Introduction to Nanoscience and Nanotechnology ENS 463 2. Principles of Nano-Lithography by Alexander M. Zaitsev alexander.zaitsev@csi.cuny.edu Tel: 718 982 2812 Office 4N101b 1
Lithographic patterning Lithographic patterning is a major fabrication method in Top-Down nanotechnology. Lithography has been and remains a major technology of fabrication of microdevices and has good potential to be also one of the most important technologies of nanodevices. Lithography refers to making patterns on surfaces by masking technique. The most common approach uses covering the surface of a sample by a layer of resist material and expose this layer to energetic radiation (photons, electrons or ions). 2
Lithographic processing steps Example of a typical sequence of lithographic processing steps (with no post-exposure bake in this case), illustrated for a positive resist. 3
Positive and negative resists The irradiated resist (usually a polymer), undergoes structural or chemical modifications, which change its solubility. If the irradiated resist become more soluble it is called as positive resist. The resist which becomes less soluble is called negative resist. After the exposure the sample is put into a solvent to remove more soluble areas and to leave a pattern of less soluble resist. This step of the lithographic process is called developing. 4
Lithography techniques differ in the ways of: - Exposure. The resist may be exposed in serial (parallel) writing mode or direct write mode. In this case the resist is irradiated with a focused beam point by point (one feature of the pattern at a time). Parallel exposure is performed over the whole pattern area simultaneously. - Energetic particles used. UV radiation, X-ray, energetic electrons, energetic ions can be used for lithography. - Creation of exposure pattern. Mask-based and maskless techniques. Electron Beam Lithography: Direct write mode Energetic electrons Mask-based 5
Parameters of Lithography - Resolution - the size of the least fabricated feature. - Throughput - the number of features of the pattern created per second. - Field - the area covered with single exposure procedure without mechanical motion of the target surface. Field size influences the throughput. - Alignment/Registration - the ability and precision to align features to each other. Alignment considerably influences the resolution and packaging density. - Radiation source quality/technology - intensity (high throughput), monochromacity (resolution), time stability (resolution), wavelength (resolution). - Resist quality - sensitivity (low dose exposure, throughput), contrast (abrupt transition at critical dose, resolution). - Cost - a lithography technology of a modern microelectronic fabrication plant may have over half of its total cost. It may attain $ 3,000,000,000 at a plant capable of fabrication of Pentium processors. 6
Comparison of primary nanolithography techniques 7
Resist materials Most common resists are radiation sensitive polymers. Radiation may act in two ways: - to lengthen the polymer chains or crosslink the polymer chains (radiationstimulated polymerization, - increase molecular weight, - decrease solubility, negative resists). Developer removes the irradiated areas of positive resist: - radiation-induced depolymerazation. Developer removes the non-irradiated areas of negative resist. - radiation-induced polymerazation. Crosslinked PMMA (polymethylmethacrylate), or KTFR (Kodak company). 8
Radiation-induced de-polymerazation Radiation shortens the polymer chains (breaking molecules, radiation-induced depolymerazation). This process is characteristic for positive resists. In this case the developer removes the irradiated areas. PMMA (Polymethylmethacrylate). 9
Resist contrast General requirement for resists is capability to form high resolution image. To achieve this the chemical response to the irradiation must be highly nonlinear (high contrast). High contrast sharpens a blurred exposure pattern. 10
Exposure curve of positive tone resist Resist contrast γ is defined as: γ = 1/lg(D 0 /D c ) γ may range from 2 to 15. High γ is a characteristic of chemically amplified resists (Shipley UV, e.g. UV-6). 11
Conventional resists In conventional resists, energy of the irradiation is directly converted into chemical transformation. PMMA (polymethylmethacrylate) is a most conventional positive tone conventional resist (molecular weight 500 to 1000 k(amu). γ may attain 3. It is sensitive to UV, electrons and ions. DNQ (diazonaphthoquinone) resist is commonly used in integrated circuit fabrication. It works with UV light at an exposure dose of 0.1 J/cm 2. γ is of 6. Resolution of 350 nm can be achieved. 12
Chemically amplified resists Chemically amplified resists use an intermediary catalytic reaction (usually driven by heat) which is carried out prior to development. Chemically amplified resists contain a photo acid generator compound. It needs only 5 mj/cm 2 dose of UV irradiation. Contrast is as high as 15. Evaluate contrast of a resist using its exposure curve shown on the graph. 13
Photoresist Coating A thin, uniform coating of photoresist at a specific, well controlled thickness is accomplished by the seemingly simple process of spin coating. The photoresist, rendered into a liquid form by dissolving the solid components in a solvent, is poured onto the wafer, which is then spun on a turntable at a high speed producing the desired film. Photoresist spins speed curves for different resist viscosities showing how resist thickness varies as spin speed to the -1/2 power. Stringent requirements for thickness control and uniformity and low defect density call for particular attention to be paid to this process. volume of the resist dispensed and properties of the resist (such as viscosity, percent solids, and solvent composition) and the substrate (substrate material and topography) play an important role in the resist thickness uniformity. 14
Post-processing trimming of resist Enhanced resolution can also be achieved by post-processing trimming. Using positive resists of somewhat increased thickness one can etch away the walls of the feature (scarifying the height) and reduce the actual width. 100 nm features can be made as small as 60 nm with this method. Advantages: (I) inexpensive and simple, (ii) aspect ratio attains a value of 10. Disadvantages: (I) applicability for isolated features only, (ii) poor reproducibility of the etch process. 15
Aspect ratio of resist pattern Aspect ratio A is the ratio of depth d to width w of a feature: A = d/w Aspect ratio of resist pattern is an important parameter of lithography. It defines maximum thickness of the resist film. Aspect ratio and maximum thickness are usually limited by the residual stress in the resist film. Small aspect ratio may considerably restrict the postlithography processing. 16
Etching of exposed resist The pattern defined on the resist covering the mask is developed by wet etching. It is good for 800 nm features (200 nm final feature). Wet etching has no anisotropy, however the aspect ratio is very low of 0.1. Dry etching (plasma or gas) has better aspect ration, but it produces much more defects because of a greater selectivity and anisotropy of the process. 17
Lithography mask Lithography masks are used to produce patterned irradiation of the resist. Projection masks do not touch the processed substrate. Reduction masks are used for projection photo- (UV), electron- and ion- lithography. Usually the pattern is formed with 4 to 5 times reduction. Reduction masks are much cheaper and easier to fabricate as they require lower resolution for their fabrication (fabrication with laser beam is possible). Masks are fabricated using direct writing lithography. 18
Proximity lithography masks Proximity lithography masks are put directly on the resist layer. Proximity lithography masks are 1:1 masks size of mask features equals to the size of features on resist. The proximity masks are usually used for X-ray lithography. 19
Mask Fabrication A common mask for photolithography consists of a glass plate coated with 80 to 100 nm thick Cr film as a light absorber. Cr has very good adhesion. Mask is fabricated by an electron or laser primary pattern generator. Laser systems are cheep, stable, fast and very precise but inferior to electron systems in resolution. The pattern defined on the resist covering the mask is developed by wet etching. It is good for 800 nm features (200 nm final feature). Wet etching has no anisotropy and aspect ratio is very low of 0.1. Dry etching (plasma or gas) has better aspect ration but gives much greater defects because greater selectivity. 20
Mask defects Defect inspection and repair is the final step in the mask fabrication procedure. 21
Repair of mask defects Opaque defects (excessive features) are removed by laser or ion beam ablation (milling). Clear defects (missing absorber) are less common and harder to repair. They can be removed by ion beam or laser-induced selective CVD deposition. 22
Protection of lithography masks After repair and re-inspection the lithography masks is placed in a transparent flat box (pellicle). The pellicle walls stand off the mask a few millimeters to keep dust particle out of the image focal plane. Thus small dust particle on the pellicle do not produce features in the final image. 23