Linewidth control

After consideration of all factors which limit resolution such as exposure hardware, resist systems, registration, alignment, and linewidth control, there is a general consensus that the useful resolution limit of photolithography may lie somewhere between 0.4 and 0.8 /im and depends on such factors as the implementation of short wavelength UV and the ability to accurately place images from a projection tool onto a silicon wafer. 

LINEWIDTH CONTROL Lens imperfections Source stability Mask dimensions Beam shape Mechanical stability Developing Etching Baking Plasma descum Film thickness Film uniformity Swelling... 

C Linewidth Control, This parameter refers to the necessity of maintaining the correct features size across an entire substrate and from one substrate to another. This is important since the successful performance of most devices depends upon control of the size of critical structures, as for example in the gate electrode structure in an MOS device. As feature size is decreased and circuit elements packed closer together, the margin of error on feature size control is reduced. The allowable size variation on structures is generally a fixed fraction of the nominal feature size. A rule of thumb is that the dimensions must be controlled to tolerances of at least 1/5 the minimum feature size. Linewidth control is affected by a variety of parame-... 

Figure 6. Linewidth control data for a typical process line. Weekly averages are shown for a 5.0 pm control feature. The dashed lines represent limits. Note weeks 15-17 represent a processing problem (etching in this case) that was corrected in week 18.

After development, the pattern should be inspected thoroughly for both defects and linewidth control. Further, pattern quality on a process line should be continuously monitored after the developing step in order to ensure that the desired quality is consistently maintained. 

With any resist, the ideal conditions for obtaining high resolution and good linewidth control are a flat surface and a thin resist (<400 nm). The flat surface means that the resist will have very little variation in thickness and, as a result, there will be little variation in resist linewidth. However linewidth variations stemming from resist thickness variations do occur when lines traverse a step as a result of exposure and development effects. Indeed, such... 

Lithographic Characterization. Electron-beam exposures were conducted on an EBES system operating at 20 kV, with a beam address and spot size both equal to 0.25 ym. Electron response parameters were evaluated using linewidth control patterns. P(SI-CMS) was spray developed after exposure using an APT Model 915 resist processor in toluene-methanol (1 1) for 30 sec followed by a methanol rinse for 45 sec. Aqueous solutions of tetramethylammonium hydroxide (TMAH, 25% in water, Fluka Inc.) were used for the novolac resist development. Exposed films were dip-developed for 20 sec. in TMAH-water (1 2.5) solutions. 

Inorganic resists have many advantages over conventional polymeric resists, especially in regard to resolution, linewidth control latitude, focus depth tolerance, and dry processability. These features make them very suitable for application to submicron VLSI fabrication. Efforts to improve processing technologies will open the way for practical use in LSI fabrication. 

The SME passivation pretreatment has two major drawbacks 1) it introduces additional process complexity in cases where the RIE system cannot be conveniently converted to a sputtering mode, and 2) it causes a relatively large decrease in the resist thickness which may lead to loss of linewidth control. 

Photochemically generated acid must diffuse in resist film to catalyze desired reactions and to provide a gain mechanism for amplification. However, excessive diffusion (into the unexposed areas) destroys the linewidth control and eventually the resolution. Thus, as the minimum feature size becomes smaller and smaller, the control of acid diffusion plays a more important and difficult role. Therefore, investigation of acid diffusion in chemical amplification resist film is one of the most active areas of research today. A number of experimental procedures to measure acid diffusion length have been reported [67-88] ... 

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