86 is deposited on the inner wall faces of the trenches 82 and the bottom of the depression 84, and the void spaces in the trenches and the depression are filled with polysilicon 88. When the thickness of the first wafer 80 is reduced by grinding and subsequent chemical-mechanical polishing to the extent of exposing the bottom of the trenches 82 as shown in FIG. 6, the polishing is terminated.
The method illustrated in FIGS. 5(A) to 5(C) and FIG. 6 includes complicated process steps and entails an increased cost of production because of forming trenches in one of the two wafers to be bonded together. Besides, in the case of producing a bonded SOI substrate in which an active layer has a thickness larger than about 50 .mu.m, it is difficult to form sufficiently deep trenches with good uniformity and accuracy of depth.