Copolymer resist

Top-Down Lithographic Patterns with Block Copolymer Resist Materials... 

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. 

Figure 4. The effect of proton beam energy on the sensitivity (QH) of the MOTSS copolymer resists.

Figure 7. The E-beam sensitivity (Qe) of MOTSS copolymer resists as a function of the proton beam sensitivity (Qp) with 90 and 125 keV protons.

Table VII. Lithographic Properties of Methacrylate Copolymer Resists...

Bearinger, J.P. Terrettaz, S. Michel, R. Tirelli, N. Vogel, H. Textor, M. Hubbell, J.A. Chemisorbed poly(propylene sulphide)-based copolymers resist biomolecular interactions. Nat. Mater. 2003, 2, 259-264. 

Reported experimental data on sensitivities for various copolymer resists are summarized in Figure 2 ( sensitivity is expressed by D M-N)MW(M-N) ). This figure shows that sensitivity vs. component ratio curves are divided into two groups. 

Figure 2. Experimental data on copolymer resist sensitivities in relation to constituent monomer mole fraction. The following references and abbreviations are used P(ST-CMS), chloromethylated polystyrene (8) P(ST-CS), chlorinated polystyrene ( ) P(ST-IS), iodi-nated polystyrene (100 P(ET-CE), chlorinated polyethylene ( 6)a P(ET-BE), brominated polyethylene C165a P(ST-GMA), poly (styrene glycidyl methacrylate)...

Optimum design was made theoretically on P(CMS-2VN) system. Design method was developed based on theoretical analysis for copolymer resist sensitivity and on dry etch rate dependence on polymer structure obtained by a series of experiments. P(CMSq -2VN9i) (Mw =7.8 x 10, Mw /Mp = 1.3) proved to have high sensitivity (Dg=0.9 jtC / cm2) and high resolution (<0.5 /nm) as well as high dry etch resistance. This resist is suitable for microfabrication especially for dry etching processes. 

A copolymer approach can provide more flexibility to the resist design because all the necessary functions do not have to reside on one component. Today s advanced positive deep UV resists are exclusively based on this concept with 4-hydroxystyrene as one component. However, early copolymer systems and some of the 193-nm resists consisted of lipophilic components only. Incorporation of 4-acetoxystyrene to poly(4- er -butoxycarbonyloxystyrene sulfone) has already been mentioned. This section deals with copolymer resists composed of lipophilic comonomers first and then the currently dominant hydroxystyrene copolymers. Co- and terpolymers for ArF excimer laser lithography will be described in a separate section. 

A new type of copolymer resist named ESCAP (environmentally stable chemical amplification photoresist) has recently been reported from IBM [163], which is based on a random copolymer of 4-hydroxystyrene with tert-butyl acrylate (TBA) (Fig. 37), which is converted to a copolymer of the hydroxystyrene with acrylic acid through photochemically-induced acid-catalyzed deprotection. The copolymer can be readily synthesized by direct radical copolymerization of 4-hydroxystyrene with tert-butyl acrylate or alternatively by radical copolymerization of 4-acetoxystyrene with the acrylate followed by selective hydrolysis of the acetate group with ammonium hydroxide. The copolymerization behavior as a function of conversion has been simulated for the both systems based on experimentally determined monomer reactivity ratios (Table 1) [164]. In comparison with the above-mentioned partially protected PHOST systems, this copolymer does not undergo thermal deprotection up to 180 °C. Furthermore, as mentioned earlier, the conversion of the terf-butyl ester to carboxylic acid provides an extremely fast dissolution rate in the exposed regions and a large... 

The aforementioned l,4-bis(2-hydroxyhexafluoroisopropyl)cyclohexane has been combined with the 2-trifluorometylacrylic structure [290,305,307]. The fluorodiol was half-protected with an ethoxymethyl group and reacted with 2-trifluoromethylacryloyl chloride in the presence of triethylamine to afford 2-[4-(2,2,2-trifluoro-l-ethoxymethoxy-l-trifluoromethylethyl)cyclo-hexane]hexafluoroisopropyl 2-trifluoromethylacrylate. This heavily fluorinated acrylate was copolymerized with 2-methyladamanty 2-trifluoromethylacrylate by anionic initiation with potassium acetate/18-crown-6 [307] as described in the literature [303] (Fig. 90). The copolymer (made from a 1 1 feed) was unexpectedly transparent with OD157 of 1.6/pm. However, imaging of the copolymer resists was sluggish perhaps due to their low Tg. Radical copolymerization was also performed with norbornene derivatives [290,305]. 

In the above condensation resist designs, the phenolic resin offers a reaction site as well as base solubility. Self-condensation of polymeric furan derivatives has been utilized as an alternative crosslinking mechanism for aqueous base development (Fig. 126) [375]. The copolymer resist is based on poly[4-hydroxy-styrene-co-4-(3-furyl-3-hydroxypropyl)styrene], which was prepared by radical copolymerization of the acetyl-protected furan monomer with BOCST followed by base hydrolysis. The furan methanol residue, highly reactive toward electrophiles due to a mesomeric electron release from oxygen that facilitates the attack on the ring carbons, readily yields a stable carbocation upon acid treatment. Thus, the pendant furfuryl groups serve as both the latent electrophile and the nucleophile. Model reactions indicated that the furfuryl carbocation reacts more preferentially with the furan nucleus than the phenolic functionality. 

A thermogravimetric analysis (TGA) profile of a typical alicyclic copolymer resist resin, poly(CBN-co-NBCA), is shown in Fig. 7.15. All of the alicyclic resist co-and terpolymers show similar TGA profiles. The deprotection temperature and decomposition temperature for the polymers are roughly 250°C and 400°C, respectively. At the deprotection temperature, roughly 25% weight loss associated with the deprotection event and corresponding to the loss of isobutylene and carbon... 

Forman et compared a series of linear and star-block copolymers resist, prepared using the same scheme, for exposure at 193 nm (ArF resist). The star copolymers developed had as a core an oligoinitiator based on saccharose from which three standard ArF photoresist monomers, a-y butyrolactone methacrylate (GBLMA), methyladamantyl methacrylate (MAMA), and hydroxyl adamantyl methacrylate... 

Accelerated tests have shown that EVA copolymers resist degradation by weathering better than conventional polyethylene. However, chemical resistance and barrier properties of EVA copolymers are inferior. EVA polymers accept high degrees of filler loading without serious degradation of their physical properties. 

Fig. 7.17. Crosslinking on baking methyl methacrylate copolymer resist (see...

---