New Opportunities For Poly-functional Photoresists
Previously the chemistry of light-sensitive compositions developed in a basic evolutionary way. As a rule, researchers improved upon available light-sensitive materials. These are typically based on diazonaphthalenone derivatives for positive-printing compositions and arylazides and onium salts for negative compositions. These new light-sensitive compositions have resulted in significant expansion in the spectral sensitivity of photoresists moving from wavelengths of 1000nm down to 157nm.
Until recently the main function of photoresists was to simply transfer a pattern from a photomask onto the surface of the silicon wafer or other substrate. This is achieved by means of various operations: imaging the pattern onto the photoresistive layer, exposure with the desired pattern and then the develop process removing the resist from selected areas. The exposed areas of the wafer are then processed with etch, deposition, diffusion, ion implant and other equipment. These conditions have determined the basic requirements for photoresist composition.
Along with improving standard photoresists as the basis for ordinary lithography processes, fundamentally new opportunities are emerging in microelectronics to simplify planar diffusion processes. The microrelief pattern formed by the photoresist can be used as a thermostable low-profile layer, a conformal coating structure or interlayer dielectric for electrical devices. With some dye combinations, the photoresist may be used as the filter for liquid crystal displays (LCD). Other specially modified photosensitive materials could be used to carry out doping into semiconductor structures directly from the photo-microrelief.
One needs to find chemical modifications for new photoresist functionality that are both positive and negative printing. Discovering such basic methods for creation of multifunctional photoresists and the development of their molecular design is one of the primary interests of modern photochemistry. An example of a poly-functional photoresist is the heat-resistant light-sensitive composition based on polyamic acid and polyheteroarylenes (Figure 1).
The patterned material left by such a photoresist can be used as a dielectric layer. The new thermostable positive pattern was formed using a composition of poly (o-hydroxyamide), and a product condensation of 5-sulphoester-2,1-diazonaphtalenone with diphenylolpropane. It is resistant to chemical etching and has been used to carry out metallic thin film deposition and spot weld processes at 400¡C in one factory laboratory. If the light-sensitive component is changed, direct and inverse images can be obtained with such photoresists. Unfortunately these films have unsatisfactory adhesion to silicon surface layers such as SiO2 or Si3N4.
To improve the adhesion of these heat-resistant materials, Si fragments have been introduced into light-sensitive components and polymers. These modifications change some of the physical and chemical properties of the resist: adhesion is improved, plasticity is decreased, coefficients of thermal expansions of the film and substrate are almost identical and the hydrophilic-hydrophobic balance of the composition is changed. For example, the high hydrophilic property of the silicon-containing sulphoester of 2,1-diazonaphthalenone allows a decrease in the concentration of alkaline developer (Figure 2).
In an analogous way, the modification of azido-containing photoresists can be used to create photoresists containing dopants for diffusion processing. These diffusable substances are chemically connected with the components forming the light-sensitive layer. The diffusible atoms are precisely deposited in the lithography process and then used as a source in the basic diffusion operation. The surface concentration of diffusible atoms must be determined to achieve the necessary depth of diffusion into the semiconductor wafer. Such a material allows for the creation of interesting semiconductor architectures.
Diazidobenzyliden derivatives of cyclo-ketones can be used as a component of azido-containing compositions. Insertion of heteroatoms into the cyclo-ketone fragment appreciably changes the oleophilic (hydrophobic) properties of the whole molecule. For example, insertion of a g-piperidone cycle (X=NH) into the diazide structure (Figure 3) increases its hydrophilic properties so that it may be easily combined with water-soluble polymers such as poly-vinyl-pyrrolidone or poly-vinyl alcohol. Thus the polymer matrix maintains its homogeneous properties, which are essential for microrelief creation.
The hydrophobic properties of the whole molecule can be significantly increased by insertion of a silicon atom into the diazide structure. Thus this diazide (Figure 3, X=Si(CH3)2) may be combined with highly oleophilic rubbers to make silicon polymers.
The variation of the oleophylic properties of these light-sensitive compositions permits the use of organic polymers to make chemical materials with unique properties.
Photosensitive azido-containing compositions with B, Al, Ga, Au, Pt, P, As and Sb have been produced for diffusion processes into silicon. Compositions containing Mg, Zn, Sn and Ni may be used for doping processes to make AlGaAs /GaAs and InGaAs(P)/InP heterostructures.
Homogeneity of the photoresist composition is important for initial exposure and final development. Homogeneous photoresist-diffusants that contain elements of the VA group of the Periodic Table (P, As, Sb) may be produced on the basis of tris-(3-azidoaryl)-derivatives.
For example, a series of P-containing compounds with a common formula (Figure 4) were synthesised. These compounds combine well with the majority of hydrocarbons and organic polymers. Changing the side chains of the compound allows a range of spectral sensitivities (210-500nm) to be attained. The quantum yield of photolysis in the polymers is rather high (0.25-0.6) and close to that of the quantum yield of photodisintegration of 2,6-bis(4-azidobenzyliden)-4-methyl-cyclohexanone (Figure 3, R=H, X=CH-CH3), which is used even now in many industrial photoresist compositions.
The compound in Figure 4 provides diffusion of phosphorus at a surface concentration of N=1016-1017 atoms/cm3. Thus the level of doping is raised by up to 1018-1019 atoms/cm3.
Parallel with this, new methods of synthesising cyclorubbers containing elements such as P, As and Sb based on poly(cycloisoprene) analogues have been developed. Variations of oligomers and triazides are also possible. The polymers include 20% by mass of the target element, which allows high dopant concentrations without the diffusion process damaging the crystal.
A higher surface concentration of phosphorus (N=1020-1021 atom/cm3) is achieved by using alkyl-poly-phosphonates and poly(phosphazenes). Azide-containing complexes - such as compounds (VI?) with quasi-aromatic fragments - can be used as doping sources for group II, III and IV elements.
The synthesis of the complex compound in Figure 5 was carried out according to standard methods described in the literature for synthesising metal-chelate-b-diketonates. The insertion of a metal ligand into the molecule was performed at pH no more than 7.5 by use of (Ph?)3B, Ga(NO3)3, Zn(NO3)2, Al(NO3)3, Cu(NO3)2 , Ni(NO3)2, Fe(NO3)3 or Pb(NO3)4.
Poly-carboranes were selected as B-containing polymers for compositions with the largest percentage by mass of boron. Note that all of these newly elaborated light-sensitive polymer diffusants are more stable than other compounds previously used for diffusion such as 'dissolved glasses'. The improvement of adhesion and the forming of uniform films are achieved by insertion of the carbo-chain or carbo-cyclic oligomers and polymers usually used in photoresists. Film-forming admixtures with aromatic fragments, as a rule, are not used because they form soot in high-temperature diffusion.
The phototechnical characteristics of photoresist-diffusants are complemented by structural data concerning the spatial linkages that arise from photochemical reactions (Table 1). These determine maximum swelling (Q), average molecular weight of a piece of a chain between the contiguous transverse linkages (M in grams/mole) and the average number of pieces of a chain in 1cm3 of cross-linked polymer (N). The photo-structured matrixes have the same structure and vulcanisation networks as tightly cross-linked trimeric vulcanisates.
Doping profile resolution and precision is excellent not only on the surface, but also in the body of semiconductor wafers. The small dependence of photosensitivity on the contents of organic aryl-azides in a layer permits alterations in concentration without changing the lithography exposure parameters. This property of organic aryl-azide layers unifies the technologies needed for their application. These series of compositions allow varying concentration of doping atoms in the range 1015-1021 atoms/cm3 - that is, in all doping concentrations demanded from semiconductor applications. It is also possible to attain exact levels of impurity and to insert several doping elements simultaneously into the polymeric diffusant structures.
New photoresist functions can be given not only by structural modification, but also by insertion of additional chemical substances into the light-sensitive compositions. Adding organic 'pure tone' dyes or colorants can be used to essentially raise contrast and improve sharpness of the image in the lithography process. The selection of colorant is carried out according to spectral characteristics and chemical and physical-chemical interactions with other photoresist components.
In the past, colorants have also been used to create high-resolution matrix colour-filters for liquid crystal diode (LCD) screens. For this purpose photoresists become the colour-filters with the required topology. However the technology needed to make these layers is very high and difficult to attain. On the one hand, achieving high-resolution elements needs highly-sensitive photoresists with pure tone dyes. On the other hand, a large amount of LCD filter colorant is also needed in the structure of the photoresist. However, these LCD dyes should not come into contact with the pure tone dyes during assembly of the photoresist layers.
There are two approaches to solving of this problem. The first approach uses the photoresist in imaging where the future filter will be. The pattern is then coloured by sorbing of colorant from suitable solvents, which does not dissolve the polymeric pattern. Different colours are obtained by sorbing with many different colorants to different fragments of the pattern depending on the screen requirements. The second method colours the photoresist composition itself.
The first way is the most widespread, using gelatine, acrylate polymers, acrylate co-polymers with compounds containing oxy-ethylene groups, epoxy-groups as the polymer source of the photosensitive compositions. However, there is one main disadvantage - previously imaged layers of the pattern must be protected against sorbing of subsequent colorants. Sorbing multiple times also affects the spectral characteristics of the colour filter.
Therefore it is much more convenient to create the patterns directly from light-sensitive compositions already containing colorants. In this case it is important to carefully select the components - polymer, colorant, sensitiser, structural agent - that provide well defined patterns with the given spectral characteristics.
One way to achieve this is through polyheteroarylene polymeric layers with colorants that have molecular masses equal to molecular masses of polyheteroarylene fragments. Chemical reactions between a polymer and specially chosen thermo-hardeners results in the heterocyclisation and the formation of an insoluble polyheteroarylene to prevent diffusion of the colorants out of the layer. This provides high-contrast transparent images and high pure tone colour-filters.
Dr Svetlana Gomon, ELCOTEQ, Estonia. Dr Tatyana Yourre, Natalia Klimova, Dr Ludmila Rudaya, St Petersburg State Technological Institute, Russia.