Contaminants, chemically amplified

Recently, nonionic acid precursors based on nitrobenzyl ester photochemistry have been developed for chemically amplified resist processes (78-80). These ester based materials (Figure 8) exhibit a number of advantages over the onium salt systems. Specifically, the esters are easily synthesized, are soluble in a variety organic solvents, are nonionic in character, and contain no potential device contaminants such as arsenic or antimony. In addition, their absorption characteristics are well suited for deep-UV exposure. 

From these highly idealized reactions, one can gain an understanding of some potential diffculties and process related concerns. For this system to work satisfactorily, it would be necessary for the radiation generated acid concentration, [H+], to remain constant. However, in most chemically amplified systems, undesired side reactions occur that prematurely destroy the acid, i.e., reactions with contaminants such as water, oxygen, ions or reactive sites on the polymer (reactions 2 and 3). 

The rates of these reactions depend upon the contaminant concentration and the inherent rate constants of the reactions. While the exact nature of these reactions differ for each type of chemically amplified system and are not fully understood, this generalized discussion is sufficient to understand many of the process issues. 

Skeptical observers might wonder if the DNA found in ancient bodies or bones are actually contaminants. PCR amplifies any DNA in the sample, whether it came from the artifact or not. DNA introduced into the sample by humans who handle the artifact is a common headache for ancient DNA research, particularly for bones, which are porous. Bone porosity increases about four or five times after death, which provides a rough field test for archaeologists to gauge the age of a bone—old bones are much more porous. But the increased porosity means that these bones soak up solutions and chemicals, some of which can contain DNA. For the bones of ancient humans, including the Iceman, in which the DNA sequence is expected to be quite closely related to modern humans, it may be difficult to decide if the DNA is ancient or if it is a modern contaminant. 

The problems met by chemically amplified photoresists are a) poor stability to environmental contaminations such as airborne amines b) sensitivity of lithographic parameters to PEB temperature variations c) poor stability during storage both after coating and after exposure d) side-directed diffusion... 

Vigil, J.C. Barrick, M.W. Grafe, T.H. Contamination control for processing DUV chemically amplified photoresists. Proc. SPIE 1995, 2438, 626-643. 

From the time of the inception, chemical amplified positive resists suffered from formation of a surface-insoluble layer, which typically showed up as a skin or T-top profile (Fig. 49). The problem has been ascribed to contamination by... 

W.D. Hinsberg, S.A. MacDonald, N.J. Clecak, C.D. Snyder, and H. Ito, Influence of polymer properties on airborne chemical contamination of chemically amplified resists, Proc. SPIE 1925, 43 (1993). 

Chemically amplified resist system is a promising technology to attain hi resolution and high sensitivity for sub-quarter micron device fabrication. However, air-borne contamination (1-3), such as ammonia mainly generated from conventional adhesion promoter, hexamethyldisilazane (HMDS), severely affects this kind of resist. It causes surface insoluble layer of resist patterns, which results in failure of the pattern fabrication. 

We have developed a new non-ammonia generating adhesion promoter, 4-trimethylsilo)Q -3-pentene-2-one (TMSP). Its adhesion cq>ability for a Si substrate is superior to IPTMS and HMDS owing to its hi reactivity. It is applicable to various substrates. We obtained hi aspect ratio and precise chemically amplified resist patterns on Si and TiN substrates using this new adhesion promoter without the anxiety about air-borne contamination and substrate dependency. This process is very promising for actual device production in terms of its simplicity and environmental safety. 

The metal-containing onium salts are generally not preferred in modem photoresists as they will contaminate the device fabrication processes. Instead organic onium salts are preferred in chemically amplified photoresist formulations. Table 57.15 shows extinction coefficients at 248 nm, 254 nm, and the absorption maxima as well as thermal stability of some organic onium salts [43]. 

As with all chemically amplified resists, a major concern is, however, the latent image stabflity and the susceptibility to environmental conditions. With t-BOC deprotecdon systems, the influence of airborne nudeophilic contaminants has been recently demonstrated (23) the observadon of surface residues in a number of such materials (23, 24) may be traced back to the presence of ppb amounts of volatile bases. In the case of the acetal systems (19-21), the influence of trace bases is less pronounced, as even amine hydrochlorides are sdll sufficiendy addic to have some catatydc activity. Linewidth dianges with the interval between exposure and post exposure bake have been observed for both the t-BOC and the acetal systems. In the case of the t-BOC tystems, long intervals (several hoius) between exposure and post-e]q)osure bake will lead to a decrease of apparent sensidvity, which manifests itself as a linewidth inCTease, or, in extreme cases, as faUure to open the imaged areas. These effects are normally due to contaminadon by base traces, or, in cases where the presence of even ppb amounts of bases can be excluded, may be assumed to be the result of the same, unspecified chain terminadon (add annihilation) mechanism which is responsible for the containment of the calalytic reacdon to the immediate vicinity of the imaged resist. 

Collecting samples of ancient nucleic acids is a delicate operation that requires what are basically surgical procedures. It is advantageous, whenever possible, that the samples be collected at excavation sites and precautions taken to ensure that they do not become contaminated with other, particularly more recent, nucleic acids. At high temperatures and humidity, nucleic acids decay quickly. Well-preserved ancient nucleic acids can, therefore, be expected in sites where low temperatures and a dry environment prevail, as, for example, in cold, desert areas of the world. Once collected, the samples need to be isolated from any other remaining materials until they can be amplified by PCR and their chemical composition and structure can then be studied. 

Amplified DNA is identified by solution hybridization of two nonisotopically labeled oligonucleotides to one strand of the amplified DNA. Following hybrid formation DNA is bound to a solid phase and detected by enzyme-labeled specific antibodies. Because only one strand is used for detection, the other one is washed off and constitutes the main source of laboratory contamination and carryover. Take precautions to avoid the spread of these molecules to other rooms (e.g., work in a chemical fume hood, decontaminate used wash buffer with acid or sodium hypochloride, and so on). 

A third type of contamination is unique to PCR and other amplification methods, such as the ligase chain reaction. It involves the inadvertent contamination of a new reaction with the aerosolized products of a previous reaction. As shown in Table 2, as little as 10"7 pi of a tube of amplified DNA can contain 103 molecules of target (C4). Recommended precautions (K13) involve the use of positive-displacement pipets and the physical separation of areas where PCR reactions are analyzed from those where new reactions are setup. In laboratories that use these precautions, contamination is infrequent, and, when it does occur, is usually at the 1- to 100-molecule level. However, in addition to these procedural measures, it would be useful to have chemical or enzymatic methods of selectively inactivating amplified DNA—similar to the sterilization procedures used to inactivate large numbers of cultured viruses or bacteria. 

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