Abstract:
A semiconductor device has a transparent dielectric substrate such as a sapphire substrate. To enable fabrication equipment to detect the presence of the substrate optically, the back surface of the substrate is coated with a triple-layer light-reflecting film, preferably a film in which a silicon oxide or silicon nitride layer is sandwiched between polycrystalline silicon layers. This structure provides high reflectance with a combined film thickness of less than half a micrometer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application of application Ser. No. 11/252,632 filed on Oct. 19, 2006, which is hereby incorporated by reference in its entirety for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a process for forming a semiconductor device on a transparent dielectric substrate such as a sapphire substrate, more particularly to the formation of a reflective film on the transparent dielectric substrate to enable the substrate to be recognized optically. 
     2. Description of the Related Art 
     Semiconductor integrated circuits formed in silicon films grown on sapphire substrates are advantageous for applications in environments in which radiation poses a hazard. Such silicon-on-sapphire (SOS) integrated circuits are generally formed by use of conventional fabrication equipment of the type that creates semiconductor integrated circuits in semiconductor substrates. In conventional fabrication processes, the fabrication equipment often uses optical sensors to detect the position of the semiconductor substrate. The position of a sapphire substrate cannot be detected in this way because sapphire is transparent light passes straight through the substrate instead of being reflected back to the sensor. One known solution to this problem is to coat the sapphire substrate with a light-reflecting film. 
     Japanese Patent Application Publication No. 7-283383 and the parent U.S. Pat. No. 5,877,094, for example, describe a sapphire substrate coated on its backside with a layer of polycrystalline silicon (polysilicon) at least about two micrometers (2 μm) thick, which reflects light and can be detected optically. Phosphorous ions are also implanted into selected regions of the polysilicon film to form conductive doped regions that can be detected electrically. 
     One problem with this substrate is that forming a polysilicon layer at least about 2 μm thick is a time-consuming and therefore expensive process. Moreover, in reflow and other subsequent heating steps, the large difference in thermal expansion coefficients between sapphire and polysilicon may cause the sapphire substrate to warp. Such warping interferes with the fabrication process and may lead to the formation of cracks in the sapphire substrate, particularly if the sapphire substrate is thin, which is the current trend. 
     Japanese Patent Application Publication No. 11-220114 describes an SOS substrate having an optically reflecting polysilicon coating 0.5 μm to 3.0 μm thick on its backside. A pattern of cuts is formed in the reflective coating so that the difference in thermal expansion coefficients does not cause the substrate to warp or crack. The thickness of the polysilicon coating must be at least 0.5 μm because a thinner film would lack the necessary reflectivity, as pointed out in paragraph 0009 of the above disclosure. 
     Due to the trend toward thinner sapphire substrates, there is a continuing need for still thinner reflective films. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is accordingly to provide a semiconductor device having a transparent dielectric substrate with a reflective coating film that can be thinner than 0.5 μm and still provide adequate reflectivity for optical detection. 
     The term ‘semiconductor device’ as used herein refers to an electronic device such as a semiconductor integrated circuit chip or to a wafer from which such electronic devices may be manufactured. 
     The semiconductor device has a dielectric substrate transparent to light, a first film disposed on the back surface of the dielectric substrate, a second film disposed on the first film, and a third film disposed on the second film. The first film and the second film have different reflective characteristics, enabling one film to reflect light not reflected by the other film. 
     The first, second, and third films combine to form a triple-layer light-reflecting film that has a higher reflectance than the conventional single-layer light-reflecting film and can be made thinner than the conventional single-layer light-reflecting film. 
     One method of fabricating the semiconductor device includes: preparing a dielectric substrate that is transparent to light and has a front surface and a back surface; forming a first film on the back surface of the dielectric substrate; forming a second film on the first film; heating the second film; and forming a third film on the heated second film. Another method of fabricating the invented semiconductor device includes: preparing a dielectric substrate that is transparent to light and has a front surface and a back surface; forming a first film on the back surface of the dielectric substrate; forming a second film on the first film by heating the first film; and forming a third film on the second film. 
     In one aspect of both methods, the second film has a lower refractive index than the first and third films. 
     In another aspect of both methods, the first, second, and third films have an aggregate thickness less than 0.5 μm. 
     In another aspect of both methods, the first film includes polysilicon, the second film includes silicon oxide, and the third film includes polysilicon. 
     In a further aspect of the preceding aspect, the first film is 42 nanometers thick, the second film is 110 nanometers thick, and the third film is 42 nanometers thick. 
     In another aspect of both methods, the dielectric substrate includes sapphire. 
     Another aspect of both methods also includes exposing the front surface of the dielectric substrate. 
     Another aspect of both methods also includes forming a fourth film on the front surface of the dielectric substrate. 
     The fourth film may include polysilicon. 
     Another aspect of both methods also includes forming a fifth film on the fourth film. 
     The fifth film may include silicon oxide. 
     In another aspect of both methods the dielectric substrate has side surfaces; this aspect also includes forming a sixth film covering the side surfaces of the dielectric substrate. 
     The sixth film may include polysilicon and silicon oxide. 
     When the second film includes silicon oxide (SiO 2 ), the above fabrication processes improve the crystalline structure of the silicon oxide. An attendant advantage is that in further fabrication steps involving etching by hydrofluoric acid, there are fewer crystal lattice defects through which the hydrofluoric acid can invade the silicon oxide film. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of a semiconductor device according to the present invention; 
         FIG. 2  is a graph indicating the reflectance of a wafer with a triple-layer reflective coating as a function of wavelength; 
         FIG. 3  is a graph indicating the reflectance of a wafer with another triple-layer reflective coating as a function of wavelength; 
         FIG. 4  is a more detailed sectional view showing an example of the structure of the substrate in  FIG. 1 ; 
         FIG. 5  is a more detailed sectional view showing another example of the structure of the substrate in  FIG. 1 ; 
         FIG. 6  is a more detailed sectional view showing yet another example of the structure of the substrate in  FIG. 1 ; 
         FIGS. 7 to 11  illustrate steps in a fabrication process for the semiconductor device in  FIGS. 1 and 6 ; and 
         FIGS. 12 to 16  illustrate steps in another fabrication process for the semiconductor device in  FIGS. 1 and 6 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the invention will now be described with reference to the attached drawings, in which like elements are indicated by like reference characters. 
     First Embodiment 
     Referring to  FIG. 1 , the first embodiment is a semiconductor device or wafer having a silicon-on-sapphire (SOS) substrate  101 , the major surfaces of which are a front surface and a back surface  101   a . The back surface  101   a  and side surfaces  101   b  of the SOS substrate  101  are covered with reflective films  102 ,  103 ,  104  to permit optical sensing by wafer sensing light. The second reflective film  103  has a lower refractive index than the first and third reflective films  102 ,  104 . The first reflective film  102  is, for example, a polysilicon film formed on the back and side surfaces of the SOS substrate  101 . The second reflective film  103  is, for example, a silicon oxide film formed on the first reflective film  102 . The third reflective film  104  is, for example, a polysilicon film formed on the second reflective film  103 . 
     Next the determination of the film thicknesses of the reflective films will be described. 
     Let the refractive index of the space through which the wafer sensing light travels before entering the SOS substrate  101  be n 0 , the refractive index of the sensed material be n x , and the refractive index of the space on the far side of the sensed material, through which the light travels if it passes through the sensed material, be n s . In order for the sensed material to have high reflectance, its refractive index n x  must be the higher than the space indices n 0  and n s . 
     If the refractive index of the sensed material is higher than the space indices and the sensed material comprises a triple-layer film consisting of a first reflective film  102 , second reflective film  103 , and third reflective film  104 , a reflectance close to unity can be achieved for the triple-layer film as a whole if the refractive index of the second reflective film  103  is lower than the refractive index of the first reflective film  102  and third reflective film  104 . 
     Let the wavelength of the wafer sensing light be λ and the refractive indices of the first reflective film  102 , second reflective film  103 , and third reflective film  104  be n 1 , n 2  and n 3 , respectively. If the wafer sensing light impinges normal (at a 90° angle) to the front surface of the SOS substrate  101 , then to achieve still higher reflectance, the thickness d of each film, the wavelength λ of the wafer sensing light, and the refractive index n of the film must satisfy the following equation (1) for some integer N: 
     
       
         
           
             
               
                 
                   
                     
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     A reflectance as close as possible to unity is achieved when the thicknesses and refractive indices n 1 , n 2  and n 3  of all three films satisfy this equation (1) and the refractive indices also satisfy the relationship mentioned above (n 2 &lt;n 1  and n 2 &lt;n 3 ). 
     If the wavelength of the wafer sensing light is six hundred forty nanometers (λ=640 nm) and the first and third reflective films  102 ,  104  are polysilicon films, their refractive indices are both 3.80 (n 1 =n 3 =3.80). If the second reflective film  103  is a silicon oxide film, its refractive index at this wavelength is 1.45 (n 2 =1.45). In order to achieve the minimum film thickness, N should be equal to zero (N=0). The thickness of the first and third reflective films  102 ,  104  can then be calculated from the above equation (1) as d=42.1 nm, while the film thickness of the second reflective film  103  can be calculated as d=109.8 nm. If the first and third reflective films  102 ,  104  are polysilicon films and the second reflective film  103  is a silicon nitride (SiN) film, then its refractive index is 2.02 (n 2 =2.02), the film thickness of the first and third reflective films  102 ,  104  is still d=42.1, and the thickness of the second reflective film  103  is d=79.2 nm from equation (1) with N=0. 
     The reflectance of the wafer as a whole to the wafer sensing light can be calculated from the following equation (2), where as above, n 0  is the refractive index of the space through which the wafer sensing light travels before entering the SOS substrate  101 , n 1 , n 2 , and n 3 , are the refractive indices of the first, second, and third reflective films  102 ,  103 ,  104 , and n s  is the refractive index of the space behind the third reflective film  104 , assuming that the wafer sensing light impinges onto the front surface of the semiconductor substrate at a normal (90°) angle. 
     
       
         
           
             
               
                 
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     In the above example, as the dielectric substrate is transparent, its refractive index may be set equal to the refractive index (n 0 ) of the space through which the wafer sensing light travels before entering the SOS substrate  101 . Equation (2) indicates that in order to achieve higher reflectance than that can be achieved by a single-layer light-reflecting film made from a high-index material, the first and third reflective films  102 ,  104  should be made from a material with a comparatively high refractive index, while the second reflective film  103  should be made from a material with a relatively low refractive index. 
     The refractive index n of each material varies according to the wavelength λ of the wafer sensing light, so from equation (2), the reflectance R of the wafer as a whole also varies according to the wavelength. A plot of the reflectance R of the wafer as a whole versus the wavelength λ of the wafer sensing light is shown in  FIG. 2  for the case in which the first and third reflective films  102 ,  104  are polysilicon films, and the second reflective film  103  is a silicon oxide film. 
     If the wafer must have a reflectance R not less than 0.8 in order to be recognized by the wafer sensing light, the wavelength λ of the sensing light should be about 640 nm±100 nm. The corresponding thickness of the first and third reflective films  102 ,  104  can be calculated from equation (1) as d=42.1±6.6 nm (hereinafter referred to as about 42 nm). This can be taken as the allowable thickness range of the first and third reflective films  102 ,  104 . The thickness of the second reflective film  103  can be calculated from equation (1) as d=109.8±17.2 nm (hereinafter referred to as about 110 nm). This can be taken as the allowable thickness range of the second reflective film  103 . 
     A plot of the reflectance R of the wafer as a whole versus the wavelength λ of the wafer sensing light is shown in  FIG. 3  for the case in which the first and third reflective films  102 ,  104  are polysilicon films, and the second reflective film  103  is a silicon nitride film. If the requirement for recognition of the wafer is relaxed to a reflectance R not less than 0.7, the wavelength λ of the sensing light should again be about 640 nm±100 nm. The thickness of the first and third reflective films  102 ,  104  can again be calculated as d=42.1±6.6 nm (about 42 nm) from equation (1), the thickness of the second reflective film  103  can be calculated as d=79.2±12.4 nm (hereinafter referred to as about 80 nm), and these can the taken as the allowable thickness ranges of the respective films. 
     Next, the structure of the SOS substrate  101  will be described. The SOS substrate  101  is fabricated by depositing various films on a sapphire substrate. In this embodiment, the SOS substrate  101  may be of any one of the following three types. 
     The first type of SOS substrate  101 , shown in  FIG. 4 , comprises a sapphire substrate  105  (dielectric substrate) and a device formation film  106  (a fourth film) formed on the sapphire substrate  105 . The sapphire substrate  105  in  FIG. 4  is six hundred micrometers (600 μm) thick; the device formation film  106  formed on the sapphire substrate  105  is 100 nm thick. The device formation film  106  can be made from silicon, which is the material from which transistors are typically made. 
     The second type of SOS substrate  101 , shown in  FIG. 5 , comprises a sapphire substrate  105 , a device formation film  106  formed on the sapphire substrate  105 , and a silicon oxide film  107  (a fifth film) formed on the device formation film  106 . The sapphire substrate  105  in  FIG. 5  is 600 μm thick, the device formation film  106  formed on the sapphire substrate  105  is 100 nm thick, and the silicon oxide film  107  formed on the device formation film  106  is 10 nm thick. The device formation film  106  may again be made of silicon. The SOS substrate  101  shown in  FIG. 5  has the advantage that the silicon oxide film  107  protects the device formation film  106  during wafer processing steps performed prior to the formation of circuit elements, resulting in less variation in the electrical characteristics of the circuit elements. 
     The third type of SOS substrate  101 , shown in  FIG. 6  comprises the sapphire substrate  105 , device formation film  106 , and silicon oxide film  107  described above, and a protective film  108  (a sixth film) covering the side surfaces of the device formation film  106  and silicon oxide film  107  and the back surface of the sapphire substrate  105 . The sapphire substrate  105  in  FIG. 6  is 600 μm thick, the device formation film  106  is 100 nm thick, the silicon oxide film  107  is 10 nm thick, and the protective film  108  is 700 nm thick. The device formation film  106  may again be made of silicon. The protective film  108  may be made from a combination of a silicon nitride film and polysilicon. The SOS substrate  101  in  FIG. 6  has the same advantages as the SOS substrate  101  in  FIG. 5 , and the additional advantage that the sides of the device formation film  106  can be protected from invasion by hydrofluoric acid, thus preventing flaking of the device formation film  106  and silicon oxide film  107 . Furthermore, this structure can prevent unwanted diffusion during doping steps in the formation of circuit elements. 
     Any one of the three types of SOS substrate  101  described in  FIGS. 4 to 6  can be used, according to the needs of the particular application. 
     Because of its triple-layer structure, the light-reflecting film of a semiconductor device according to the first embodiment of the invention can be thinner than a conventional single-layer light-reflecting film. Semiconductor chips can be fabricated by coating part or all of a wafer with a light-reflecting film according to the invention, forming circuit elements on the semiconductor substrate and interconnecting them by using conventional semiconductor fabrication equipment, and then dicing the wafer into individual chips. If the wafer sensors in the fabrication equipment illuminate only selected parts of the wafer, the triple-layer light-reflecting film only has to cover the selected parts. For example, the triple-layer light-reflecting film may cover only the peripheral parts of the wafer. Then after the wafer is divided into chips, none of the chips includes any portion of the light-reflecting film, so the thickness of the semiconductor chips can be further reduced. 
     Next, a process for fabricating a semiconductor device of the above type will be described with reference to  FIGS. 7 to 11 . 
     Among the SOS substrates shown in  FIGS. 4 to 6 , a fabrication process using the SOS substrate shown in the  FIG. 6  will be described. For numerological consistency, the component parts are numbered as shown in  FIG. 7 . The SOS substrate  201  in  FIG. 7  comprises a transparent dielectric sapphire substrate  205 , a device formation film  206  formed on the sapphire substrate  205  as a silicon film, a silicon oxide film  207  formed on the device formation film  206 , and a protective film  208  formed on side surfaces of the sapphire substrate  205 , the device formation film  206  and the silicon oxide film  207 , and the back surface of the sapphire substrate  205 . 
     Next, the fabrication process of the SOS substrate  201  will be summarized. First, a sapphire substrate  205  is obtained and a silicon film is formed thereon by chemical vapor deposition (CVD). Next, the part of the silicon film near the interface with the sapphire substrate  205  is transformed into amorphous silicon by an implantation process. Then the silicon close to the interface is crystallized by heating in an oxygen atmosphere to form the device formation film  206 , and the silicon oxide film  207  is formed by oxidizing the remaining silicon film simultaneously. Next, the circumference is coated with a polysilicon CVD film; then the circumference is coated with a silicon nitride film. Next, the silicon oxide film  207  is exposed and the protective film  208  is formed to complete an SOS substrate  201  of the same type as shown in  FIG. 6 . 
     The SOS substrate  201  can have various structures other than the structure described above. For example, a substrate comprising the sapphire substrate  205  and the device formation film  206 , or a substrate comprising the sapphire substrate  205 , the device formation film  206  and the silicon oxide film  207  can be used. A silicon-on-insulator substrate comprising fused silica instead of sapphire is also usable instead of an SOS substrate, but the following description will continue to assume an SOS substrate. 
     As shown in the  FIG. 8 , a first reflective film  202  is formed to cover all sides and surfaces of the SOS substrate  201 . The first reflective film  202  is a film comprising polysilicon formed by CVD, and has a film thickness adjusted to 42 nm. 
     Referring to the  FIG. 9 , a second reflective film  203  is formed to cover the first reflective film  202 . The second reflective film  203  is a silicon oxide film formed by CVD, and has a film thickness adjusted to 110 nm. Next, the second reflective film  203  is heated in a nitrogen (N 2 ) atmosphere at 950° Celsius for 20 minutes. The CVD process used to form the second reflective film  203  forms a silicon oxide film with poor crystallization, containing much vapor, which could be easily invaded by hydrofluoric acid during wet etching steps. The subsequent heating step, however, readily eliminates the vapor from the silicon oxide film, giving the silicon oxide film an improved crystalline structure that prevents invasion by hydrofluoric acid. 
     Referring to the  FIG. 10 , a third reflective film  204  is formed, covering the second reflective film  203 . The third reflective film  204  is a polysilicon film formed by CVD, and having a film thickness adjustable to 42 nm by the time the light-reflecting film is needed for wafer detection. That is, if the thickness of the third reflective film  204  will be reduced by fabrication steps carried out after the formation of the three films  202 ,  203 ,  204 , the third reflective film  204  may originally be made thicker than 42 nm in order to obtain the desired film thickness of 42 nm at the time of wafer detection. 
     Referring to the  FIG. 11 , the silicon oxide film  207  of the SOS substrate  201  is exposed by removing the first, second, and third light-reflecting films  202 ,  203 ,  204  from the front surface of the substrate. The first, second, and third light-reflecting films may be removed by dry etching. 
     The above process fabricates a semiconductor wafer device according to the second embodiment of the invention. After the triple-layer light-reflecting film has been formed, semiconductor integrated circuit devices can be fabricated by using conventional semiconductor IC fabrication equipment with optical wafer sensors to form any desired circuitry in and on the device formation film  206 , and then dicing the wafer into chips. 
     In a variation of the second embodiment, the triple-layer light-reflecting film does not cover the entire back surface of the wafer. In particular, if the optical wafer sensors illuminate only selected parts of the wafer, the triple-layer film can be removed from the other parts of the wafer to reduce the thickness of the chips. 
     In the fabrication process of the second embodiment, the heating step improves the crystalline structure of the second light-reflecting film. Furthermore, the problem of unintended detachment of the third light-reflecting film can be avoided. This problem occurs when a triple-layer light-reflecting film is formed by sequentially depositing a first light-reflecting film, a second light-reflecting film, and a third light-reflecting film made from polysilicon, silicon oxide and polysilicon, respectively, on the back surface of a dielectric substrate by CVD. In this method, in subsequent steps using hydrofluoric acid, the acid reacts with the silicon oxide film material of the second light-reflecting film, thereby invading the silicon oxide film. If the invasion proceeds far enough, eventually the third light-reflecting film becomes detached. By avoiding this problem, the second embodiment maintains the desired optical properties of the light-reflecting film and prevents detached fragments of film from contaminating the fabrication equipment. 
     Next, a semiconductor device fabrication process according to a third embodiment of the invention will be described with reference to  FIGS. 12 to 16 . Steps similar to steps in the second embodiment will not be described in detail. 
     Referring to  FIG. 12 , an SOS substrate  301  comprising a sapphire substrate  305 , a device formation film  306 , a silicon oxide film  307 , and a protective film  308  is obtained. A detailed description of this step will be omitted, as the SOS substrate  301  is similar in structure and fabrication to the SOS substrate described in the second embodiment, or any of the SOS substrates described in the first embodiment. 
     Referring to  FIG. 13 , a first reflective film  302  is formed to cover all sides and surfaces of the SOS substrate  301 . The first reflective film  302  is a polysilicon film formed by CVD. Part of the first reflective film  302  will become a silicon oxide film as described below. To allow for a doubling of the thickness of this part when the polysilicon is oxidized, the thickness of the first reflective film  302  is reduced to 100 nm. 
     Referring to  FIG. 14 , a second reflective film  303  is formed covering the first reflective film  302 . The second reflective film  303  is formed by heating the first reflective film  302  at 950° Celsius in an oxygen atmosphere for an appropriate length of time to oxidize substantially the outer 58 nm of the first reflective film  302 . The oxidization process approximately doubles the thickness of the oxidized material, creating a second reflective film  303  substantially 110 nm thick. A second reflective film  303  formed in this way has a better crystal lattice structure than a silicon oxide film formed by CVD, and can better prevent invasion of hydrofluoric acid in subsequent wet etching steps. The remaining part of the first reflective film  302  is substantially 42 nm thick. 
     Referring to  FIG. 15 , a third reflective film  304  is formed covering the second reflective film  303 . This step will not be described in detail because it is similar to the corresponding step described in the second embodiment. 
     Referring to  FIG. 16 , the silicon oxide film  307  of the SOS substrate  301  is exposed. This step is also similar to the corresponding step in the second embodiment, and will not be described in detail. 
     This completes the fabrication of a semiconductor wafer device according to the third embodiment of the invention. As in the second embodiment, semiconductor integrated circuit devices can be fabricated by forming desired circuitry in and on the device formation film of the SOS substrate, using conventional semiconductor fabrication equipment with optical wafer sensors, and then dicing the wafer into individual chips. As noted in the second embodiment, before the circuitry is formed, the triple-layer light-reflecting film can be removed from parts of the wafer not illuminated by light from the optical wafer sensors, to reduce the thickness of the chips. 
     The third embodiment has effects similar to those of the second embodiment, and the additional advantage of reduced cost, compared to the second embodiment, because the second light-reflecting film is formed by heating in an oxygen atmosphere, so one CVD step can be omitted from the process described in the second embodiment. 
     The invention is not limited to a silicon-on-sapphire substrate. It is applicable to a semiconductor device with any type of transparent dielectric substrate, and may include any type of semiconductor material. 
     Those skilled in the art will recognize that further variations are possible within the scope of the invention, which is defined in the appended claims.