Abstract:
A tunable CO 2  gas laser is used to selectively heat various SiO 2  -based materials to elevated temperatures while maintaining an active device region at relatively low temperatures, to, for example, induce densification and/or flow of the SiO 2  -based material to round off sharp edges and stops.

Description:
This is a continuation of application Ser. No. 143,983 filed Apr. 28, 1980, abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to heating of SiO 2  -based material in the semiconductor art. 
     The phosphosilicate glass flow process is used primarily to round off sharp edges of etched cuts on phosphosilicate glass to help prevent cracking at the edges of subsequently deposited metal or polycrystalline silicon fill and also to smooth the overall surface of a layer of phosphosilicate glass for subsequent controlled etching thereof. The flow process is normally induced by high temperature (approximately 950° C. or above) furnace anneal. 
     In general, the phosphosilicate glass layer is part of a structure initially formed by providing a silicon substrate with a thermally deposited silicon dioxide (SiO 2 ) layer thereon. A layer of P 2  O 5  -SiO 2  is then chemically vapor deposited on the layer of thermally deposited SiO 2  by, for example, reaction of a phosphorous-silane mixture and oxygen at low temperature, so that a top layer of phosphosilicate glass is provided. 
     A major problem with the prior art approach of inducing flow by high temperature anneal is that when device structures, i.e., structures which may be of material in close proximity to the phosphosilicate glass, are heated to a relatively high temperature in the furnace, their properties often undesirably change due to dopant diffusion, alloying and contamination. Another problem is that P 2  O 5 , typically in the concentration of 7-9 mol %, must be added to the SiO 2  base composition to lower the flow temperature. This subsequently enhances corrosion of the metal interconnections. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a highly efficient method of inducing densification and/or flow of phophosilicate glass or the like so as to overcome the problems set forth above. 
     Broadly stated, the invention comprises a method of inducing flow or densification of a first, SiO 2  -based portion of a structure in close proximity to a second portion of the structure, the first portion being of the type wherein flow or densification is induced therein upon application of heat thereto, comprising heating the first portion to an extent substantially greater than the second portion. 
     Broadly stated, the invention also comprises a method of inducing flow or densification of a first portion of a structure in close proximity to a second portion thereof and of the type wherein flow or densification is induced therein upon application of heat thereto, comprising applying a laser beam to the first portion to heat the first portion. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects will become apparent from a study of the following specification and drawings, wherein: 
     FIG. 1 is a schematic cross section of a semiconductor structure being treated with a laser beam. 
     FIG. 2 is a view similar to FIG. 1 but showing a smooth cavity in the surface of the semiconductor structure. 
     FIG. 3 is a view similar to FIGS. 1 and 2 but showing the surface of the semiconductor structure after the structure of FIG. 2 has been etched to form a concavity with sharp edges. 
     FIG. 4 is a view similar to FIG. 3, but showing the concavity after being heated with a laser beam to smooth the sharp edges of the concavity. 
     FIG. 5 is a view similar to FIGS. 1-4 but showing a complete device with metal contacts deposited over the smooth edges. 
     FIG. 6 is a graphical view of a theoretical temperature profile of phosphosilicate glass as a function of incident power density for a 10 -3  second laser pulse or dwell time corresponding to pulsed or Q-switched and continuous wave (CW) laser modes, respectively. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As explained above, FIGS. 1-5 illustrate the steps of the present invention. As shown in FIG. 1, a semiconductor structure 10 typified by an MOS device includes a first, SiO 2  -based portion 12 which is in the form of vapor deposited silicon oxide, grown phosphosilicate glass, which is in close proximity to a second structure portion 14. The second portion includes a silicon substrate 16 having a source/drain portion 18 formed therein, an oxide region 20, and a polycrystalline silicon interconnect 22. Isolation oxide 24 is also included. Typically, the direction &#34;A&#34; may be of the order of 1.0-1.5 μm, the dimension &#34;B&#34; may be of the order of 0.4 μm-0.6 μm, and the dimension &#34;C&#34; may be of the order of 0.4 μm-0.6 μm. 
     As described above, and as will be further described, it is desirable to apply heat to the phosphosilicate glass 12 to induce densification and/or flow thereof. As also discussed above, it will be seen that it is desirable to maintain the portion 14 at a relatively low temperature so as to avoid undesired dopant diffusion, alloying, and/or contamination. 
     The basis for the present process is the SiO 2  absorption maxima centered at about 1080 cm -1  (approximately 240 cm -1  band width FWHM) with an absorption coefficient of about 3×10 4  cm -1 . This absorption band is well suited for coupling to the 9.261 μm (1080 cm -1 ) emission of a suitable CO 2  scanning laser as shown at 30, which may be for example a Model 560 CO 2  gas laser as manufactured by Apollo Lasers, Inc., 6357 Arizona Circle, Los Angeles, CA 90045. For the composition SiO 2  up to 20 mol % P 2  O 5 , the 1080 cm -1  absorption band decreased monotonically with an increase in mol % of P 2  O 5 . Comparable concentrations of B 2  O 3 , As 2  O 5  or Sb 2  O 5  in SiO 2  will exhibit similar absorption characteristics. Additions of Si 3  S 4  to SiO 2  will also behave similarly. Thus, any SiO 2  based material with a greater than approximately 80 mol % SiO 2 , whether it be crystalline or amorphous, can be effectively coupled to the output of a CO 2  gas laser, and preferably one which is tunable. 
     At the wavelength under consideration, metallization has exhibited reflecting losses greater than about 95%, so that essentially no heat conduction thereby takes place. Also, the large thermal conductivity of metallization will act as a heat sink, pulling heat away from the active device. For silicon such as the substrate material 16, absorption occurs via photoexcitation of free carriers with a small contribution due to multi-phonon excitation. The absorption is in proportion to the dopant concentration and inversely proportional to the carrier mobility. Therefore, the coupling efficiency is significantly lower for both metal and silicon relative to SiO 2  -based material 12. 
     The temperature T in a solid heated by a laser beam of incident intensity I o  is given [1] as: ##EQU1## where D is the thermal diffusivity, ρ is the mass density, C p  is the specific heat, R is the reflectivity, α is the absorption coefficient, Z is the coordinate parallel to the incident radiation, and ##EQU2## where X, Y and Z are the cartesian coordinates of the solid. In the case of laser induced phosphosilicate glass flow, α -1  &lt;&lt;(2Dt) 1/2 , so that heat is created essentially at the surface and transported into the bulk by heat conduction. Assuming D, C p , and R to be temperature invariant, I to be spatially and temporally uniform, and that no latent heat due to phase transitions is involved, the solution to Eq. [1] is: ##EQU3## where T o  is the initial temperature, and T(Z,t) is the temperature at a point Z and time, 0≦t≦t I , where t I  is the pulse width or the dwell time of the incident radiation. 
     For phosphosilicate glass, the following material constants are assumed: 
     D=6×10 -3  cm 2  sec -1   
     p=2.27 g cm -3   
     C p  =1.0 J g -1  °C. -1   
     R=0.525 
     Under these conditions, in FIG. 6 is plotted the calculated temperature profile, T(Z,t)-T o  as a function of Z, for t=0.001 sec and I o  =10, 11, 12 and 13 MW cm -2 . These curves suggest that for phosphosilicate glass thicknesses greater than about 1 μm, large thermal gradients will exist which maintain the surface of the substrate at a relatively cool temperature during flow. Within the normal phosphosilicate glass thickness range of 0.5 to 1.5 μm, FIG. 2 suggests that maximum phosphosilicate glass thickness will be optimal for this purpose and that a significant increase in thickness will not be beneficial. In support of this, consider that 12 MW cm -2  will produce a surface temperature of 1156° whereas the temperature at depths of 0.98, 1.47, and 2.45 μm will be 792°, 664° C., and 409° C., respectively. The underlying portion 14 is then never subjected to elevated temperatures. 
     If the substrate is heated, it will be noted that T o  is increased, and the power density necessary to provide a certain T(Z,t)-T o  decreases. 
     It should be recognized that the present method has the most significant advantage that the portion 12 can be induced to flow and/or densify by application of heat thereto through use of the laser 30, but with it being understood that the portion 12 is heated to an extent substantially greater than the portion 14. As discussed above, it will be seen that the surface of the phosphosilicate glass 12 can be heated to an elevated temperature, whereas the phosphosilicate glass at a depth of about 1.47 μm will be heated to a much lower temperature. 
     The result of the method described above is shown in FIG. 2. It will be seen that a great degree of smoothing of the surface 13 of the phosphosilicate glass 12 has been achieved. This is highly desirable for purposes of etching since the placement of the etching window defined by well-known photoresist techniques can be accurately achieved. This is to be compared with FIG. 1, wherein, if etching was to take place without such flow, the photoresist edges would have to be placed approximately where the &#34;shoulders&#34; 13A, 13B are defined by the phosphosilicate glass 12. Also, because of the relatively smooth shape of the phosphosilicate glass 12 as shown in FIG. 2, the etching can be controlled as chosen to a higher degree. 
     The result of such etching of the FIG. 2 structure is shown in FIG. 3. It is to be noted that while etching down to the substrate 16 in the chosen place and in the chosen manner has been achieved, sharp corners 13C, 13D are now defined by the phosphosilicate glass 12. The process as described above is again undertaken in the same manner, causing the phosphosilicate glass 12 to again flow, so as to smooth all the phosphosilicate glass surface 13 as shown in FIG. 3, and in particular to smooth the corners 13C, 13D shown in FIG. 3. The resulting structure is shown in FIG. 4. 
     The smoothing of the phosphosilicate glass 12 into the form shown in FIG. 4 allows for the proper placement of the aluminum leads 32, 34 as shown in FIG. 5, it being noted that FIGS. 1 through 4 have in fact shown the process as undertaken on the right-hand half of the structure of FIG. 5, which will readily be seen to be an MOS device. It is thus insured that cracking of the aluminum leads 32, 34 will not take place. 
     It is to be understood that although an MOS structure is shown in the illustration of the present process, such process can readily be used in bipolar structures. As also discussed above, the process is intrinsically independent of the mol % of P 2  O 5 , As 2  O 5 , or B 2  O 3  up to about 20 mol %. 
     Through the use of a laser as described above, it will also be seen that extremely small areas can be made to flow as desired.