LITHOGRAPHY IN THE TOP-DOWN PROCESS - NEW CONCEPTS

Lithography In the Top-Down Process – New Concepts Learning Objectives To identify issues in current photolithography To quantify the needs of nanomanufacturing To define improvements in photolithography To explore new lithography processes To define the limitations of these new processes in top-down nanomanufacturing

What are the limitations of current photolithography processes? Light sources from traditional mercury vapor lamps have little “deep UV” spectra Finer feature sizes require shorter wavelength sources Photoresist must be sensitive to appropriate wavelengths of light Lenses and optical components have limited numeric aperture The cost of an integrated circuit is proportional to its size, and the yield of good die from a wafer drops as the die size increases. For improved functionality, more devices per die are required, so the solution is to pack the devices more densely. To do this, improved resolution in lithography is required to create the features needed. In nanotechnology, applications such as filters, chemical analysis devices, and other devices require nanometer resolution. As earlier discussed, the wavelengths of the UV light from mercury vapor sources used in traditional photolithography limits resolution. The photoresist must match the properties of the light for proper exposure. A relationship between the wavelength of light and the numeric aperture of the lens used in photo-reduction systems is also a factor. Each of these items will be discussed separately, along with contemporary solutions that are being applied or will be applied.

Why Shorter Wavelengths? Minimum Feature Sizes are dictated by the following relationship: F = K (λ/NA) Where F = Feature Size in nM λ = Wavelength (nM) K = Process Constant NA = Numeric Aperture From this relationship, it is clear that shorter wavelengths will allow for smaller feature sizes. The process constant K typically is in the range of 0.5 – 0.6 and the numeric aperture for the lens is typically less than 1. If no further improvements in process occur, this defines the minimum feature size.

Shorter Wavelength Sources Replace the mercury vapor lamp an excimer (exciplex) laser source with shorter wavelength emission ArF – 193 nM – Shorter wavelength than so-called “deep UV” peak of 248nM F2 Laser – Low output but at 157nM Matching photoresist that is sensitive to this spectra is also required. Laser sources under development – 13.5 nM! (extreme UV or EUV range) When certain inert and reactive gases are combined through application of energy, the resulting bond is weak. Removal of the energy source that forced the temporarily excited state results in disassociation, giving off the excess energy in the form of intense stimulated emission of light. Gases such as argon and fluorine provide spectra in the extreme UV range. The F2 laser provides an even shorter wavelength, but at present, the power attained in this laser is too low.

Numerical Aperture Light passing through the mask will be subject to diffraction. The numerical aperture of the lens used determines its capability to bring the diffracted pattern into a single point of focus. NA = n sin θ where n = index of refraction of the media in which the lens is working (air) and θ is the angular spacing between objects making up the image The lens material that makes up a projection photolithography system is subject to diffraction as the light from the source passes through it. The resolving power (ability of the lens to keep adjacent image components separate) is limited by the index of refraction of the lens material and is a function of its radius

Numerical Aperture (2) sin θ = 1.22 λ/D where θ is the angular spacing between objects and D = diameter of the lens A larger diameter lens helps, but is difficult to manufacture Depth of Field Issues limit the use of larger diameter as a solution Increased NA reduces depth of field To counter diffraction effects from the mask, gathering together as much of the emerging beam is important. A larger diameter lens provides an increased focusing capability, but it is difficult to manufacture a large lens without optical defects that limit its use. Even if the manufacturing issues for larger diameter lenses were non-existent, there would still be a challenge in that higher NA lenses have reduced depth of field. That is to say that the higher NA lens, due to its wider angle, focuses over a limited range. This puts the added requirement in that the wafer and photoresist coating be exceptionally flat and thin so that the feature being projected will be in focus. Planar surfaces on the wafer are achieved through lapping and polishing. Photoresist coatings for deep UV and EUV must be thinner in formulation to provide a thin, even coating, and this requires careful processing in the original spin coating process.

Improving the Index of Refraction Conventional photolithography processes operate in air. The index of refraction of air is 1, which limits the numerical aperture. If, instead, the media the process operated in was water (n = 1.44), the numerical aperture is increased. This can improve feature resolution by 30 – 40%, but it is not without issues, as contaminants in the water, possible damage to the photoresist, and microbubbles from motion are possible, but immersion lithography is a path to the 45nM process step that is being used. Higher refractive index liquids are under study.

Improving the Photomask Sharp edges in photomasks are not well reproduced as feature sizes shrink Optical proximity correction techniques put borders on corners and edges to correct for this As feature sizes and wavelengths shrink, projection lithography tends not to “fill” corners and edges of patterns. Adding relieved border “corners” to the mask set allows for further exposure openings and squarer features.

Click once for each question. Practice Questions Click once for each question. 1. What limits feature sizes in photolithography? Wavelength of the light source used Numerical aperture of the lens 2. What effect causes blurring in photomasks? Diffraction of the light source 3. What is the limitation that occurs when numerical aperture is increased? Depth of field is decreased

Alternative Exposure Methods – Electron Beam Lithography Use of exposure sources other than UV light has been studied for some time. An electron beam is exceptionally “narrow”, and does not require a mask λ=h/P Phase Shift Mask An electron beam has exceptionally narrow bandwidth, and, by DeBroglie, becomes narrower as its energy increases. No masking would be necessary if the patterning is done by such a narrow source. Utilizing an e-beam source, however, can result in the area being exposed generating secondary electrons, a phenomenon commonly used in scanning electron microscopy to examine the object under study. In this situation, though, instead of collecting the electrons in the scanner, the secondary emission tends to expose areas adjacent to the desired area, causing blurring of the pattern. Lower energies may result in lower secondary emission proximity effect problems, but are not without problems as the wavelength increases. Although typical beam widths of 1nM are possible, proximity effect and other problems limit e-beam lithography to 10nM. The issue from a manufacturing process with E-Beam etching is that the throughput is very poor as this is line by line scanning.

Electron Beam Lithography (2) E-beam lithography also serves as a tool for mask making Throughput is not an issue in this case, since the masks are made once, and used many times. Sub-50 nM feature sizes are possible The issue of throughput from a manufacturing standpoint exists with e-beam lithography, but the E-Beam tool can also be used to create photolithography masks of extreme precision, with feature sizes in the 30nM range. Since masks are reusable and photolithography processes operate in more of a “batch” mode, throughput is not an issue.

X-Ray Lithography Synchrotron radiation sources can be used Masks use “absorber” materials on a membrane X-rays pass through the membrane PMMA photoresists can be used A thin membrane passes the X-rays from the synchrotron source. In place of chromium as would be used in a conventional photomask, an “absorber” of x-rays must be used. These materials can include gold, tungsten, or titanium. The thin membrane masks are easily damaged by the x-ray exposure, making for short mask life. This is an issue for X-ray lithography that keeps it in the R&D stage.

X-Ray Lithography Issues Spacing, mask dimension, and wavelength are critical So-called “sweet spot” will provide small feature size for a given wavelength exposure and defined mask feature New research into this method indicates that a “sweet spot” that will provide sharp features exists when the critical gap above is met. This requires specific features sizes in the transparent mask. Repetitive features could be sharply defined if these conditions are met. See also X-Ray Lithography on the Sweet Spot, Antony Bourdillon and Yuli Vladimirsky, (2006) book published by UHRL, P.O.Box 700001, San Jose, CA 95170, nfxrl@xraylithography.us

Nano-Imprint Lithography Concept – To use a “stamp” of precise dimension to create features in resist Advantages High Throughput No issues in diffraction No secondary emission Can be carried out in non-vacuum environment N

Nano-Imprint Lithography (2) T-NIL (Thermal Nano-Imprint Lithography) PMMA resist similar to that used in X-ray lithography is spin coated onto surface Stamp is pressed into contact with surface Substrate is heated to glass temperature of resist Pressure is applied to “stamp” imprint Substrate cools and stamp can be removed Thermal imprint nanolithography uses a spin-on resist coating on a wafer substrate. The stamp, often made from silicon, is pressed into the resist surface. Heat is applied to make the resist “plastic” and the stamp transfers by displacing resist in the areas where it projects. The substrate is cooled and the pattern remains. The advantage is in simplicity, but since the silicon stamp is not transparent to light, alignment can be difficult. http://www.nilt.com/Default.asp?Action=Details&Item=219

UV-Nano Imprint Lithography UV-NIL (Ultra-violet Nano-Imprint Lithography) A UV sensitive resist is used The stamp must be UV-transmissive UV light is applied through the stamp Pressure is applied to “stamp” imprint UV-imprint nanolithography uses a UV sensitive spin-on resist coating on a wafer substrate. The stamp, often made from , is pressed into the resist surface. Heat is applied to make the resist “plastic” and the stamp transfers by displacing resist in the areas where it projects. The advantage is in simplicity and lower temperatures used. Alignment of the mask is easier as it is transparent to UV light.

Issues in Nanoimprint Lithography Alignment of layers can be more difficult than with projection lithography “Proximity effect” of having large stamp areas near small features may cause uneven feature sizes Residual layer thickness and profile may vary Patterned areas may “stick” to stamp In projection photolithography, alignment of masks is easier to perform. Misalignment of layers can cause defects. When the stamp is pressed into the surface, the resist is displaced. If wide areas are adjacent to narrow ones, the displacement may not be even, affecting the width of the narrow feature. Pressing the stamp into the surface should displace the resist, but if correct pressure and temperature are not present, the displaced depth may not be uniform, making developing more difficult. Finally, when the stamp is removed, the resist may stick to the stamp surface, which can cause uneven features or defects in subsequent stamping if the resist is not cleaned off the stamp surface prior to the next operation. This is, in general, a promising technology due to its simplicity.

AFM Probe Lithography An atomic force microcscope cantilever writes a pattern in resist Extremely precise “Scratching the surface” Nanolithography can be accomplished using an AFM probe tip to scratch the resist coating on a wafer substrate. The throughput of the process is poor, as it is literally writing a line at a time. http://www.pacificnanotech.com/afm-modes_active-modes.html

AFM “Dip Pen” Lithography An atomic force microcscope cantilever writes a pattern in resist Extremely precise “Scratching the surface” Nanolithography can be accomplished using an AFM probe tip to scratch the resist coating on a wafer substrate. The throughput of the process is poor, as it is literally writing a line at a time. http://www.pacificnanotech.com/afm-modes_active-modes.html