Leading Edge of Moore’s Law Demands New Etch Capabilities

 
Although traditional wet and dry etch methods remain the backbone of the semiconductor industry, leading-edge microchips with greater structural complexity and ever-increasing aspect ratios are creating manufacturing challenges that these methods cannot overcome. These challenges include lack of selectivity, inadequate penetration into small spaces, and collapse of high aspect ratio structures. 
 
Extreme selectivity is becoming supremely important in advanced etch technology to ensure that only a specific target material is removed without affecting others in the film stack or degrading the overall integrity and stability of the structure. This concept is illustrated in the series of images below (Fig. 1). Traditional etch processes can cause corner rounding or damage and leave unwanted residues. 
 
As devices scale, some structures are becoming so tiny that it is fundamentally impossible for wet chemistry molecules to penetrate them. This limitation is based upon the electrostatic charge that is built up in the ionic solutions commonly used by the industry. These ions interact to form an electrical double layer that effectively constricts a narrow channel, restricts movement, and inhibits chemical refresh (Fig.2). Consequently the etch rate drops dramatically (Fig 3.) and ultimately the etch stops completely. This limitation is becoming more and more common as infinitesimally small structures are becoming the norm in DRAM and advanced logic. 
 
Fig. 2. An electrical double layer forms in ionic solution used for wet etch. Image courtesy of Sony.
Fig. 2. An electrical double layer forms in ionic solution used for wet etch. Image courtesy of Sony.
 
Fig. 3. Etch rate as a function of feature width. Image courtesy of Sony.
Fig. 3. Etch rate as a function of feature width. Image courtesy of Sony.
As devices scale, they tend to get taller and narrower, which causes them to become more fragile and susceptible to distortion from stiction (capillary forces) during the drying step that follows wet etch.  This distortion leads to line bending and even pattern collapse (Fig. 4 and 5).
 
Fig. 4 DRAM STI Collapse (Side View).
Fig. 4 DRAM STI Collapse (Side View).
  
Fig. 5 DRAM STI collapse (Top View).
Fig. 5 DRAM STI collapse (Top View).
The precision with which devices are patterned or cleaned is crucial for their ultimate reliability and performance. Incomplete removal of a material or enlargement of critical dimensions caused by inadequate selectivity can have detrimental consequences to device performance.
 
In my next post, I’ll focus specifically on the importance of complete, selective removal of a target material in the complex multi-patterning schemes being used to extend current lithography technology.
 

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