Abstract:
A method of forming a composite silicon oxide layer over a semiconductor device. The composite silicon oxide layer is formed between the semiconductor device and a doped silicate glass layer. The composite silicon oxide layer comprises two silicon oxide layers, each having a different silicon/oxide composition. The oxygen-rich oxide layer or silicon dioxide layer is formed directly above the semiconductor device, and the silicon-rich oxide layer is formed above the silicon dioxide layer next to the doped silicate glass layer. Both the silicon dioxide layer and the silicon-rich oxide layer are formed in the same plasma deposition chamber.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional application of, and claims the priority benefit of, U.S. application Ser. No. 09/515,015 filed on Feb. 29, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to a method of forming a semiconductor device. More particularly, the present invention relates to a method of forming a composite silicon oxide layer over a semiconductor device. 
     2. Description of Related Art 
     In most semiconductor devices, doped silicate glass such as borophosphosilicate glass (BPSG) is generally used as an inter-layer dielectric (ILD). This is because doped silicate glass has a low annealing temperature, thereby possessing and ability to lower the thermal budget of fabrication. However, due to the high porosity of doped silicate glass, some of the dopants inside the silicate glass can easily diffuse into neighboring layers at a moderately high temperature. Thus, semiconductor devices under the silicate glass layer may be contaminated in the back-end process, leading to reliability problems. 
     To prevent the contamination of a semiconductor device by dopants inside silicate glass, an isolating layer is often formed between the semiconductor device and the silicate glass layer. In other words, before doped silicate glass is deposited to form the inter-layer dielectric (ILD), undoped ozone-TEOS oxide (USG), plasma-enhanced chemical vapor deposition (PECVD) oxide, or silicon-rich oxide (SRO approximated formula SiO x , x&lt;2) is first deposited over the semiconductor device. Among USG, PECVD oxide and silicon-rich oxide, silicon-rich oxide is the best material for preventing device contamination. 
     However, using silicon-rich oxide as an isolation layer has its own problems too. In depositing silicon-rich oxide, hydrogen molecules are often produced. These hydrogen molecules are often retained inside the silicon-rich oxide after the reaction, leading to a possible contamination of nearby semiconductor devices. Consequently, hot carrier degradation, current leakage and resistivity change of semiconductor devices occur more frequently. 
     SUMMARY OF THE INVENTION 
     Accordingly, one object of the present invention is to provide a method of forming a composite silicon oxide layer over a semiconductor device so that hot carrier degradation, current leakage and resistivity change of the device are minimized. 
     To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a method of forming a composite silicon oxide layer. The composite silicon oxide layer is formed between the semiconductor device and a doped silicate glass layer. This composite silicon oxide layer is composed of two silicon oxide layers, each having a different silicon-to-oxygen ratio. The silicon oxide layer that contacts the doped silicate glass layer is a silicon-rich oxide layer (SiO x , x&lt;2), while the other silicon oxide layer that contacts the semiconductor device is a silicon dioxide (SiO 2 ) layer. 
     In addition, the two silicon oxide layers having different silicon/oxygen ratios are formed in the same plasma deposition step. The plasma deposition is carried out using, for example, SiH 4 —N 2 O plasma. The method of controlling the silicon/oxygen ratios in each of the two silicon oxide layers includes changing the ratio of SiH 4  to N 2 O in the SiH 4 —N 2 O plasma. 
     In this invention, since the silicon-rich oxide layer of the composite layer is in contact with the doped silicate glass layer, dopants inside the doped silicate glass layer are prevented from diffusing into and contaminating the semiconductor device underneath. On the other hand, since the silicon dioxide layer in the composite layer is attached to the semiconductor device, any residual hydrogen molecules in the silicon-rich oxide is prevented from crossing into the underlying semiconductor device. Therefore, resistivity of the semiconductor device can be maintained and leakage current from the semiconductor device can be minimized. 
     Furthermore, since the two silicon oxide layers are formed by plasma deposition reactions in the same plasma chemical vapor deposition chamber, there is no additional complication other than that of forming a single silicon-rich oxide layer. 
     In brief, the composite silicon oxide layer of this invention is capable of preventing dopants in the doped silicate glass layer as well as residual hydrogen molecules in the silicon-rich oxide layer from contaminating the semiconductor device. Hence, quality of the semiconductor can be maintained. Moreover, since the two silicon oxide layers are formed in the same reaction chamber, no additional steps need to be taken. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings, 
     FIG. 1 is a flow chart showing the steps for forming the composite silicon oxide layer over a semiconductor device according to this invention; 
     FIG. 2 is a schematic cross-sectional view showing a composite silicon oxide layer over an N-type metal oxide semiconductor (NMOS) structure according to one preferred embodiment of this invention; 
     FIG. 3 is a schematic cross-sectional view showing a composite silicon oxide layer over a P-type sheet resistor according to one preferred embodiment of this invention; 
     FIG. 4 is a three-dimensional diagram showing thickness variation of the silicon dioxide layer and refractive index variation (silicon/oxygen variation) of the silicon-rich oxide layer versus yield in leakage current test for an NMOS device with a composite silicon oxide layer thereon according to this invention; and 
     FIG. 5 is a bar chart showing the effect of refractive index variation (silicon/oxygen variation) of the silicon-rich oxide layer on resistivity for a P-type resistor with a composite silicon oxide layer thereon according to this invention. 
    
    
     Note that the silicon/oxygen ratio in the silicon-rich oxide layer is indicated by the refractive index (RI). In other words the higher the silicon/oxygen ratio, the higher the refractive index will be. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 
     FIG. 1 is a flow chart showing the steps for forming the composite silicon oxide layer over a semiconductor device according to this invention. First, as shown in FIG. 1, a substrate having a device such as an NMOS transistor, a PMOS transistor, or a P-type sheet resistor therein is provided. Steps for forming the composite silicon oxide layer are next conducted. In other words, a silicon dioxide layer is first formed over the substrate, and then a silicon-rich oxide layer is formed over the silicon dioxide layer by changing the deposition reaction conditions. Finally, a doped silicate glass layer that functions as an inter-layer dielectric is formed over the silicon-rich oxide layer. In FIG. 1, the refractive index (RI) is an indication of silicon/oxygen ratio inside the silicon oxide layers. For instance, the silicon dioxide layer has a refractive index of 1.46 while the silicon-rich oxide (SiO x , x&lt;2) layer has a refractive index greater than 1.46. Moreover, the greater the value of the refractive index, the higher the ratio of silicon to oxygen will be. In the embodiment of this invention, refractive index of the silicon-rich oxide layer is between about 1.50 and 1.55. 
     In this invention, the silicon dioxide layer and the silicon-rich oxide layer of the composite silicon oxide layer are formed in the same plasma deposition step. The plasma used in the plasma deposition is preferably SiH 4 —N 2 O plasma. By varying the ratio between SiH 4  and N 2 O in the SiH 4 —N 2 O plasma, the silicon dioxide layer and the silicon-rich oxide layer with a pre-defined silicon/oxygen ratio can be formed in sequence over the semiconductor device in the same step. For example, the silicon dioxide layer is formed in a plasma chamber by setting the flow rate of SiH 4  to 125 cm 3 /min and the flow rate of N 2 O to 2000 cm 3 /min. On the other hand, to form the silicon-rich oxide layer having a refractive index of 1.52, the flow rate of SiH 4  is set to 150 cm 3 /min while the flow rate of N 2 O is set to 680 cm 3 /min. 
     In the following description, a composite silicon oxide layer is formed over an NMOS transistor and a P-type sheet resistor, respectively, to illustrate some applications (in FIGS. 2 and 3) of this invention. The advantages of this invention are illustrated through some testing figures (in FIGS.  4  and  5 ). 
     FIG. 2 is a schematic cross-sectional view showing a composite silicon oxide layer over an N-type metal oxide semiconductor (NMOS) structure according to one preferred embodiment of this invention. As shown in FIG. 2, the NMOS transistor  200  includes a P-type silicon substrate  210 , a polysilicon gate  220  and source/drain regions  230 . A silicon dioxide layer  240 , a silicon-rich oxide layer  250  and a doped silicate glass layer  260  (inter-layer dielectric) are sequentially formed over the NMOS transistor  200 . The silicon dioxide layer  240  and the silicon-rich oxide layer  250  together form the composite silicon oxide layer of this invention. 
     FIG. 3 is a schematic cross-sectional view showing a composite silicon oxide layer over a P-type sheet resistor according to one preferred embodiment of this invention. As shown in FIG. 3, the P-type sheet resistor  300  includes an N-type well  310  and a P-type diffusion region  320 . A silicon dioxide layer  340 , a silicon-rich oxide layer  350  and a doped silicate glass layer  360  (inter-layer dielectric) are sequentially formed over the P-type diffusion region  320 . The silicon dioxide layer  340  and the silicon-rich oxide layer  350  together form the composite silicon oxide layer of this invention. 
     In the examples of FIGS. 2 and 3, the silicon-rich oxide layers  250  and  350  have a thickness of about 1000 Å to 2000 Å while the silicon dioxide layers  240  and  340  have a thickness of about 200 Å to 1000 Å. 
     FIG. 4 is a three-dimensional diagram showing thickness variation of the silicon dioxide layer  240  and refractive index variation (silicon/oxygen variation) of the silicon-rich oxide layer  250  versus yield in leakage current test for the NMOS device  200  with a composite silicon oxide layer thereon according to this invention. As shown in FIG. 4, when the refractive index of the silicon-rich oxide layer  250  is fixed (at a fixed silicon/oxide ratio), a greater thickness of the silicon dioxide layer  240  means a smaller leakage current. When the refractive index of the silicon-rich oxide layer  250  is at the maximum value of 1.55, leakage current is considerably lowered by a silicon dioxide layer  240  that is just 500 Å thick. In other words, current leaks are effectively stopped by the silicon dioxide layer  240  in the composite oxide layer of this invention. 
     FIG. 5 is a bar chart showing the effect of refractive index variation (silicon/oxygen variation) of the silicon-rich oxide layer  350  on resistivity for a P-type resistor  300  with a composite silicon oxide layer thereon according to this invention. As shown in FIG. 5, the effect of the composite silicon oxide layer on the resistance of the P-type sheet resistor  300  is smaller (that is, error range of resistance value is smaller) compared with a P-type sheet resistor protected by just one silicon-rich oxide layer. In other words, the composite silicon oxide layer is better able to stabilize the resistance of the P-type sheet resistor  300 . The error range of the resistor is lowered considerably, especially when the refractive index (or silicon/oxygen ratio) of the silicon-rich oxide layer is between about 1.52 and 1.55. 
     In summary, since the silicon-rich oxide layer of the composite layer is in contact with the doped silicate glass layer, dopants inside the doped silicate glass layer are prevented from diffusing into and contaminating the semiconductor device underneath. On the other hand, since the silicon dioxide layer in the composite layer is attached to the semiconductor device, any residual hydrogen molecules in the silicon-rich oxide layer is prevented from crossing into the underlying semiconductor device. Therefore, resistivity of the semiconductor device can be stabilized and leakage current from the semiconductor device can be minimized. Furthermore, since the two silicon oxide layers are formed by plasma deposition in the same plasma chemical vapor deposition chamber, there is no additional complication than that of forming a single silicon-rich oxide layer. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.