Abstract:
A method of improving high aspect ratio etching by reverse masking to provide a more uniform mask height between the array and periphery is presented. A layer of amorphous carbon is deposited over a substrate. An inorganic hard mask is deposited on the amorphous carbon followed by a layer of photodefinable material which is deposited over the array portion of the substrate. The photodefinable material is removed along with the inorganic hard mask overlaying the periphery. A portion of the amorphous carbon layer is etched in the exposed periphery. The inorganic hard mask is removed and normal high aspect ratio etching continues. The amount of amorphous carbon layer remaining in the periphery results in a more uniform mask height between the array and periphery at the end of high aspect ratio etching. The more uniform mask height mitigates twisting at the edge of the array.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application as a continuation of U.S. patent application Ser. No. 11/758,714 filed Jun. 6, 2007 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to improvements in high aspect ratio etching and, in particular, relates to reverse masking profiling to provide a more uniform mask height between the array and periphery portions of a memory cell to mitigate twisting during high aspect ratio etching. 
     Today&#39;s semiconductor-based integrated circuits and micro-electro-mechanical systems (MEMS) are pushing the limits of many deep etch processes with their need for increasingly deeper and narrower contacts that have aspect ratios greater than 40:1. High aspect ratio (HAR) etching could be key in the future development of devices with high device/feature densities on a semiconductor wafer such as, for example, dynamic random access memory (DRAM) container capacitors and FLASH contacts. 
     However, as the aspect ratio of the plasma etch increases, twisting is increasingly becoming an issue. “Twisting” is the lateral offset of the bottom of an etched feature from the top. In a cross section, the twisted features bends in the X or Y direction, i.e., to the left and right of the page (X-direction) or in and out of the page (Y-direction). During plasma etching, as the aspect ratio increases, twisting becomes more common. The twisting is caused by asymmetric feature charging, which results in a lateral electrical field. In general, feature charging is due to the electrons having an isotropic velocity distribution, i.e., the thermal velocity is larger than the directed velocity, while the ions have an anisotropic velocity distribution, i.e., the directed velocity is much larger than the thermal velocity. For ions, the directed velocity is normal to the wafer, due to their acceleration by the plasma sheath. This means that most of the electrons will deposit their charge near the top of an HAR feature while the ions deposit their charge more toward the bottom. This results in the top of the feature charging negatively and the bottom positively. If this vertical charging becomes azimuthally asymmetric than the lateral electric field results, causing twisting. Asymmetric charging is caused by asymmetric mask geometry, which results in different view angles for electron and ion fluxes at different locations around the circumference of the contact or container. Differential electric charge builds up on the mask, causing local distortion of the ion trajectory at the edge of the array. This is often stochastic in the array, due to small variation in polymer deposition or lithographic induced asymmetries. At the edge of the array, systematic twisting is frequently observed, wherein the last several (up to 40) features twist in the direction of the edge of the array. One common, and problematic, example of twisting is in a DRAM container oxide etches. During oxide etching, twisting can result in “open” capacitors when the DRAM container does not land on the contact. Alternatively, twisting can cause shorts (doublebits) when two containers twist together. 
     Theory and computer simulation have shown that the twisting at the edge of the array is caused by different hard mask heights between the periphery and array portions of the semiconductor wafer. As described below, the different mask heights are caused by the faceting of the hard mask. The different mask heights result in a lateral electric field toward the periphery. This electric field pushes ions in the same direction. It is believed that this causes, or at least contributes, to the systematic twisting seen toward the moat at the edge of the array. In other words, for plasma etching with a strong ion energy component, i.e., the etch is as much or more physically driven than it is chemically driven, facets naturally develop because the peak angular yield of incident ions occurs at off-normal incidence. Typically, this is about 60 degrees. Oxide etch chemistries are typically done at high bias and the dominant ion is argon (Ar+). This means that the oxide etch ions are, in fact, quite physically driven, and prone to faceting. 
     In the array, the facets “come together” due to the small critical dimension of the space (the “line”) between the dynamic random access memory (DRAM) containers. In doing so, the etch rate of the mask in the array is naturally increased as compared to etch rate of the open, peripheral areas due to these geometric considerations. In addition, the difference in open area (i.e., the area to be etched) in the array versus the open area in the periphery contributes to a loading difference that tends to increase the mask loss in the array as compared to periphery. These two effects together result in less mask remaining in the array portion of the semiconductor wafer than in the periphery portion toward the end of a high aspect ratio etch. It is at the time that the systematic twisting typically occurs. 
     Therefore, it is important to reduce the relative height differential between array and periphery, which results in a lateral electric field and, therefore twisting. This could be done by reducing the faceting of the mask during high-aspect-ratio etches. However, the problem is overconstrained and the high bias and chemistries needed to drive an oxide etch at high aspect ratios results in a fairly fixed level of mask faceting and, therefore, result in a difference in mask height between the periphery and the array. 
     Therefore, there is a need to provide a solution to the problem of twisting at the edge of an array portion of a semiconductor wafer during high aspect ratio etching by reducing the difference in mask heights between the periphery and the array. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The following detailed description of specific embodiments of the present invention can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which: 
         FIG. 1  is a schematic top plan view of a memory device according to an embodiment of the present invention. 
         FIGS. 2-6  are schematic cross-sectional views of the formation of a masking level according to an embodiment of the present invention. 
         FIGS. 7A-C  are schematic cross-sectional views of the formation of a masking level according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION  
     In the following detailed description of the embodiments, reference is made to the accompanying drawings that form a part hereof, and in which are shown by way of illustration, and not by way of limitation, specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and that logical, mechanical and electrical changes may be made without departing from the spirit and scope of the present invention. 
     The term ‘substrate’ is to be understood as a semiconductor-based material including silicon, silicon-on-insulator (SOI) or silicon-on-sapphire (SOS) technology, doped and undoped semiconductors, epitaxial layers of silicon supported by a base semiconductor foundation, and other semiconductor structures. Further, in the discussion and claims herein, the term ‘on’ used with respect to two layers, one ‘on’ the other, means at least some contact between the layers, while ‘over’ means the layers are in close proximity, but possibly with one or more additional intervening layers such that contact is possible but not required. Neither ‘on’ nor ‘over’ implies any directionality as used herein. 
     Referring initially to  FIG. 1 , a top view of an integrated circuit  100  such as, for example, a memory cell is illustrated. A central region  110  of the integrated circuit  100 , the “array,” is surrounded by a peripheral region  120 , the “periphery.” The array  110  is typically densely populated with conducting lines and electrical devices such as, for example, transistors and capacitors. The periphery  120  typically is comprised of features larger than those found in the array  110 . Consequentially, typically, high aspect ratio etching is performed in the array  110 , whereas low aspect ratio is performed in the periphery  120 . Alternatively, the periphery  120  may contain no features for a given masking level. 
     Referring to  FIG. 2 , a layer of amorphous carbon  200  is deposited over a substrate  210 . Typically, the amorphous carbon layer  200  can have a thickness of about 4000 Å to about 10000 Å. As shown in  FIG. 3 , a hard mask layer  400  is then deposited over the amorphous carbon layer  200 . This hard mask layer can be an inorganic material such as, for example, silicon oxynitride anti-reflective coating (SiON ARC). This hard mask layer  400  typically have a thickness of between about 200 Å to about 500 Å. Typical photolithography can then be performed where a bottom anti-reflective coating (BARC) (not shown) can be deposited over the SiON ARC layer  400  to control light reflections. As shown in  FIG. 4 , a photodefinable material layer  410  is deposited on the BARC and SiON ARC  400  layers. The photodefinable material  410  typically can have a thickness of between 500 Å to about 1500 Å. The photodefinable material  410  can be photoresist material or any other suitable photodefinable material known in the art. The array  110  is then patterned with, for example, contacts and containers. The photodefinable material layer  410  can then be exposed and developed. The BARC layer can be consumed, leaving the SiON ARC layer  400  over the amorphous carbon layer  200 . Photolithography is again performed leaving another layer of photodefinable material layer  500  over the array and exposing the periphery  120 . The SiON ARC layer  400  can be then etched away from the periphery  120  portion as shown in  FIG. 5 . 
     As shown in  FIG. 6 , a portion of the amorphous carbon layer  200  can then be etched away in the exposed periphery  120  portion resulting in a thicker layer of amorphous carbon  200  in the array  110  portion than the periphery  120 . Typically, approximately half of the thickness of the layer of amorphous carbon  200  is etched away in the periphery  120  portion. For example, if the original amount of amorphous carbon deposited over the substrate  210  is 8000 Å, approximately 4000 Å would be etched away in the periphery  120  portion. The photodefinable material layer  500  can have a thickness that is approximately equal to the amount of amorphous carbon etched from the periphery  120 . This photodefinable material thickness is due to the fact that the photodefinable material layer  500  etches at least as fast as the amorphous carbon  200 . For example, if approximately 4000 Å amorphous carbon is to be etched, the photodefinable material layer  500  can have a thickness of approximately 4000 Å. 
     A portion of the amorphous carbon layer  200  remains over the periphery  120  after etching. The amount of the amorphous carbon layer  200  remaining is adjusted depending on the consumption of the amorphous carbon layer  200  during the HAR etch such that the heights of the array  110  and the periphery  120  matched toward the end of the HAR plasma etch. The photodefinable material layer  500  is exposed and developed away through exposure to light at the appropriate wavelength. 
     Typical HAR plasma etch can then be performed. The SiON layer  400  remaining over the amorphous carbon layer  200  in the array  110  will be consumed during the HAR plasma etch resulting in the layer of amorphous carbon  200  of variable thickness covering entire surface of the substrate  210 . However, by the end of the HAR plasma etch, the amorphous carbon layer  200  will have approximately the same thickness over the entire surface of the substrate  210 . Alternatively, a fill material may be used over the layer of amorphous carbon  200  in the periphery  120  before the start of the HAR plasma etch in order to reduce any topography issues caused by the varying thickness of the amorphous carbon layer  200 . It will be appreciated that the layers described above can be formed by various methods known in the art. For example, chemical vapor deposition can be used to form the hard mask layers, spin-on-coating processes can be used to form the photodefinable material layers, and the amorphous carbon layer  200  can be formed by chemical vapor deposition using a hydrocarbon compound, or mixtures of such compounds, as carbon precursors. 
     At the start of the high aspect ratio plasma oxide etch, the layer of amorphous carbon  200  will be thicker over the array  110  portion of the substrate  210  than over the periphery  120  portion. However, this thinner amount of amorphous carbon  200  in the periphery  120  does not cause issues due to the fact the amorphous carbon  200  etch rate in the periphery  120  portion, as mentioned above, is lower than in the array  110  portion. At the end of the high aspect ratio plasma oxide etch, the mask heights in the periphery  120  and the array  110  portions should be similar. In other words, the amount of the amorphous carbon layer  200  remaining over the periphery  120  portion results in a more uniform mask height between the array  110  and periphery  120  portions at the end of high aspect ratio plasma oxide etching. This more uniform mask height across the memory device reduces the lateral charging difference and, therefore, mitigates twisting toward the moat at the edge of the array  110  portion. 
     Alternatively, both the BARC layer and the SiON ARC layer  400  can be etched after the array  110  has been patterned, leaving only the layer of amorphous carbon  200  over the substrate  210 . In this embodiment, after a portion of the amorphous carbon layer  200  is etched away in the periphery  120 , another photodefinable material layer/photolithography process step can occur which exposes the amorphous carbon layer  200  in the array portion  110  while leaving a layer of photodefinable material over the amorphous carbon  200  in the periphery portion  120 . Another layer of inorganic material such as, for example, SiON ARC, can then be used as a hard mask to etch the amorphous carbon  200  in the array  110  while the photodefinable material layer protects the periphery  120  from further etching. Normal HAR plasma etch can then occur. The amount of amorphous carbon  200  and photodefinable material over the periphery  120  can be adjusted to equalize the mask height of the material in the array  110  and the periphery  120  after the end of the HAR etch. Again, it will be appreciated that the layers described above can be formed by various methods known in the art. For example, chemical vapor deposition can be used to form the hard mask layers, spin-on-coating processes can be used to form the photodefinable material layers, and the amorphous carbon layer  200  can be formed by chemical vapor deposition using a hydrocarbon compound, or mixtures of such compounds, as carbon precursors. 
     It is noted that terms like “preferably,” “commonly,” and “typically” are not utilized herein to limit the scope of the claimed invention or to imply that certain features are critical, essential, or even important to the structure or function of the claimed invention. Rather, these terms are merely intended to highlight alternative or additional features that may or may not be utilized in a particular embodiment of the present invention. 
     Having described the invention in detail and by reference to specific embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims. More specifically, although some aspects of the present invention are identified herein as preferred or particularly advantageous, it is contemplated that the present invention is not necessarily limited to these preferred aspects of the invention.