Patent Publication Number: US-6713348-B2

Title: Method for forming an etch mask during the manufacture of a semiconductor device

Description:
This is a division of U.S. Ser. No. 09/677,267 filed Sep. 26, 2000 and issued Nov. 12, 2002 as U.S. Pat. No. 6,479,861, which was a division of U.S. Ser. No. 09/370,064 filed Aug. 6, 1999 and issued Sep. 26, 2000 as U.S. Pat. No. 6,124,167. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to the field of semiconductor processing, and more particularly to a method for forming an etch mask and exemplary uses therefor. 
     BACKGROUND OF THE INVENTION 
     During the manufacture of a semiconductor device a large number of transistors and other structures are formed over a semiconductor substrate assembly such as a semiconductor wafer. As manufacturing techniques improve and transistor density increases as feature size decreases, one manufacturing step which can create difficulties is photolithography, as there is a limit to the minimum feature size which can be formed with conventional equipment. 
     Various attempts have been made to overcome the limitations of conventional photolithography. For example, U.S. Pat. No. 5,750,441 by Figura et al., assigned to Micron Technology, Inc. and incorporated herein by reference in its entirety, describes various patterning techniques which have been developed in an attempt to decrease the allowable feature size using conventional lithographic equipment. 
     Using an oxygen plasma treatment to alter the etch characteristics of a material has been demonstrated. Hicks, et al. (S. E. Hicks, S. K. Murad, I. Sturrock, and C. D. W. Wilkinson, “Improving the Resistance of PECVD Silicon Nitride to Dry Etching Using an Oxygen Plasma,”  Microelectronic Engineering , 35, pp. 41-44, 1997) teaches the treatment of a silicon nitride layer to increase its resistance to an etch. After the silicon nitride layer is formed using plasma enhanced chemical vapor deposition, it is subjected to a treatment in a reactive ion etch chamber comprising a radio frequency power of 50 Watts, an oxygen flow rate of 25 standard cubic centimeters, and a gas pressure of 100 millitorr thereby resulting in a direct current bias of 110 volts. Hicks teaches a layer which is homogeneously densified. The etch rate of an untreated silicon nitride layer using SF 6  reactive ion etching was demonstrated to be up to 1,000 times greater than a treated silicon nitride subjected to the same etch conditions. 
     A patterning technique which can form device features smaller than those allowable by conventional photolithography equipment would be desirable. 
     SUMMARY OF THE INVENTION 
     The present invention provides a new method which decreases the minimum device feature size that can be formed with conventional photolithography equipment. In accordance with one embodiment of the invention a semiconductor substrate assembly is provided and a layer of polycrystalline silicon (poly) is formed thereover. A silicon nitride layer is formed over the poly layer, and a silicon dioxide layer is formed over the silicon nitride. The nitride and oxide layers are patterned with an etch mask having a width, thereby resulting in cross sectional sidewalls in the silicon nitride. The sidewalls of the silicon nitride are treated with an oxygen plasma which alters the etch characteristics of the silicon nitride sidewalls. 
     The silicon dioxide is removed, as is the untreated portion of the silicon nitride layer thereby resulting in pillars of treated silicon nitride having a width less than the width of the mask. The poly layer is etched using the silicon nitride pillars as an etch mask, thereby resulting in poly features, each of which has a width less than the width of the original etch mask. 
    
    
     Objects and advantages will become apparent to those skilled in the art from the following detailed description read in conjunction with the appended claims and the drawings attached hereto. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross section depicting a semiconductor substrate assembly having a polycrystalline silicon (poly) layer, a nitride layer, an oxide layer, and a resist layer formed thereover; 
     FIG. 2 is a cross section depicting the FIG. 1 structure after patterning the nitride and oxide layers; 
     FIG. 3 depicts the FIG. 2 structure after removal of the resist layer and the treatment of the nitride with an oxygen plasma; 
     FIG. 4 depicts the FIG. 3 structure after removal of the oxide and the untreated nitride which results in pillars of treated nitride; 
     FIG. 5 depicts the FIG. 4 structure after etching of the poly layer using the pillars as a mask; 
     FIG. 6 depicts the FIG. 5 structure after removal of the nitride pillars; 
     FIG. 7 is a cross section depicting a first step in the formation of a floating gate device comprising a semiconductor substrate assembly having a gate oxide, a floating gate poly, a nitride, an oxide, and a patterned resist thereover; 
     FIG. 8 depicts the FIG. 7 structure after removal of the resist and treatment of the nitride layer; 
     FIG. 9 depicts the FIG. 8 structure after etching of the oxide and removal of the untreated nitride; 
     FIG. 10 depicts the FIG. 9 structure after etching of the poly layer using the pillars as an etch mask; 
     FIG. 11 depicts the FIG. 10 structure after the removal of nitride pillars, and the formation of an intergate oxide, control gate poly, and patterned resist; 
     FIG. 12 depicts the FIG. 11 structure after etching the control gate poly and intergate dielectric; and 
     FIG. 13 depicts an alternate floating gate device. 
     It should be emphasized that the drawings herein may not be to exact scale and are schematic representations. The drawings are not intended to portray the specific parameters, materials, particular uses, or the structural details of the invention, which can be determined by one of skill in the art by examination of the information herein. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A first embodiment of the invention which decreases the device feature size that can be produced with conventional semiconductor photolithography equipment is depicted in FIGS. 1-6. FIG. 1 depicts a semiconductor wafer substrate assembly  10  which can comprise a semiconductor wafer and one or more layers, such as an overlying dielectric, depending on the use of the invention. FIG. 1 further depicts a layer  12  such as polycrystalline silicon (poly) which is to be patterned. A 2,000 angstrom (Å) poly layer can be formed in a low pressure chemical vapor deposition (LPCVD) furnace at about 620° C. in about 30 minutes using an atmosphere of silane gas (SiH 4 ), or by using another workable method. 
     Next, a layer of plasma enhanced (PE) silicon nitride (Si 3 N 4 )  14  is deposited by chemical vapor deposition (CVD). The deposition can be performed in an Oxford PlasmaTechnology μP80 Plus by placing the wafer substrate assembly on the grounded electrode and heating the assembly to between about 200° C. and about 600° C., preferably about 400° C. The other electrode is driven at a frequency of about 13.56 MHz with a power of between about 100 watts (W) and about 800 W, preferably about 500 W. Silane gas, for example having a purity of 99.999%, ammonia (NH 3 ) having a purity of 99.995%, and nitrogen (N 2 ) having a purity of 99.995% are each pumped into the chamber. Silane can be pumped at a flow rate of between about 5 standard cm 3  (sccm) and about 300 sccm, more preferably between about 10 sccm and about 100 sccm, and most preferably about 20 sccm, ammonia can be pumped at between about 5 sccm and about 300 sccm, more preferably between about 10 sccm and about 150 sccm, and most preferably about 20 sccm, and nitrogen can be pumped at a flow rate of between about 1,500 sccm and about 5,000 sccm, preferably at about 4,000 sccm. Silicon nitride forms at a rate of between about 500 Å/min and about 5,000 Å/min, and thus the process can be continued for between about 1 minute and about 10 minutes, depending on the flow rates, power, etc., to form a 5,000 Å thick layer. 
     Next, a layer of silicon dioxide  16  is formed using a PE tetraethyl orthosilicate (TEOS) source. For example, an SiO 2  layer between about 1,000 Å and about 3,000 Å thick, for example about 2,000 Å thick can, be formed by providing a supply of liquid TEOS at a temperature of between about 200° C. and about 550° C., preferably about 400° C. for about 20 seconds. 
     Finally, a patterned photoresist layer  18  is formed over the TEOS layer  16  using means known in the art. For purposes of illustration, the photoresist layer provided has a width which is the minimum allowable by current photolithography. 
     The structure of FIG. 1 is etched with a dry anisotropic etch to result in the structure of FIG.  2 . An etch which would adequately remove the SiO 2  layer  16  and the Si 3 N 4  layer  14  having the thicknesses described above includes C 4 F 8  at a flow rate of between about 5 sccm and 50 sccm, preferably between about 5 sccm and about 20 sccm, and more preferably about 10 sccm, and argon at a flow rate of between about 50 sccm and about 500 sccm, preferably about 300 sccm, for between about 20 seconds and 200 seconds, preferably about 100 seconds. This etch has a selectivity to poly, which facilitates stopping the etch on the underlying poly layer  12 . In other uses of the invention where layer  12  is not poly, the etch of the FIG. 1 structure may be performed with another suitable etch which facilitates stopping on the selected layer. After etching, the resist layer  18  is removed according to means known in the art. 
     The structure of FIG. 2 with resist  18  removed is treated with an oxygen plasma. The structure to be treated is placed in a down flow plasma chamber. The operating conditions include a radio frequency (RF) power of between about 300 W and about 3,000 W, and more preferably between about 700 W and about 2,000 W, most preferably about 1,000 W, an oxygen flow rate of between about 50 sccm and about 5,000 sccm, preferably between about 50 sccm and about 1,000 sccm, most preferably about 200 sccm, and a gas pressure of between about 10 millitorr (mTorr) and about 1,000 mTorr, preferably about 100 mTorr for a duration of between about 1 minute and about 10 minutes for the structure described above. After the oxygen plasma treatment the structure of FIG. 3 results wherein the silicon nitride layer comprises treated areas  30  along the sidewalls and untreated areas  32  in the center where the treatment does not penetrate. The width of the treated material is self-limiting as it forms and protects the center portion of the nitride so that the center portion remains untreated. Treating the silicon dioxide does not substantially alter the etch characteristics of the material over an untreated portion. 
     Contrary to the process and results of Hicks et al. previously described which uses a layer treated throughout, the instant embodiment of the invention uses an untreated or minimally treated center portion. Thus a shorter duration and a lighter treatment may be used to prevent or reduce treatment of the center portion of the silicon nitride layer. In further contrast to Hicks, an overlying layer (in the instant embodiment, TEOS layer  16 ) reduces treatment of the center portion. It is believed that an untreated to treated etch ratio of at least 30:1 will be obtained with the process and structure as described above. 
     Subsequently, the structure of FIG. 3 is etched, for example with an anisotropic RIE etch using SF 6  at a flow rate of from about 5 sccm to about 50 sccm, more preferably between about 5 sccm and about 30 sccm, and most preferably about 20 sccm, an RF power of between about 50 W and about 300 W, more preferably between about 50 W and 200 W, most preferably about 100 W, and a pressure in the range of from about 10 mTorr to about 200 mTorr, more preferably from about 20 mTorr to about 200 mTorr, most preferably about 100 mTorr. Generally, increasing the pressure will decrease the etching of the treated Si 3 N 4  and increase etching of the untreated Si 3 N 4 . Both the treated and untreated portions of the silicon dioxide layer  16  are readily etched, as is the untreated silicon nitride  32  after the oxide has been removed. Etching the oxide  16  and untreated silicon nitride  32  result in the structure of FIG. 4 including pillars of treated silicon nitride  40 . 
     The FIG. 4 structure is subsequently etched to pattern the poly layer using the Si 3 N 4  pillars  40  as an etch mask to result in the poly structures  50  of FIG.  5 . An anisotropic etch using Cl 2  at a flow rate of between about 10 sccm and about 200 sccm, preferably about 80 sccm and a pressure of between about 2 mTorr and about 50 mTorr, preferably about 10 mTorr will clear most of the exposed poly. The etch characteristic can be altered to produce a more isotropic etch to ensure removal of the residual material in the narrow areas between pillars  40 . 
     Finally, the Si 3 N 4  pillars  40  are removed to result in the structure of FIG. 6. A hydrofluoric acid (HF) wet etch in a 10% HF acid for between about 5 minutes and about 20 minutes will sufficiently remove the treated silicon nitride without excessively removing the poly. 
     It can be seen that the above process yields poly features having a width less than the width of the photoresist  18  of FIG.  1 . The width of the pillars  40  depends in part on the length of time the silicon nitride is treated. For example, treating 40% of the nitride at each sidewall results in 20% untreated nitride in the center, and thus the width of the pillars will be 40% of the width of the original mask  18 . Treating 25% of the nitride at the sidewalls will similarly result in pillars having a width which is 25% of the width of the original resist mask. Further, the pillars can be narrowed after their formation using an isotropic etch which etches but does not completely remove them. 
     Various other uses and embodiments can be determined for this process by one of ordinary skill in the art. For example, another embodiment of the invention which is used to form a floating gate memory structure is depicted in FIGS. 7-12. Floating gate memory devices such as erasable programmable read-only memories (EPROMs) electrically-erasable PROMs (EEPROMs), and flash EEPROMs are well known in the art. For example the following US Patents by Roger R. Lee, assigned to Micron Technology, Inc. and incorporated herein by reference in their entirety, describe various read-only memory cells and their methods of manufacture: U.S. Pat. No. 5,089,867 issued Feb. 18, 1992; U.S. Pat. No. 5,149,665 issued Sep. 22, 1992; U.S. Pat. No. 5,192,872 issued Mar. 9, 1993; U.S. Pat. No. 5,241,202 issued Aug. 31, 1993; U.S. Pat. No. 5,260,593 issued Nov. 9, 1993; U.S. Pat. No. 5,444,279 issued Aug. 22, 1995; U.S. Pat. No. 5,658,814 issued Aug. 19, 1997. 
     FIG. 7 depicts a semiconductor wafer assembly  70 , a gate oxide layer  72  such as a tunnel oxide, a floating gate poly layer  74  between about 2,000 Å and about 8,000 Å thick, a silicon nitride layer  76  between about 2,000 Å and about 8,000 Å thick, a TEOS layer  78  between about 500 Å and about 2,000 Å thick, and a patterned resist layer  80 . The TEOS oxide layer  78  and Si 3 N 4  layer  76  are etched using the resist layer  80  as a pattern to result in a structure similar to that of FIG.  2 . The resist layer is removed and the structure is treated with an oxygen plasma as described above for between about 3 minutes and about 30 minutes to result in the structure of FIG. 8, which comprises a Si 3 N 4  layer having oxygen plasma treated portions  82  and an untreated portion  84 . 
     An anisotropic etch is performed on the FIG. 8 structure to remove the TEOS layer and the untreated Si 3 N 4  layer and to result in the structure of FIG. 9 having treated Si 3 N 4  pillars  90 . An RIE etch as described above using SF 6  at a flow rate of from about 10 sccm to about 30 sccm, an RF power of between about 50 W and about 500 W, and a pressure of between about 20 mTorr and about 200 mTorr would be sufficient. 
     Next, the FIG. 9 structure is etched to result in the structure of FIG. 10 using the Si 3 N 4  pillars  90  as an etch mask to result in the poly structure  100  of FIG.  10 . In this embodiment the floating gate comprises a generally U-shaped vertical cross section. An anisotropic etch using Cl 2  at a flow rate of between about 10 sccm and about 200 sccm, preferably 80 sccm, and a pressure of between about 2 mTorr and about 50 mTorr for about 2 minutes will clear most of the exposed portion of an 8,000 Å poly layer and will leave a horizontally-oriented poly “stringer” bridging or spanning the two vertically-oriented poly pillars as depicted. Subsequently, the Si 3 N 4  pillars  90  are removed, for example with a wet HF etch as described above. 
     As depicted in FIG. 11, an intergate dielectric layer  110 , for example an oxide-nitride-oxide stack, is formed, and a control gate layer  112  is formed according to means known in the art. A layer of tungsten silicide (not depicted) can be formed according to means in the art to decrease the sheet resistance of the control gate. A patterned photoresist layer  114  is provided which will define the control gate and further define the floating gate. The FIG. 11 structure is etched to form the structure of FIG. 12, then the resist is removed. As depicted in FIG. 12, the control gate has a lower surface which is generally conformal with the floating gate. Next, wafer processing continues according to means in the art which will result in a quasi split gate memory device. 
     In an alternate embodiment, the poly layer  112  of FIG. 11 is formed thick enough to impinge on itself within the concave surface formed by the floating gate. This effectively forms a thicker poly over the floating gate than between adjacent transistors. The mask  114  is omitted thereby eliminating a masking step. An anisotropic etch of the poly is performed to stop on oxide  110  and to clear the poly from between adjacent transistors. The etch is timed such that a portion of the poly  112  remains in the recess defined by the floating gate as depicted in FIG.  13 . In this embodiment the floating gate and control gate each have an upper surface, and the upper surfaces are generally coplanar as depicted. Further, the floating gate has a generally concave profile and the lower surface of the control gate is conformal with the concave profile. A process using chemical mechanical polishing (CMP) can also be used to form the structure of FIG. 13 from the description herein by one of ordinary skill in the art. 
     While this invention has been described with reference to illustrative embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as additional embodiments of the invention, will be apparent to persons skilled in the art upon reference to this description. It is therefore contemplated that the appended claims will cover any such modifications or embodiments as fall within the true scope of the invention.