CHEMICALLY AMPLIFIED RESISTS FOR ELECTRON BEAM LITHOGRAPHY

This thesis describes the development of chemically amplified resists for electron beam lithography. The techniques and concepts oflithography are discussed and the motivations for the development of chemically amplified resists are examined. The experimental techniques used in this work are then described. Two groups of resists, derivatives of fullerene and derivatives of triphenylene, were tested for chemical amplification and the results obtained from the research are presented. A systematic study of the response of several methanofullerenes and polysubstituted triphenylene derivatives before and after chemical amplification is presented. Films of the compounds were prepared by dissolving the resists in solvents such as chloroform and adding to the solution various concentrations of certain photoacid generators and crosslinkers, and spin coating the mixture on hydrogen terminated silicon wafers. The films were irradiated using 20 keY electrons. Post exposure bakes between 90 to 120 'C for 30 to 180 s were applied to the resists before development with non-polar solvents such as monochlorobenzene. Most of the chemically amplified resists showed sensitivity enhancement compared to their pure counterparts. Fullerene derivative, 3' H-cyclopropa [I, 9, 5, 6] fullerene-C60-Ih - 3', 3'- carboxylic [ 2-2-(2-hydroxyethoxy) ethoxyl ethyl] ester (a mixture of adducts) demonstrated the highest sensitivity enhancement with the incorporation of an epoxy novolac crosslinker and bis[4-di(phenylsulfonio) phenyl]sulfide bis(hexafluorophoshate) as photoacid generator with a sensitivity of -8 ~Clem' and a resolution of -24 nm. The polysubstituted triphenylene derivative, 2,6,10-trihydroxy-3,7,11- tri(pentyloxy) triphenylene, showed a sensitivity of -5 ~Clem' when the crosslinker hexamethoxymethylmelamine and the photoacid generator triphenylsulfonium triflate were added to the compound. However, fine patterning in the resist was not very successful due to acid diffusion. An alternative triphenylene derivative similar to 2,6, 1 0-trihydroxy-3,7, 11- tri(pentyloxy) triphenylene, with epoxides incorporated into the structure showed better results with the photoinitiator bis[ 4-di(pheny lsulfonio) phenyl]sulfide bis(hexa fluorophoshate). The chemically amplified C51epoxide demonstrated a sensitivity of ~9 f..!C/cm2 and a resolution of 40 nm. The etch durabilities of these chemically amplified resists for dry plasma etching with SF6 are reasonably high, comparable to a conventional high durability novolac resist

Advanced resist materials for next generation lithography

With the advancement in technology the minimum lithographic feature size decreases more and more for every generation. The development of lithographic techniques and resist materials capable of meeting the requirements for the up- graded technology (resolution, sensitivity, roughness) started to play a trivial role. However, the issue represents a fundamental principle in lithography (the RLS trade-off) and it proves difficult to overcome. Addition of quenchers in chemically amplified resists reduces the acid diffusion length and improves the line edge roughness and increases the resolution of the patterned features, but decreases the sensitivity. The current most commonly researched approach to boost the sensitivity in organic resists is the addition of metals embedded in the molecular structure by covalent bonds. This approach was investigated in this thesis, and an extension towards high-Z organic additive compounds and high-Z cross-linkers was conducted. Furthermore as feature sizes less than 20 nm are routinely required, pattern col- lapse driven by the capillary forces upon development has become a serious limiting factor, independent of the lithography technique involved. Alongside with constantly developing the resist platforms there is also the need to improve the adhesion of the resist material to the silicon substrate, reducing pattern collapse and allowing for ultra high resolution and high aspect ratio patterning. In this thesis I will present the research I have undertaken in order to implement a resist platform suitable for next generation lithography and I will introduce and describe the new multi-trigger mechanism concept developed for this resist system. I will also present a study on active underlayes investigated for improved adhesion between the resist and the substrate

Anisotropic acid catalyst displacement in a chemically amplified photoresist via application of an electric field

Electrostatic force theory and Fickean diffusion theory both predict that anisotropic movement can be conveyed to chemically amplified resist (CAR) acid catalysts via an electric field during the post exposure bake (PEB). A demonstration of this effect was attempted through photoresist thickness loss measurements with and without the addition of an electric field. The experimental design underwent multiple evolutionary iterations and measured as much as 86 nm of additional thickness loss with the presence of an electric field compared to a PEB control. Repeatability was inconsistent and several principal assumptions were found to be in violation. From measurements of current across exposed photoresist during an electric field enhanced post exposure bake (EFE-PEB), it was concluded that acid pileup at the electrode/photoresist interfaces caused charge separation which reduced the internal electric field; this effect was relieved by acid neutralization via electrons from the cathode

Patternable materials for next-generation lithography

One of the salient truths facing the microelectronics industry today is that photolithography tools are unable to meet the resolution requirements for manufacturing next-generation devices. In the past, circuit feature sizes have been minimized by reducing the exposure wavelength used for patterning. However, this strategy failed with the worldwide dereliction of 157 nm lithography in 2003. Extreme ultraviolet (EUV) lithography still faces many technical challenges and is not ready for high volume manufacturing. How will the microelectronics industry continue to innovate without regular advances in photopatterning technology? Regardless of which paradigm is adopted, new materials will probably be required to meet the specific challenges of scaling down feature sizes and satisfying the economic ultimatum of Moore’s Law. In the search for higher resolution patterning tools, device manufacturers have identified block copolymer (BCP) lithography as a possible technique for next-generation nanofabrication. BCP self-assembly offers access to sub-5 nm features in thin films, well beyond the resolution limits of photolithography. However, BCP materials must be carefully designed, synthesized, and processed to create lithographically interesting features with good etch resistance for pattern transfer. In this dissertation, we describe a pattern transfer process for 5 nm BCP lamellae and a directed self-assembly (DSA) process for aligning 5 nm structures in thin films. To achieve defect-free alignment, the interfacial interactions between the BCP and pre-patterned substrate must be precisely controlled. We also discuss a new process for selectively modifying oxidized chromium films using polymer brushes, which could further improve the aforesaid DSA process. To facilitate better pattern transfer of BCP structures, several new BCPs with “self-developing” blocks were synthesized and tested. These materials depolymerize and evaporate in strongly acidic environments, leading to developed BCP features without the need for etching or solvent. “Self-developing” polymers may also be useful materials for traditional photolithography. Chemically amplified resists used in manufacturing today are fundamentally limited by a trade-off between sensitivity and pattern quality. To overcome this problem, we present a new type of photoresist that relies on depolymerization, rather than catalysis, to achieve amplification without producing significant roughness or bias in the final patternChemical Engineerin

Conductive resists for nanofabrication on insulating substrates

The purpose of this work is to support ongoing miniaturization of III-V microelectronic devices, which present a unique combination of economic and technical challenges. As miniaturization has proceeded photolithography has been able to meet required 20 nm feature sizes through the use of increasingly complex optical engineering techniques. However, this is not economically viable for low volume fabrication. The most promising low-volume technique here electron beam lithography (EBL). However, on insulating substrates, (e.g. for III-V devices), charging during EBL leads to pattern distortion and resolution is limited. Whilst charge mitigation strategies exist, they introduce process complexity, and resolution limits. A new approach using aconductive triphenylene electron beam resisthas been investigated. Triphenylenes form well-ordered hexagonal columnar discotic liquid crystals that show fast hole mobility (e.g. 10$^-$$^3$ cm$^2$V$^-$$^1$s$^-$$^1$) along columns. The triphenylene based chemically amplified resist investigated here has a conductivity of ~10$^-$$^6$ S/m at room temperature. It has demonstrated high sensitivity in EBL, requiring a patterning dose of ~14 μC/cm2on silicon and ~10 μC/cm$^2$ on fused silica substrates at 50 keV exposure. The resist has demonstrated high-resolution patterning including 20 nm half pitch lines on silicon, and 55 nm isolated lines on glass at 30 keV exposure

Materials and processes for high-resolution patterning

Advancement in microelectronic devices today has been possible with the continual push of the resolution limit of optical lithography tools. However, the current 193 nm immersion photolithography is not able to meet the resolution requirements for manufacturing the next-generation microelectronic devices. Multiple patterning methods such as self-aligned double patterning exist to extend the resolution of current photolithography further but they come at the expense of additional cost and extra processes. Very recently, extreme ultraviolet (EUV) lithography has entered the semiconductor market as the next-generation lithography technique to pattern 7 nm and smaller technology nodes. However, due to tool reliability, high maintenance cost, and sensitivity of chemically amplified EUV resists, high-resolution patterns with good line edge roughness have not yet been demonstrated. In this dissertation, several methods are described to extend the resolution of the current lithography tools without involving high cost or additional process requirements. Chapter 1 provides an overview of lithography used in the microelectronic industry. Chapter 2 discusses the design of pitch division resist using photobase generator to improve the resolution of 365 nm (or 355 nm) photolithography tools by a factor of two. Chapter 3 provides an overview of block copolymer (BCP) lithography (or directed self-assembly (DSA)) as an alternative patterning technique to achieve patterns smaller than 10 nm. Chapter 4 presents the DSA using photo-patternable guidelines and a fundamental study on photoacid diffusion in thin photo-patternable films. Chapter 5 discusses a revised hybrid DSA process to improve the alignment control and achieve higher pattern density multiplication. Chapter 6 describes a strategy to reduce the defect density of high-chi silicon-containing BCPs to be used as alternative materials for BCP lithography. Chapter 7 focuses on developing an etch process for pattern transfer and fabrication of high-resolution nanoimprint templates using 5 nm lamellae forming BCPs.Chemical Engineerin