1. Field of the Invention
The present invention pertains to a method for etching of patterned layers. The method is useful in high temperature etch processes where the layers are high temperature masking materials or are metal-comprising layers such as copper, platinum, iridium, and barium strontium titanate. The method is also useful in lower temperature etch processes in which organic polymeric layers, such as low k dielectrics, are etched to form damascene process structures. The present method may also be used for lower temperature etching of metal layers such as aluminum or tungsten, when the pattern geometry is small (less than about 0.25 .mu.m) and etch selectivity is a problem.
2. Brief Description of the Background Art
In the field of semiconductor device fabrication, particularly with the continuing trend toward smaller device feature sizes, the etch processes which are used to construct conductive features such a metal interconnects and contacts have become particularly critical. The new devices, having feature sizes in the range of about 0.25 .mu.m and smaller, place an emphasis on both the precise profile achievable during pattern etching and on the avoidance of any residue remaining after etch which causes problems during subsequent processing or problems in long term device function.
We previously worked to develop a plasma etching system which reduces and avoids the formation of residue on the surface of a copper layer during pattern etching of the copper. The etching systems useful in etching of the conductive material are described in copending application Ser. Nos. 08/891,410, filed Jul. 9, 1997, and 08/911,878, filed Aug. 13, 1997, both of which are hereby incorporated by reference and both of which are assigned to the assignee of the present application.
We have simultaneously been working to develop an etching process which permits the development of patterning masks which can transfer a desired pattern to adjacent layers in a manner which reduces or avoids the formation of mask residue on the etched structure. Further, we have worked to develop an etching process useful in etching organic polymeric materials for damascene processes, where the conductive layer is applied over the patterned surface of a dielectric layer to form desired conductive interconnect and contact structures.
FIGS. 1A-1E show a schematic cross-sectional view of a typical plasma etch stack for etching a metal-comprising layer at temperatures in excess of about 150.degree. C. as it progresses through a series of steps including the etching of both dielectric and conductive layers. This etch stack is of the kind known and used prior to the present invention. FIG. 1A shows a complete etch stack, including, from bottom to top, Substrate 102, which is typically a dielectric layer overlying a semiconductor substrate (such as a silicon wafer substrate) or which may be the semiconductor material itself, depending on the location on a given device surface. Barrier layer 104, which prevents the diffusion and/or migration of material between conductive layer 106 and substrate 102; conductive layer 106, which is typically aluminum or copper, but might be tungsten, platinum, or iridium, for example. Anti-reflective-coating (ARC) layer 108, which is typically a metal-containing compound and which enables better imaging of an overlying patterning layer. Pattern masking layer 110, which is typically a layer of silicon dioxide or similar inorganic material which can withstand the high temperatures encountered during etching of conductive layer 106, and which can be patterned and used as a mask during such etching. And, photoresist layer 112 which is typically an organic-based material which is stable at low temperatures and which is used to pattern masking layer 110, which is stable at higher temperatures. In FIG. 1A, photoresist layer 112 has already been patterned to provide the feature shape desired to be transferred to pattern masking layer 110.
FIG. 1B shows the stack described in FIG. 1A, where the pattern in photoresist layer 112 has been transferred to pattern masking layer 110, using a standard plasma etching technique. When masking layer 110 comprises a silicon-containing material, such as silicon dioxide, the etch plasma typically comprises a fluorine-generating species. Preferably the plasma selectivity is for the silicon dioxide over the photoresist material.
FIG. 1C shows the next step in the process of etching conductive layer 106, where the photoresist layer 112 has been stripped from the surface of pattern masking layer 110. This stripping procedure may be a wet chemical removal or may be a plasma etch which is selective for the photoresist layer 112 over the pattern masking layer 110. Stripping of photoresist layer 112 is carried out for two reasons. The organic-based photoresist materials typically used for layer 112 would melt or become distorted in shape at the temperatures commonly reached during the etching of conductive layer 106. This could lead to distortion of the pattern which is transferred to conductive layer 106. In addition, polymeric species generated due to the exposure of the surface of photoresist layer 112 to the etchant plasma tend to contaminate adjacent surfaces during the etching of conductive layer 106, thereby decreasing the etch rate of conductive layer 106.
The procedure of using a photoresist material to pattern an underlying silicon oxide patterning layer is described in U.S. Pat. No. 5,067,002 to Zdebel et al., issued Nov. 19, 1991. Zdebel et al. mention the need to remove the photoresist material prior to etching of underlying layers, to avoid contamination of underlying surfaces with the photoresist material during etching of such underlying layers. David Keller describes the use of an ozone plasma for the purpose of dry etch removal of a photoresist mask from the surface of an oxide hard mask in U.S. Pat. No. 5,346,586, issued Sep. 13, 1994. Mr. Keller also mentions that it is easier to etch selectively to a gate oxide when there is no photoresist present during a polysilicon gate oxide etch step.
FIG. 1D shows the next step in the etching process, where the desired pattern has been transferred through ARC layer 108, conductive layer 106, and barrier layer 104. Typically all of these layers are metal comprising layers, and a halogen containing plasma can be used to etch the pattern through all three layers. At this point, the problem is the removal of the residual silicon dioxide hard masking material and the removal of residue deposits of the silicon dioxide masking material from adjacent surfaces. The residual hard masking material is present as residual masking layer 110, and the residue deposits as 114 on the surface of the patterned conductive layer 106 and the surface of substrate 102.
In the case of the deposit 114 on the surface of patterned conductive layer 106, deposit 114 can trap residual chemical etch reactants under deposit 114 and against the surface of patterned conductive layer 106, leading to subsequent corrosion of conductive layer 106. That corrosion is shown on FIG. 1D as 116.
In addition, when substrate 102 is a low dielectric constant material, for purposes of reducing electrical interconnect delays, residual masking layer 110 which remains after pattern etching through layers 108, 106, and 104 (as shown in FIG. 1D) can deteriorate device performance or cause difficulties in future processing steps (particularly during contact via etch). This makes it important to remove any residual masking layer 110 from the surface of ARC layer 108.
Further, when a dielectric layer 118 is applied over the surface of the patterned conductive layer 106, as shown in FIG. 1E, if residual masking layer 110 is not removed, a non-planar surface 120 is produced. A non-planar surface creates a number of problems in construction of a multi-conductive-layered device, where additional patterned conductive layers (not shown) are constructed over the surface 120 of dielectric layer 118.
With the above considerations in mind, we wanted to develop a patterning system, including a multi-layered structure and a method for its use which would provide for the easy removal of residual masking layer material after completion of the patterning process. During the development of the etching method we discovered a particular etch chemistry which is equally applicable to etching organic dielectric layers for formation of contact structures in conventional metallization processes, and in the formation of damascene and dual damascene structures.