12 Electron Beam Lithography – I

Dr. Ajit K. Mahapatro

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Learning Objectives

From this module students may get to know about the following

i. Overview of Lithography Techniques

ii. Electron Beam Lithography

iii. Working Principle of Electron Beam Lithography

1. Introduction

1.1. Lithography

Lithography comes from the Greek words lithos means stone and grapho means write. Low dimensional structures are defined with writing patterns at micro/nano-meter scale designed by printing text or graphics on the surface of materials. Lithography is currently one of the most feasible techniques used for patterning micro/nanoscale features of desired designs with definite shape, size and material compositions, selectively on the material of choice for device fabrication. Several lithographic methods are developed with different resolution, reproducibility, speed, simplicity for processing, and acquisition costs, etc., by using different sources of energy. The process of utilizing light source for patterning is termed as photolithography and utilization of energetic beams as source are nomenclatured according to the type of energy source.

During semiconductor processing, fabrication of micro/nano-structures rearranges the additives by deposition, subtractives by etching, and surface dopants by annealing. Etching out the unwanted materials and depositing desired materials for fabrication of a complete nanosctructure or device structure is the key concept of microfabrication processing. Lithography is used for defining the wanted and unwanted regions by using a polymer chemical called resist layer over top of the surface to be patterned. The etching of unwanted material or metallization of new materials are pursued with the patterned resist on the surface, followed by resist cleaning to fabricate the desired structure. Simple solvents includes acetone, methyl ethyl ketone (CH3COC2H5), methyl isobutyl ketone (CH3COC4H9) for chemicals used for resists stripping/lift-off process. Additional O2 plasma cleaning is effective for removing residual resist or any organic contaminations during lithography process.

1.2. Types of Lithography

In general, the ability to view or fabricate nano-sized objects depends on availability of strongly focused particle beams. Since, diffraction limits the spot size of the exposure, smaller the wavelength of the beam particle produces smaller spots. Here in the tabular are few beam energies correlating to particle wavelength (Å) at various energies that decide the spot size are tabulated in Table 1.


Table 1. Particle wavelength (Å) at various energies

 

1.3 Basic components and working principle of lithography

 

1) Energy Source- Modifies the resist dissolution rate.

2) Mask – Allows patterning by shining direct energy to the resist.

3) Exposure Systems/Aligner – Aligns the mask to patterns to be written on wafer.

4) Resist – Transfers the image from mask to wafer.

Schematic of the photolithography technique is demonstrated in Figure-1. Generally, photolithography utilizes a light source that illuminates over an area of larger dimension compared to the dimension of the features to be designed. This requires a photo mask of same design with the feature sizes to restrict light to pass through and fall on to the photoresist on the substrate. The photo mask is the master pattern of the designs that transfer to wafers. The region, where light falls, could change the chemical decomposition of the polymer used as photoresist and could be dissolved out using another solvent during the lift-off process.

Figure 1. Components of Lithography

 

1.3.1. Steps for Photolithography

  • Sample surface preparation
  • Resist coating (Spin casting)
  • Pre-bake (Soft bake)
  • Alignment (Photo mask and substrate)
  • Exposure
  • Post-bake (Hard bake)
  • Development
  • Processing using the resist masking layer (Etching, Metallization, …)
  • Lift-off of resist layer

 

 

2. Electron Beam Lithography

The difference between photolithography and E-beam lithography are pointed here.

 

Electron beam lithography is a mask-less technology developed in 1950s. The aim is to create nanometer scale structures in the resist that can subsequently be transferred to the substrate material and the resists used as a patterned mask for semiconductor processing during micro-fabrication processes. The final pattern is created directly from a digital representation on computer and scanning the electron beam on the resist- coated substrate following the computer patterning. This is pursued with scanning the beam over the resist coated substrate by controlling its deflection from a computer. The exposed regions (in the case of a positive resist) or unexposed regions (in the case of a negative resist) are then selectively removed (developed). It’s primary advantage is it’s ability to draw custom patterns (direct-write) with sub-10 nm resolution. This form of maskless lithography has high resolution and low throughput, limiting its usage to photomask fabrication, low-volume production of semiconductor devices, and research and development.

Example of gold (Au) metallization using resist coating and e-beam lithography is sketched in Figures 2 – 6. The polymethyl methacrylate (PMMA) as positive e-beam resist with chlorobenzene or anisole as casting solvent, is spin coated over the substrate. Electron beam exposure breaks the polymer into fragments that we dissolve in a developer (1:1 of methyl isobutyl ketone and isopropyl alcohol).

 

Figure 2. Chemical reaction of the resist polymer after exposure to energetic beam

 

Figure. 3. Exposure to electron resist cross-section: The electron beam causes chemical changes in the exposed areas

 

 

Figure 4. Development of electron resist cross-section: Chemically changed e- resist dissolved in a specific solvent (positive lithography)

 

With the removal of the exposed resist from the substrate, a thin metallic layer is deposited which sticks to the substrate, while on the unexposed areas the metal sticks to the resist surface (Fig. 5). After metal deposition the remaining (unexposed) resist is dissolved in an aggressive solvent and lift off). The metal sticking to the resist loses footing and only the metal sticking to the substrate remains after lift-off process (as in Fig. 6).

 

 

Figure 5. Electron resist cross-section after metal deposition

 

 


Figure 6. Circuit cross-section after metal deposition

 

 

 

3. Formation of Thin Films Resists using Spin Coating

Spin coating is a process used to deposit or coat a liquid or a suspension in the form of a thin film by introducing the solution at the center of the rotating substrate (as shown in Figure 7). In spin coating a substrate is placed on the chuck (sample holder) of the spin coater that connects to a vacuum pump to hold the substrate. A small amount of resist is poured at the center of the substrate with help of a dropper. Further this substrate is rotated at a constant angular speed for specific time period. The spinning at high speed using a motor allows the coating material to spread out over the substrates by centrifugal force and is continued till the desired amount of thickness is achieved. The steps to be followed during the spin coating process are indicated in Figure 7. Repeating the spin coating process consecutively for few times can form highly thick films.

Generally, the spinning speed and spinning time of the substrate are controlled according to the required thickness of the film. Viscosity and volatility of solvent, surface tension, and amount of solid content in the suspension are additional factors that affect the thickness of the coated film. Centrifugal force, viscous force, and evaporation rate of the solvent are the forces involved in the spin coating process.

 


Figure 7: Steps to follow during spin coating process.

 

 

Figure 8: Variation of thickness of the spin coated films w. r. t. the (a) spin speed (b) executing time (c) viscosity

 

The thickness of the film depends on the viscosity and concentration of the solution. Thinner films of suspension material can be formed by using low viscos and/or non-volatile solvents spin-coated at higher speeds. Figure 8a and 8b represents the general trend for the expected variation in the film thickness with speed and time as parameters. For most of the resist materials, the final thickness is inversely proportional to the spin speeds and spins time,

?= 1/√w

where, ‘t’ is the thickness of the material and w is the angular speed. Figure 8c shows that for low viscous material the thickness is low and for high viscous material the thickness is high.

 

 

4. Electron Beam Lithography System

 

Electron-beam lithography systems can be classified according to both the beam shape and beam deflection. Earlier systems scanned Gaussian-shaped beams in a raster fashion. Nowadays, systems use shaped beams, which may be deflected to various positions in the writing field (this is also known as vector scan)

Figure 9. Electron Beam Exposure Systems

An electron beam lithography system (as shown in Figure 9) consists of an electron source, a lens system, an electron beam deflection system, a motorized stage, and computers and software to control all the elements. The beam deflection system is used to scan the beam within a writing field, typically 100 µm x 100 µm up to 1 mm x 1 mm. The movement between writing fields is performed by the motorized stage, controlled with a laser interferometer to achieve the smallest possible stitching error between those writing fields.

 

4.1. Electron Beam Sources

 

Figure 9. Work Function and Fermi level of metals

 

Techniques for electron emissions from the source metal are listed below:

 

4.1.1. Types of Sources

I) THERMIONIC EMITTERS

In thermionic emitters, electrons are given thermal energy to overcome the barrier (work function) of the emitting material. Current is given by the Richardson-Dushman equation, which relates the current density of a thermionic emission to the work function (φ0) and temperature (T) of the emitting material by mathematical relation,

III) PHOTO EMITTERS

In  photo  emitters,  energy  is  given  to  electrons  by  incident  radiation  (photons).  Only  photo-electrons generated close to the surface are able to escape from the material.

 

4.1.2. Cathode Materials

  • Thermionic cathodes: Thoriated tungsten (1700 C & 3 A/cm2); WC+ThO2 W + Th+ CO2 (bad vacuum) lanthanum hexaborid; oxide coated (750 C & 0.5 A/cm2); Ni coated by Sr-O, Ba-O, Ca-O; tungsten sponge filled with Ba/Ca aluminate (1100 C & 5 A/cm2) are used as source materials.
  • Field-Emission cathodes (best but most expensive): The emission occurs in lobes because of crystal facets present at the surface emission can be unstable.

Various types of cathode materials used in e-beam lithography are tabulated below.

Table 4. Various materials used as cathodes to generate electron beams

 

4.2. Lenses

Both electrostatic and magnetic lenses are used in electron beam lithography system. However, electrostatic lenses have more aberrations, hence these are not used for fine focusing. There is no current mechanism to make achromatic electron beam lenses, so extremely narrow dispersions of the electron beam energy are needed for finest focusing.

 

4.3. E‐ beam Lithography Write Time

The minimum time to expose a given area for a given dose is estimated from D. A = T. I, where T is the time to expose the object (can be divided into exposure time/step size), I is the beam current, D is the dose, and A is the area exposed. For example, assuming an exposure area of 1cm2, a dose of 10−3 Coulomb/cm2, and a beam current of 10−9 Amperes, the resulting minimum write time would be 106 seconds (about 12 days).

 

 

5. Stage and Substrate Positioning in E-beam Lithography

 

5.1. Positioning the Sample and Stage

The scanning of beams is controlled by its deflection from a computer (sequential exposure) coded program. Exposure in a spot takes ~10-7 s and a mechanical stage is used to move the samples to have a beam exposure along larger distances.

Alignment is made for each stage positions, either by checking for registration marks, or using laser interferometers to precisely measure the stage movement.

A  good  quality  substrate  for  electron  beam  lithography  must  satisfy  various  conditions  stated  below:

1) It should be conductive, or else the wafer would build up an electric charge, which would deflect the electron beam, thus distorting the drawn pattern.

2) The base for electronic circuits should be insulating (to avoid the entire circuit would be short circuited). The substrate should be as close to insulating as possible without distorting the drawn pattern.

The most widely used substrate is silicon, which is semiconducting with a thin insulating layer of silicon dioxide on top. Other substrates that can be used are glass plates coated with metal chrome, widely used in mask production, in which the metal layer should be grounded before drawing.

 

5.2. Scanning of e-beam on the Resist Coated Surfaces

The e-beam scanning system for moving over the resist layer could contain single or multiple beams scanned over the resist layers in vector or raster scanning modes. Various scanning systems are categorized in Figure 10.

Figure 10. Different scanning systems utilized in e-beam lithography.

 

5.2.1. Vector scan:

The vector scanning systems deflect the beam to follow paths dictated by the pattern and is schematized in Figure 11a. A scanning technique where the beam is positioned over an area of the substrate, and the features in that area are drawn out using lines according to the pattern data. The beam remains on at all times. The Raster scan is performed by following the three steps:

  • An area of an individual die is aligned under the electron beam
  • The beam is deflected so as to draw out the features in that area of the chip
  • The chip is completed either by a step-and-repeat method, or by continually moving the substrate  The alignment of features in adjacent scanning area is important. A measure of misalignment is termed as the butting error

 

5.2.2. Raster Scan

The raster-scanning systems cover the area of pattern by switching the e-beam on and off as required by the patterns shape and is schematized in Fig. 11b. A scanning technique where the beam scans line by line a section of the surface. The beam is turned on or off depending on whether the current pixel is to be exposed or not. The Raster scan is performed as follows:

• The chip is divided into stripes of ~28 pixels.

• The selected strip is written onto the substrate.

• The beam scans over the substrate line by line.

• If the current pixel is to be exposed, the beam is on.

• If the current pixel is not to be exposed, the beam remains off.

The substrate is moved perpendicular to the scan lines by a laser-controlled table.

 

 

you can view video on Electron Beam Lithography – I

 

Summary

  • Advantages of e-beam lithography includes smaller spot size < 100 Å, high speed writing with great precision of e-beam deflection and modulation, good control over the energy and dose delivered to resist provides an ultimate machine for patterning resists with high resolution, high density, high sensitivity and high reliability.
  • In e-beam lithography, the key focus is the quality of the electron optics (e.g., the ability to create a focused spot), the choice of resist, substrate and developer, and the process conditions of electron beam energy and dose, and development time and temperature, are the key parameters that defines the successful designing of nanometer scale patterns.
  • Two distinct uses of e-beam lithography, (a) direct exposure of resist for device fabrication, (b) mask fabrication for subsequent photolithography.
  • Various scanning systems are categorized by the way they scans. The vector scan features two-dimensional full-chip scan, provides a distortion free deflection, nd uses a stepped stage movement. The Raster scan features one dimensional line scan with continuously moving stage.