1. LITHOGRAPHY IN THE TOP-DOWN PROCESS - BASICS

2. Lithography In the Top-Down Process - Basics
Learning Objectives
To define lithography
To identify the need for lithography
To explain the lithography process in general
To define the limitations of the current photolithography processes in top-down nanomanufacturing

3. What is Lithography?
Lithography is a process that uses focused radiant energy and chemical films that are affected by this energy to create precise temporary patterns in silicon wafers or other materials.
Lithography is an important part of the top-down manufacturing process, since these temporary patterns can be used to add or remove material from a given area
Lithography uses light or other forms of radiant energy to change the chemical properties of thin layers of films that have been coated on a substrate. The film provides a precisely patterned “stencil” that can be used in the process to mask out certain areas of the substrate from exposure to chemical or physical deposition or etching that occurs later in the process. Since the top-down process starts with a “chunk” of material and then adds or removes other materials from it to create the desired object, defining precisely where to do this is a critical component.

4. What is Lithography (2)?
Lithography is one of the 4 major processes in the top-down model
Lithography
Etching
Deposition
Doping
In order to perform the other 3 processes, we must precisely define where to do them
Lithography Does This!
In the top-down model, we use these 4 processes to create features in certain areas of the substrate. The thin film patterns created in lithography can mask the substrate from being etched by strong chemicals or define areas where metal layers can be added for wiring. With doping, we change the physical properties of certain areas of the substrate. Lithography also masks off areas for this process.

5. Lithography’s Key Role in the Process
With multiple etch, deposition, and doping processes taking place in the fabrication of a device, the lithography process is repeated many times.
The precision and accuracy of lithography in the manufacturing process controls, to a first degree, the success in building a device.
The lithography process is repeated many times during fabrication of a device, since there are often several “layers” of materials added to a substrate. With this multilayer manufacturing method, if one layer doesn’t line up with the one below it or if the areas overlap each other, the final device will not work.

6. Overview of the Photolithography Process
Photolithography uses light energy passing through a patterned mask
The light is focused onto the photosensitive surface
Chemical changes in the surface coating occur
Subsequent chemical development creates a temporary pattern on the surface.
Photolithography is one of the lithographic methods most commonly used in the top-down process. A top down view of the photolithography process shows that the light is controlled by a masked off area and focused onto the area to be patterned through a lens. In subsequent slides, we’ll show how the photosensitive surface was placed there, and how the properties of the light and the patterns in the mask affect the finished product.

7. Steps in the Lithography Process
Silicon wafers are commonly used substrates in the top-down process.
The first step is to coat the clean surface of the wafer with a light sensitive chemical emulsion known as photoresist
Photoresist Dispensing (Spinners)
Photoresist is made up of chemical compounds whose properties are changed by exposure to radiant energy. Typically, the photoresist chemicals are in a liquid solution that is applied to a silicon wafer in a device called a spinner. The silicon wafer must be clean, flat, and dry in order for the spin process to create a flat, uniform coating of the resist. If the coating is not uniform, the pattern created will not be even (planar). Gaps or holes in the coating may allow chemicals and other physical processes to “seep through” areas where they should not be occurring and cause the device to be rejected.
The “recipe” that defines the speed, temperature of the wafer, and timing for each part of the spinning process is unique to the photoresist being applied.

8. Steps in the Lithography Process
Photoresist Dispensing (Spinners)
Baking the resist causes it to form a solid layer.
The chemical properties of the photoresist define what wavelengths of light will affect it.
Photoresist coatings are baked onto the surface after coating to harden, drive off volatile compounds, and ensure that they will adhere to the surface during processing. Since the coatings must be even across the surface, it is necessary to remove the “edge bead” that occurs during the spinning process. When the bake and edge bead removal are completed, the wafer is ready for the next step.
Photoresists differ in their chemical makeup based on the energy source that they respond to and the type of processing step that they must act as a mask for. In some of the newest processes for the top-down method known as stamp pad lithography, the resists are intentionally soft and respond to pressure from a patterned stencil that is applied into the wafer, accompanied by heat or light rather than just light alone, as is the case in photolithography.

9. Steps in the Lithography Process(2)
Exposure
A photomask, typically made of quartz with a chrome plating, controls where the radiant energy will strike the photoresist.
Photomasks are often made with electon beam patterning tools
Photomasks provide the definition of the light patterns that will strike the photoresist. A photomask can be compared to a photographic negative from which a picture is made. The photomask, just like the negative, has the image of the pattern to be “printed”. The accuracy of the shapes on the photomask defines the accuracy of the printing. Photomasks are generally manufactured from quartz plates that are especially flat and planar that have been coated with a chromium plating layer. The chromium plating blocks light from passing through. To create a photomask, electron beam patterning tools are often used, as the beam width is exceptionally small. This is a very time consuming process as a result, but the accuracy and precision of the mask is essential to the process. In some cases, the term reticle is used in place of mask.

10. Click once for each question.
Practice Questions
Click once for each question.
1. Define lithography.
Art and science of defining patterns of features on a wafer.
2. How are photography and photolithography similar?
Both processes involve exposing an image onto a chemically treated surface. Both processes involve depositing chemicals on the surface, exposing the surface to light, and developing the surface to reveal the image.
3. What is the goal of photolithography?
To define patterns of features on a wafer in preparation for further processing.

11. Steps in the Lithography Process(3) - Exposure
Exposure of the photoresist to the radiant energy pattern occurs next
There are several ways to do this
Contact/proximity printing
Projection printing (shown here)
Projection scanning
The exposure step is critical in many ways. The mask must be precisely aligned to the surface or the patterned area won’t line up with other layers that may already be in place. The light source must provide even light to the entire area to be patterned, and the pattern must be in focus. There are several exposure methods possible. Contact printing puts the mask directly in contact with the coated surface. Projection printing moves the mask away from the surface and uses a condensing lens to focus the pattern. Line by line printing uses a fine beam of light to scan across the area to be exposed. Each method has its advantages and pitfalls, and some are not applicable to nanomanufacturing due to the limitations of accuracy and feature sizes that exist. We’ll discuss each method to illustrate this as the manufacturing considerations are important.

12. Contact Printing The mask is directly in contact with the wafer
Advantages
Simple
Low Cost
Disadvantages
Poor for small features
Mask damage may occur from contact
Defects from contaminants on mask or wafer due to contacting surfaces
Contact printing has the advantage in that the alignment of the mask and wafer can be well controlled, and that since the pattern is directly on the surface, no distortion due to projection by a lens occurs. It is one of the oldest processes for photolithography. Unfortunately, optical contact printing is limited in feature sizes to approximately 2 um, which drastically limits its use in nanotechnology. Since the masks are in direct contact with the photoresist surface, it is possible that the mask or the photoresist can be damaged in the printing process, and any trapped particles between the surfaces will result in defects in the pattern and possible damage to the mask or the photoresist surface. Mask life is shorter as a result. Although traditional contact printing methods are not useful in nanomanufacturing, we’ll see that some new contact printing methods do hold promise.

13. Proximity Printing The mask is above the wafer surface Advantages
Mask damage is minimal
Good registration possible
Disadvantages
Poorer resolution due to distance from the surface
Defects from contaminants on mask or wafer due to contacting surfaces
Diffraction errors
Proximity printing has the advantage in that the mask is spaced away from the wafer so that damage to the mask or the resist is unlikely. Unfortunately, the spacing limits the resolution of the image possible as shadows form under the mask. Diffraction of light becomes a problem as pattern sizes become smaller. Feature sizes down to approximately 2.0 um are the limit, which excludes this process in most areas of nanomanufacturing.

14. Projection Printing (1)
An optical system focuses the light source and reduces the mask image for exposure on the surface
Advantages
Higher resolution
Lens system reduces diffraction error
Disadvantages
Errors due to focus of lens system may occur
Limiting factor in resolution can be due to optical system
Projection printing also has the advantage in that the mask is spaced away from the wafer so that damage to the masks is unlikely. An optical system below the mask helps to minimize diffraction errors. A mask can be made that is larger than the feature to be fabricated using the optical system, which makes mask making simpler and more precise. An image of the entire device to be fabricated is projected at once in simple projection printing. The “step and repeat” method is used to replicate the image on multiple sites on a wafer. Feature sizes in the 0.5 uM range are easily attainable, but diffraction is still an issue as feature sizes shrink smaller. The optical system can also become a limitation, both from a material and an system standpoint. The numerical aperture (NA) of the lens defines the limitation of feature size possible. The minimum feature size F = K (wavelength/NA) where K = process constant typically about 0.5. Numeric aperture is typically less than 1.

15. Projection Printing (2)
Step and repeat aligner
Lens reduction
Good throughput but resolution limited to about 0.35 uM
Cadiotropic System
Mirror, folding prisms and lenses 1:1 ratio
Less common than steppers
The “step and repeat” method is used to replicate the image on multiple sites on a wafer. Feature sizes in the 0.5 uM range are easily attainable, but diffraction is still an issue as feature sizes shrink smaller. A larger mask is used with a lens for reduction. The optical system can also become a limitation, both from a material and an system standpoint. The numerical aperture (NA) of the lens defines the limitation of feature size possible. The minimum feature size F = K (wavelength/NA) where K = process constant typically about 0.5. Numeric aperture is typically less than 1.
The cadiotropic system uses prisms and mirrors to create a system with a larger field. It isn’t as commonly used.

16. Step and Scan Aligner Uses a spherical mirror and a scanning pattern
Advantages
Improved throughput
Lens system aberration minimized
Disadvantages
Complex motion system is required for alignment and precise tracing
Light source wavelength is still an factor limiting feature size
Scanning with stepping can be used to improve somewhat on the situation. If a narrow slit of light is used and both the wafer and the mask or, in this case, the reticle, move in opposite directions, a narrower beam results. This helps in the limitation of the numerical aperture of the lens.

17. Diffraction
As feature sizes shrink in the mask, the wavelength of the light used as a source becomes a factor.
Shrinking feature sizes require shorter wavelengths of light
The photoresist must be optimized to match the light source used.
Diffraction of the light entering the mask occurs when the size of the feature being printed approaches the wavelength of the light source. A diffraction pattern on the back side of the mask results in hazy or out-of-focus shapes on the photomask. The most commonly used radiant energy source in IC manufacturing is a mercury vapor lamp.

18. Diffraction (2)
The traditional mercury vapor lamp has peaks in certain ranges.
The intensity of some UV peaks is low
The photoresist must be optimized to match the light source used.
The traditional mercury vapor lamp has spectral peaks in different UV ranges. Smaller feature sizes will require shorter wavelengths of light to reduce diffraction if traditional photolithography processes are to continue. The shortest wavelength peak from this source is 248nM, in the so-called DUV (deep UV) range. So called H-line and I-line peaks are too long of a wavelength for nanoscale feature sized, but feature sizes well below the 248nM peak are currently supported, so there must be a way!

19. Click once for each question.
Practice Questions
Click once for each question.
1. Which process puts the mask on the wafer?
Contact lithography.
2. What effect creates a “haze” around a mask pattern when feature sizes become smaller?
Diffraction
3. Which process uses a lens to reduce mask feature sizes?
Projection Lithography
4. Besides wavelength, what limits feature sizes?
Numerical Aperture of the lens