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Lecture 5: Lithography

Topics covered:OverviewImportance of clean room for lithographyLithography ProcessOptical lithography and resolution enhancement techniqueAdvantages and limitations of other lithographic methods

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OverviewLithography is the process by which circuit or device patterns are transferred from layout to Si wafer.Several methods can be used to make circuit patterns on wafers. The most common process is to make the master photomask using electron beam exposure system and replicating its image by optical printers.

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Importance of Clean Room

Various ways in which dust particles can interfere with photomask patterns.

Particle-size distribution curve for English (---) and metric (——) classes of clean rooms.

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Lithography Process

Layout functional blocks (or use previous designs) and use software tools help route or wire connections between functional blocks Tools check for design rule violations Circuit and system level simulation tools predict performance Information from design transferred to mask making machine and pattern written on a mask blank using scanning electron or laser beam

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Use mask to expose the resist using a photo alignerCreates an aerial image of mask pattern on the resist

Resist is then developed removing (exposed or exposed regions) Resist is used to transfer mask pattern onto wafer.

Ion implantation, oxide tech, metal etch, silicon etch, etc

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Lithography Process Sequence & Components

Functional Components of Lithography

•Energy—cause (photo)chemical reactions that modify resist dissolution rate

•Mask—Pattern (or direct) energy to create an aerial image of mask in resist

•Aligner—Align mask to previous patterns on wafer (to a tolerance level)

•Resist—Transfer image from mask to wafer, after development Positive resist reproduces the mask pattern, Negative resist reproduces inverse mask pattern

•Substrate—Has previous mask patterns

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Exposure ToolsThe pattern transfer process is accomplished by using lithographic exposure tool. The performance of exposure tool is determined by three parameters: resolution, registration and throughput.Resolution- is the minimum feature dimension that can be transferred with high fidelity to resist film on semiconductor wafer.Registration – is a measure of how accurately patterns on successive masks can be aligned (or overlaid) with respect to previously defined patterns on the wafer.Throughput – is the number of wafers that can be exposed per hour for a given mask level.

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Energy Sources (Waves or Particles)

Energy sources are required to modify the photoresist. The energy source is aerial imaged on the photoresist.The imaging can be done by scanning the energy beam or by masking the energy beam.Bright sources are usually required for high throughput.

λhchvE ==

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Optical Shadow Printing Techniques

The minimum linewidth(or critical dimension,CD) that can be pinted is roughly

where λ is the wavelength of the exposure and g is the gap between the mask and the wafer and includes thickness of the resists

gCD λ≅

Schematic of optical shadow printing techniques. (a) Contact printing.

(b) Proximity printing.

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Image Partitioning techniques for Projection Printing

(a) Annual-field wafer scan. (b) 1:1 step-and-repeat. (c) M:1 reduction step-and-repeat. (d) M:1 reduction step-and-scan.

The resolution of the projection systen is given by

where λ is the exposure wavelength, k1 is the process-dependent factor, and NA is the numerical aperture.

NAklm

λ1=

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Simple Image SystemThe numerical apertureis given by

where n is the index of refraction in the image medium( usually air, n=1), and θ is the half-angle of the cone of light converging to a point at the wafer (see side figure)

θsinnNA =

Also shown is the depth of focus (DOF), which cane be expressed as

where k2 is theprocess-dependentfactor

22sin2/

tan2/

NAkllDOF mm λ

θθ=

±=

±=

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Photomasks

Fabricated by e-beam direct write using a electronic database generated by the CAD tools

There are several substrate (transparent) types• Quartz, low expansion glass, sodalime glass

There are also several Opaque materials used to block light• Chrome, emulsion, iron oxide

Often, a master is made on quartz; then the the pattern is transferred to less expensive L.E. glass where it is step and repeated to create several diesTwo polarities of masks are common

Light field, LF(mostly clear) Dark field, DF(mostly dark)

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An integrated circuit photomask.

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Defects due to Masks

Mask defects can be introduced during the manufacturing of masks or during subsequent lithographic processes.Yield is defined as the ratio of good chips per wafer to the total number of chips per wafer. As the first approximation, the yield, Y for a given masking level can be expressed as,

where D0 is the average number of “fatal” defects per unit area, and AC is the defect sensitive area ( or “critical area”) of the IC chip. If D0 remains the same for all mask level, then the final yield becomes

CADeY /0−≅

CANDeY /0−≅

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Yield for a 10-mask lithographic process with various defect densities per level.

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PhotoresistPhotoresist is a radiation-sensitive compound that can be classified as positive or negative resist, depending on how it responds to radiation.Positive photoresists consists of three components: a photosensitive compound, a base resin, and an organic solution.Negative photoresists are polymers combined with a photosensitive compound. The contrast ratio is computed as

where ET is threshold energy;energy required to make the resistcompletely soluble and E1 is the energy obtained by drawing tangent at ET to reach 100% resist thickness

1

1

ln−

⎥⎦

⎤⎢⎣

⎡=

EETγ

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Exposure response curve and cross section of the resist image after development. (a) Positive photoresist. (b) Negative photoresist.

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Align/Expose/Develop Steps

(x,y,θ) alignment of mask to substrate

Uniform UV exposure illumination

Chrome on glass photomask

Photoresist (PR)

Wet chemical development

Substrate waferLatent image created in photoresist after exposure

NEGATIVE PHOTORESIST

Photoresist is photopolymerized where exposed and rendered insoluble to the developer solution.

POSITIVE PHOTORESIST

Exposure decomposes a development inhibitor and developer solution only dissolves photoresistin the exposed areas.

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Details of the optical lithographic pattern transfer process.

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The liftoff process for pattern transfer.

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Resolution Enhancement Technique

The principle of phase-shift technology. (a) Conventional technology. (b) Phase-shift technology.

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Next Generation Lithographic Methods

Electron Beam LithographyExtreme Ultraviolet LithographyX-Ray LithographyIon Beam Lithography

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Schematic of an electron beam

lithography machine.

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(a) Raster scan writing scheme. (b) Vector scan writing schemes.(c) Shapes of electron beam: round, variable, and cell projection.

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Schematic of positive and negative resists used in electron beam lithography.

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(a) Simulated trajectories of 100 electronics in PMMA for a 20-keV

electron beam. (b) Dose distribution for forward scattering and backscattering at the

resist-substrate interface.

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Schematic representation of an extreme ultraviolet (EUV) lithography system.

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Schematic representation of a proximity x-ray lithography system.

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Trajectories of 60-keV H* ions traveling through PMMA into Au, Si, and PMMA.

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The Imaging Tools windows in PROLITH.

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The resist profile for the mask feature specified