1. Field of the Invention
The present invention relates generally to semiconductor circuit processing, and more specifically to a method of field isolation using spin-on-glass (SOG) polymers.
2. Description of the Related Art
Local oxidization of silicon (LOCOS) is the conventional lateral isolation scheme. Referring to FIG. 1, in the LOCOS technique, a layer of silicon nitride 14 is deposited over a barrier oxide 12 overlying a silicon substrate 10. The barrier oxide 12 is a thin oxide which allows better adhesion between the nitride and silicon and acts as a stress relaxation layer during field oxidation. The nitride and oxide layers 14,12 are etched to leave openings 11 exposing portions of the silicon substrate where the local oxidation will take place. A boron channel-stop layer is ion implanted into the isolation regions. The field oxide 15 is grown within the openings 11 and the nitride and barrier oxide layers 14, 12 are removed. This completes the local oxidation.
When the silicon nitride openings 11 between active device regions are reduced to the sub-micrometer regime, a reduction in field oxide thickness within the openings is observed. This oxide thinning in the area where field spacing is narrow causes insufficient isolation between active regions 13. Thus, the scaling capability of the LOCOS technique is severely restricted by lateral encroachment of the oxide (often referred to as bird's beak) into the active region 13. As shown in FIG. 1, in LOCOS isolation, scalability is limited by oxide encroachment under the nitride during field oxidation. As this encroachment can typically be as high as 80 percent of the field oxide thickness [Lbb/Tfox=0.8], at spacings of 1.0 um, and a field oxide thickness of 5000 A, the bird's beak encroachment may be as high as 0.8 um, leaving only 0.2 um of active region 13.
Several enhancements to conventional LOCOS involve either additional mask levels and complex processing sequences or employ processes, such as lift-off, whose control and reliability are questionable.
One such technique is known as Side Wall Masked Isolation (SWAMI) described by K. Y. Chiu et al in IEDM Tech. Digest, pp 224-227, December 1982 and in IEEE Trans. Electron Devices, vol. ED-29, pp 536-540, 1982. In this technique, the silicon substrate is trenched before the oxidation step and thus stress, which often leads to defects in the silicon substrate, is a problem.
Another such technique is known as poly-buffered LOCOS which is described by R. L. Guldi et al. in J. Electrochem. Soc., 136, 3815, 1989; N. Hoshi et al., Proceedings of IEDM Meeting, p. 300, December 1986; and R. A. Chapman et al., Proceedings of IEDM Meeting, p. 362, December 1987. In this technique, in addition to the complexity of process steps, the danger exists of a double bird's beak if the polysilicon is allowed to be oxidized before the nitride deposition.
Yet another known technique uses a spin-on-glass polymer. A spin-on-glass (SOG) composition is a liquid, silica-based composition that can be applied to the surface of a semiconductor wafer and spun with the wafer to provide a coating with a level top surface. As reported by G. Smolinsky, et al. in J. Electrochem. Soc., 137, 229, 1990, in this technique, SOG is filled in the trenches formed in the substrate, two different masks are used to accommodate large field areas and the SOG is annealed in nitrogen and dry oxygen for 30 minutes at 900° C. SOG is unstable and with this technique, the properties of the SOG are altered only slightly without approaching the properties of a thermal oxide, as reported by M. P. Woo, J. L. Cain and C. O. Lee in J. Electrochem. Soc, 137, 196 (1990) and by E. E. Ibok and S. Garg in Abstract No. 862, Proceedings of the 183rd Meeting of the Electrochem. Soc., Honolulu, Hi. (1993). Moreover, the different thermal expansion of the spin-on-glass and the underlying substrate contributes to the cracking of the spin-on-glass, thereby yielding poor reliability.
Furthermore, when wide isolation fields are required, a dip is formed in the spin-on-glass surface and the spin-on-glass does not retain a planar shape. A known solution is to fill up the isolation trenches with multiple spins. However, the additional spin-on-glass spins also add the spin-on-glass polymer on the active regions of the substrate, thereby requiring additional processing to remove the spin-on-glass polymer from the active regions. This additional processing is expensive and thus undesirable.
It is therefore an object of the invention to provide a method for effective isolation of active regions without any reduction in the size of the active regions on the semiconductor substrate.
It is another object of the invention to provide a method for isolation of active regions which requires a minimum number of process steps.
Another object of the invention is to provide a method of forming an isolation structure which can be used in the fabrication of VLSI (Very Large Scale Integration) devices.
It is a further object of the invention to provide such a method which can be used for both wide and narrow isolation locations.
Yet another object is to provide such a method which provides negligible bird's beak encroachment in the active region of the semiconductor device.
Still another object of the invention is to provide an isolation method which uses spin-on-glass in the field region reliably by preventing any cracking in the spin-on-glass.
In accordance with the object of this invention, a method of field isolation using spin-on-glass is achieved. A silicon oxide layer is formed over the surface of silicon substrate. A layer of silicon nitride is deposited overlying the silicon oxide layer. The silicon oxide and silicon nitride layers are patterned to provide openings, exposing portions of the silicon substrate to be trenched. Trench regions are formed in the openings and the trenches are filled with spin-on-glass. The spin-on-glass is cured and annealed at a temperature which is higher than the oxidizing temperature of the spin-on-glass polymer, thereby ensuring a maximum shrinkage in the spin-on-glass and preventing the spin-on-glass from cracking. The spin-on-glass is then oxidized at the oxidizing temperature. The structure is masked with a source-drain mask with negative imaging to expose the active regions. The oxidized spin-on-glass covering the nitride and the nitride are etched off leaving the field regions.
Other objects and advantages of the invention will be apparent to those of ordinary skill in the art having reference to the following specification together with the drawings.
In the accompanying drawings forming a material part of this description, there is shown:
FIG. 1 which illustrates a cross-sectional representation of a conventional LOCOS isolation technique.
FIGS. 2-7 which schematically illustrate in cross-sectional representation, the preferred embodiment of the present invention.
FIG. 8 which compares the fourier transform infrared (FTIR) spectra for a silicon oxide with a cured and annealed spin-on-glass.
FIG. 9 which shows a cross-sectional representation of a SEM photograph showing the presence of bird's beak encroachment according to the conventional LOCOS technique.
FIG. 10 which shows a cross-sectional representation of a SEM photograph showing the absence of bird's beak encroachment according to the method of the present invention.
FIG. 11 shows the sequence of steps in the fabrication process of the spin-on-glass isolation structure in the preferred embodiment of the present invention.
FIG. 12 shows the oxidation process steps according to the method of the present invention.
The process, steps and structures described below do not form a complete process flow for manufacturing semiconductor circuits. The present invention can be practiced in conjunction with semiconductor circuit fabrication techniques currently used in the art and known to those of ordinary skill, and only so much of the commonly practiced process steps are included as are necessary for an understanding of the present invention. The figures representing cross-sections of portions of a semiconductor circuit during fabrication are not drawn to scale, but instead are drawn so as to illustrate the important features of the invention.
FIG. 11 outlines the sequence of steps in the fabrication process to form the spin-on-glass isolation structure 1 (See FIG. 7). Details of the steps according to the invention are described with particularity herein. Referring more particularly to FIG. 2, there is shown a monocrystalline silicon substrate 10. A layer 12 of silicon oxide is grown on the surface of the substrate to a preferred thickness of at least 100 angstroms. A layer of silicon nitride 14 is deposited over layer 12. A photoresist layer 16 (source-drain mask) is then formed over the silicon nitride layer 14 and patterned using conventional lithography and etching techniques to provide openings 18 and 20.
Referring now to FIG. 3, trenches 30 are formed in substrate 10 through the openings 18 and 20. Preferably, trenches 30, which are 0.5 to 10 micrometers wide and up to approximately 0.5 micrometers deep are formed using a conventional process.
Referring now to FIG. 4, the photoresist layer 16 is removed. A spin-on-glass polymer is filled into the trenches 30 and applied on top of the nitride layer 14 forming a spin-on-glass layer 40. Trenches 30 are filled with multiple, successive applications of the spin-on-glass polymer. As shown in FIG. 4, spin-on-glass layer 40 comprises portions 42 which designate the spin-on-glass polymer in the trenches 30; portions 44 which designate the spin-on-glass polymer applied over portions 42; and portions 48 which designate the spin-on-glass polymer applied on the silicon nitride layer 14. The spin-on-glass polymer is preferably a siloxane-based polymer. A silicate-based polymer may also be used.
Next, the spin-on-glass layer 40 is cured and annealed in an environment of about 10% oxygen by volume to a high temperature up to approximately 1,000 degrees centigrade. This temperature is equal to the maximum post oxidation temperature which the substrate 10 would be exposed to. The curing and annealing ensure that the spin-on-glass layer 40 is subject to a maximum shrinkage for subsequent thermal cycles in further processing of the semiconductor substrate for eliminating cracks in the spin-on-glass layer 40. Because spin-on-glass layer 40 shrinks during the high-temperature anneal, it does not introduce any crystalline defects in the trenches. The spin-on-glass layer 40 is then oxidized in an environment containing steam and at a temperature which is less than the curing and annealing temperature, for example, about 900 degrees centigrade. Preferably, the oxidation time is about 15 minutes. FIG. 12 shows the oxidation process steps. After the oxidization step, the spin-on-glass layer 40 has properties analogous to that of a thermally grown oxide on silicon.
Referring to FIG. 5, a photoresist layer 50 (negative resist, source-drain mask) is formed over portions 44 according to conventional processes to expose portions 48.
In FIG. 6, portions 48 of FIG. 5 have been etched and the photoresist layer 50 removed using conventional techniques. The negative resist portions 48 are removed without the need for separate process steps. The silicon nitride layer 14 is also etched away using conventional methods. This process results in the final field oxidation structure 1, illustrated in FIG. 7.
Referring now to FIG. 8, the FTIR spectra of both a thermal oxide and the spin-on-glass layer 40 (FIG. 4) are shown. The FTIR spectra of both the thermal oxide (curve (a)) and layer 40 (curve (b)) appear to be equivalent for the peak characteristics of SiO2 (seen at 810, 1080 and a shoulder at 1250 cm-1) and appear at virtually the same wave number. More particularly, a comparison of curves (a) and (b) shows that the 1250 cm-1 peak appeared at 1250.58 cm-1 for the thermal oxide and at 1250.23 cm-1 for layer 40. The 1080 cm-1 peak appeared at 1082.96 cm-1 for the thermal oxide and at 1081.1 cm-1 for layer 40. The 810 cm-1 peak appeared at 809.7 cm-1 for the thermal oxide and at 810.6 cm-1 for layer 40. For layer 40, the Si--CH3 peak generally observed at 1275 cm-1 was absent and the main Si--O peak at 1080 cm-1 had shifted from 1040 cm'1. Moreover, the etch rates of both films were found to be comparable--approximately 300 Å/min for the thermal oxide and 350 Å/min for the thermalized layer 40. The above data indicate virtually complete conversion of layer 40 to a film equivalent to thermal oxide.
FIGS. 9 and 10 show SEM micrographs of field oxide regions of a typical memory device fabricated according to the conventional LOCOS and the isolation structure 1 formed according to the present invention, respectively, immediately after field oxidation.
Referring to FIG. 9, a SEM photograph is shown showing the bird's beak encroachment in the isolation structure formed using a conventional LOCOS technique (FIG. 1). The field oxide thickness is approximately 4000 Å and the bird's beak length is approximately 3000 Å. The SEM photograph in FIG. 10 shows a negligible bird's beak encroachment which results when employing a method according to the invention. The field oxide thickness is approximately 5000 Å and the bird's beak length is negligible. In FIG. 9, the barrier nitride has been etched off. The film covering the wafer surface is a nitride film provided to obtain a contrast and outline the field region during SEM. The nitride layer is not part of the device structure. In FIG. 10, the nitride layer 14, has not been etched off.
With the present invention, the semiconductor circuit layout density is maximized by allowing n and p devices to butt against the trenches and good surface planarity is obtained.
While the invention has been particularly shown and described as reference to the preferred embodiment thereof, it will be understood by those skilled in the art that various changes in the form and details may be made without departing from the spirit and scope of the invention. For example, an oxide layer deposited by chemical vapor deposition may be placed under the spin-on-glass layer 40 or the spin-on-glass layer 40 may be encapsulated with top and bottom oxide layers.
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|U.S. Classification||438/435, 257/E21.546, 438/438|