Photoresist development rate

Two component, positive photoresists (see Section 3.5.b) represent systems with unusual exposure characteristics caused by the standing wave effect (see Section 2.1.f) and "bleaching" or change in optical density during exposure (see Sections 3.5 and 3.9). Both of these phenomena result in nonlinear exposure throughout the thickness of the resist film, and result in uneven developing rates as a function of film thickness, making evaluation of these systems difficult. 

Initially, the experiments apply a commercially available polyimide coating, which is well known from many microelectronic applications. The polyimide (PI 2545, HD Microsystems GmbH) is a high-temperature coating that can be patterned by a positive photoresist. It is dissolved in the same process step as the exposed resist using an alkaline photoresist developer [6], but by different etching rates. 

Thin film dissolution behavior has been the subject of stuity for many applications. In the case of positive photoresists, a number of techniques have been used to characterize the kinetics of dissolution (1-12). The earliest such e q>eriments were performed by exposing the resist to a solvent for a fixed time and then measuring the thidmess of the remaining film (2). From repeated measurements of this type, a bulk development rate could be determined. 

As a result of these efforts to design highly sensitive resists, the concept of chemical amplification and in particular chemical amplification in positive-tone resists was proposed, demonstrated, and promoted for use in DUV photoresists by Ito in 1982. The basic concept of the chemical amplification scheme as it relates to photoresist materials is that a single initial photochemical event be used to tri er a cascade of many subsequent chemical reactions and transformations in the resist that lead to the desired development rate change between the exposed and unexposed areas in the material. A basic schematic of one popular incarnation of this concept is shown in Figure 14, which shows the use of photochemically generated acid to cause a catalytic series of reactions in a... 

Making a polymer relief image commonly requires two processes. First, there is a photochemical process which alters the solubility of the exposed areas relative to the unexposed areas. This is followed by the actual dissolution of the most soluble areas during development. Historically, studies of photoresists have emphasized the photochemical aspects of image formation rather than the dissolution process. The central theme of this paper is the very Icurge effects on dissolution rate that can result from adding small molecules to the matrix. 

When working with this type of resist to make VLSI devices there are several problems that come up, particularly the need for improved thermal stability from the resist images emd a method to control light that is reflected or scattered in a nonimagewise manner. The function of a development enhancement agent is to increase the dissolution rate of the photoresist so that polymers with better physical properties but slow dissolution rates may be used. The dyes... 

Postexposure bake of the wafer. A postexposure bake (PEB) improves contrast of the photoresist before its development. The PEB process causes three effects 1) diffusion of the PAC 2) solvent evaporation and 3) thermally induced chemical reactions. In general, the dissolution rate of a resist decreases as a function of a PEB temperature. PEB becomes more important for the photoresists with a chemical amplification (CA) feature. The photoresists need the PEB to complete chemical reactions initiated by exposure. 

Along with a photochemically activated solubility switch, UV transparency and ion etch resistance, a viable photoresist polymer must exhibit some degree of hydrophilicity. This will help in the adhesive and film forming properties of the polymer as well as the dissolution rate of the exposed resist in the developer. Other considerations include glass transition temperature and mechanical proper-... 

In this work, we provide dry-process plasma etch rate ratios versus a standard for an expanded list of novel vinyl polymeric resists and commerical photoresists. The novel vinyl resists have been synthesized as part of a larger resist development program aimed towards the development of improved x-ray and e-beam lithographic resists (6). 

In order to confirm the effect predicted from the equation obtained by utilizing of data on sodium hydroxide/sodium chloride solutions, experiments were performed with the buffered Microposit series of developers (Na OH solution with NajBOj buffer) to obtain the Re (dissolution rate of exposed resist) and Ru (dissolution rate of unexposed resist). The results of these measurements are shown in Figures 6, 7, 8 as function of sodium chloride concentration. It can be seen that the effect of the molar sodium chloride solution is to enhance the dissolution rate of both the exposed and unexposed photoresist areas. When the ratio of Re/Ru is obtained (Table I), it is clear that the 5 1 developer dilution has significantly higher contrast and that the ratios are roughly constant for all sodium chloride concentration studied. 

Novolac polymers are soluble in a variety of common organic solvents. Resist films are typically cast from organic solutions (propylene glycol monomethyl ether acetate, or PGMEA, is a common solvent) to form isotropic, glassy, thin films. These polymers are also soluble in aqueous alkaline solutions due to the presence of the acidic phenolic functionalities on the polymer backbone. It is this base solubility that is exploited in the development process. In order to print a relief image in a photoresist, it must be possible to modify the dissolution rate of the resist upon exposure to radiation. It is not the... 

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