Negative resist developers

Figure 22. Scanning electron beam micrographs of a typical negative resist developed in different solvents. The dose was similar in both cases.

Development. Resist development is a critical step in lithography, because it exerts great influence on pattern quality. The traditional development method uses a liquid developer solution that preferentially dissolves either the exposed region (positive resists) or the unexposed region (negative resists). Development can be carried out by either spray or immersion tech-... 

It can be said that positive or negative resists developed for electron-beam lithography can be used for X-ray lithography. However, the sensitivity of conventional electron-beam resists is not sufficient from an economical point of view. 

Fig. 3. Lithographic process using positive- and negative-resist systems. The element (a) is (b), exposed to uv radiation (c), developed and (d), the metal is...

The negative resistance effect is observed when anodic oxides are subjected to so-called electroforming (i.e., annealing in vacuum).93 Such a treatment removes the special features of the anodic oxides (asymmetry of conduction and electric strength, electret effect, etc.), and the negative resistance effect may be explained using the general approach developed for amorphous dielectrics.5... 

Contrast curves were obtained for each resist by measuring the thickness after development of a series of 1 mm by 5 mm exposed areas the exposure dose typically varied from approximately 1 mJ/ cm2 to several J/cm2 for the slowest resists. The majority of the resists were developed in ethyl acetate for 30 to 60 sec followed by a 20-sec rinse in 2-propanol. Initially, THF or a THF/2-propanol mixture was used as the developer they were replaced by ethyl acetate because it provided superior contrast. Resist sensitivity was taken to be the incident dose which resulted in 50% exposed thickness remaining after development, Dg 5. This is the standard convention for a negative resist. 

Radiation-sensitive polymers are used to define pattern images for the fabrication of microelectronic devices and circuits. These polymers, called resists, respond to radiation by either chain scission (positive resists) or by crosslinking (negative resists). In positive resists, the exposed areas dissolve selectively by chemical developers in negative resists, the exposed areas are insoluble and remain after development. 

High resolution negative resists are needed for masked ion beam lithography (MIBL) and for the fabrication of MIBL masks by E-beam lithography (EBL). The MOTSS copolymer resists were developed to obtain the resolution of fine features that a bilevel resist can best provide. The flexibility afforded by choosing the structure of the HS, the copolymer composition, and the molecular weight allows a resist to be tailored by simple synthesis adjustments to have the particular sensitivity and etch protection which best suits the application. 

The photoresponsive properties of molecular glasses also have been applied in the design of resists for semiconductor lithography. In a resist, irradiation changes the solubility of the materials, making it more or less soluble (positive or negative resist, respectively). The search for new resist materials follows the development of lithographic techniques toward deep-UV and electron beam... 

COP, the familiar negative e-beam resist developed at Bell Laboratories, is an example of a one-component negative resist system. COP is a copolymer which has excellent film-forming characteristics, resistance to etchants, and intrinsic radiation sensitivity. 

The value in units of incident dose per unit area for either a positive or negative resist system is of little value unless accompanied by a detailed description of the conditions under which it was measured. This description should include, at the minimum, the initial film thickness, the characteristics of the substrate, the temperature and time of the post- and pre-bake, the characteristics of the exposing radiation, and the developer composition, time and temperature. The structure, copolymer ratio, sequence distribution, molecular weight, and dispersity of polymers included in the formulation should also be provided. 

It should be noted that useful, high resolution (0.75 fim) patterns can be produced in certain negative resists if the development step is followed by a sequene of rinses in solvents that have less and less affinity for the polymer structure. The rinse steps effectively reverse the swelling that occurs during development. 

Sensitivity and contrast are conveniently measured experimentally by exposing areas of resist of known size to varying radiation doses and measuring the film thickness remaining after development for each area. In the case of negative resists, gel is not formed until a critical dose, denoted as the interface gel dose Dp, has been reached. At this dose no lithographi-... 

For a positive resist, the film thickness of the irradiated region after development decreases until eventually a critical dose Dp is reached which results in complete removal of the film 8,9). The sensitivity and contrast (7p) are evaluated in a manner similar to that for a negative resist. After they have been spin-coated and prebaked, a series of pads of known area are exposed to varying doses. The substrate is developed in a solvent that does not attack the unexposed film and the thickness of the film remaining in the exposed areas measured. The film thickness is normalized to the original thickness, and this value is plotted as function of log dose, as shown in Figure 5 where Dp represents the sensitivity of the positive resist. Contrast (7p) is determined from the extrapolated slope of the linear portion of the response curve as... 

Development criteria for negative resists are essentially the same, although in this case, the original film must by necessity, be completely removed. The film thickness remaining is also never the original film thickness for negative materials since this condition would not result in the correct feature size, (see Section 4.2.a.). 

Figure 24. Swiss Cheese effect in GMC negative resist produced by shock rinsing after development.

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