1. Chapter 5: Lithography

2. Introduction
The mechanism to print 2-D patterns to a thin film layer on the wafer surface.
Masks are glass plates (soda lime or quartz glass) that contain the patterns.
The patterns are first transferred from the mask to photoresist (PR), a light-sensitive polymer.
After opening windows in the PR, the pattern is transferred to the thin film using etching techniques.
Complexity of a fabrication process is often measured by the number of photolithographic masks used in the process.

3. Introduction The concept is simple
Spin on a thin layer of light-sensitive photoresist
Selectively expose it to UV light
Causing chemical bonds to either form or break
Develop to selectively remove the lighter weight PR
The resist may be used as a mask for either etching or for ion implantation
Because of constraints of resolution, exposure field, accuracy, throughput, and defect density, the implementation is not so simple
Very expensive
Very complex

4. Introduction

5. Introduction
Mask Maker

6. Introduction

7. Introduction

8. Introduction
Mask Aligner

9. Introduction UV 1
5,6 2 6
3,4

10. Introduction Some common PRs:
7,8
Some common PRs:
1800 series (for thin)  will be developed in MF 319
9200 series (for thick)  will be developed in AZ 400 9 10
Be aware that there are two different types of PR:
Positive PR: exposed areas will be developed
Negative PR: exposed areas will not be developed

11. Alignment Markers
Once a photolith process is done, the pattern developed is used to perform some additional process selectively on the wafer
Etching trenches in Si or SiO2
Making metalization runs
Implantation of dopants
Then the wafer will come back for another photolith step
Alignment markers are registration patterns that mate from one mask to another so that the multiple pattern sets match one another.

12. Introduction
Positive resists provide better controllability for small features.
Positive resists are easier to work with and use less corrosive developers and chemicals.
Positive resists are the dominant type of photoresists today.

13. Clear Field and Dark Field Masks
Most photolith engineers prefer clear field masks when possible
Easier to detect pattern on the wafer itself as there is more clear glass in the mask

14. Introduction Demands placed on this process for
Resolution: smaller device structures
Exposure field: ever-increasing chip sizes
Placement accuracy: aligning with existing layers
Throughput: manufacturing cost
Defects: yield and cost

15. NTRS Lithography Requirements

16. Introduction
The National Technology Roadmap for Semiconductors defines the future needs
Note especially
The driving force is the reduction of feature size
For every factor of two in reduction of area, there is a reduction of 0.7 in the linear dimensions
The reduction is required every three years
The most commonly quoted feature size is not as small as isolated MOS gate lines
Critical dimension (CD) control must improve (about 10% of minimum feature size)
Alignment accuracy must be about 1/3 of minimum feature size
The printing area increases with time since we must print one full die at a time

17. Introduction
About 1/3 of the cost of a wafer cost (about $1000 for an 8-inch wafer) is associated with lithography; we have only a few hundred dollars per wafer to spend
Optical lithography is used down to m (130nm) generations
For smaller dimensions, X-ray, direct e-beam, or extreme UV (EUV) processes are used.

18. Basic Concepts We generally separate lithography into three parts
The energy source (photons or electrons)
The exposure system
The resist
The exposure tool, which includes the light source and the exposure system, creates the best image possible on the resist (resolution, exposure field, depth of focus, uniformity and lack of aberrations)
Optimization of the photoresist with the settings on the exposure tool transfers the aerial image from the mask to the best thin film replica of the aerial image

19. Light Source
Historically, light sources have been arc lamps containing Hg vapor
A typical emission spectra from a Hg-Xe lamp
Low in DUV ( nm) but strong in the UV region ( nm)

20. Light Source
A much smaller set of wavelengths used to expose the resist
to minimize optical distortion associated with the lens optics.
to match the properties of the resist
Pick the wavelength that is heavily absorbed and causes changes in resist chemical properties
Two common monochromatic selections are the g-line at 436 nm and the i-line at 365 nm.

21. UV Light Sources
To expose < 250nm wide lines, we need to use shorter wavelength light
Two excimer lasers (KrF at 248 nm and ArF at 193 nm)
These lasers contain atoms that do not normally bond, but if they are excited the compounds will form; when the excited molecule returns to the ground state, it emits UV light
These lasers must be continuously strobed (several hundred Hz) or pulsed to pump the excitation; can get several mJ of energy out

22. Excimer Lasers
Low reliability due to etching of the electrodes and the optical windows by the energitic F ions

23. E-beam Source
Field Emission Gun (3), which provides the source of the electron beam, is a W or LaF6 filament.
Condenser Lens (7) are pairs of electromagnets that are used to collimate the beam of electrons.
Beam Booster, composed of Anode (5), Vacuum Tube (6), Apertures (8), Alignment Coils (9a, b, c), Stigmator (13), and Isolating Valve (15) is used to determine the energy of the electrons and to remove the electrons moving off-axis.
Objective Lens (10,11) is another set of electromagnets that focuses the electron beam onto the specimen (12), also containing the Deflecting System (14), which is another set of electromagnetics that sweep the electrons across the field of view and off of the sample .

24. X-Ray Source
High energy electrons collide with a metal. The transfer of energy results in the release of x-rays (short wavelength photons).

25. Exposure System There are three classes of exposure systems Contact
Proximity
Projection

26. Exposure System Contact printing is the oldest and simplest
The mask is put with the absorbing layer face down in contact with the wafer
This method
Can give good resolution
Machines are inexpensive
Cannot be used for high-volume due to damage caused by the contact
Still used in research and prototyping situations

27. Wafer Exposure Systems
Proximity printing solves the defect problem associated with contact printing
The mask and the wafer are kept about 5 – 25 m apart
This separation degrades the resolution
Cannot print with features below a few microns
The resolution improves as wavelength decrease. This is a good system for X-ray lithography b/c of the very short exposure wavelength (1-2 nm).

28. Projection/Step and Repeat
For large-diameter wafers, it is impossible to achieve uniform exposure and to maintain alignment between mask levels across the complete wafer.
Masks are now called reticules
Projection printing is the dominant method today
They provide high resolution without the defect problem
The mask is separated from the wafer and an optical system is used to image the mask on the wafer.
The resolution is limited by diffraction effects
The optical system reduces the mask image by 4X to 5X
Only a small portion of the wafer is printed during each exposure
Steppers are capable of < 0.25 m
Their throughput is about 25 – 50 wafers/hour

29. Optics Basics We need a very brief review of optics
If the dimensions of objects are large compared to the wavelength of light, we can treat light as particles traveling in straight lines and we can model by ray tracing
When light passes through the mask, the dimensions of objects are of the order of the dimensions of the mask
We must treat light as a wave

30. Snell’s Law and Reflectivity
n1 sin(q1) = n2 sin(q2) 1 = T+R+A, where T is transmission R is reflection A is absorption If q1 = p/2, q2 = sin-1(n1/n2) R = [(n1-n2)/(n1+n2)]2

31. Refractive index of SiO2
Transmission through two air-glass surfaces is less than 93.1%.
R = 3.5 in air
l = 365nm

32. Snell’s Law/Antireflective Coatings
when the layer thickness,t, is
t = (m+1)l/4; m = 0,1,2…
R = 0 when n = (n1n2)1/2 n1 n2 n t

33. Young’s Single Slit Experiment
sinq = l/d

34. Amplitude of largest secondary lobe at point Q, eQ, is given by:
eQ = a(A/r)f(c)d
where A is the amplitude of the incident wave, r is the distance between d and Q, and f(c) is a function of c, an inclination factor introduced by Fresnel.

35. Young’s Double Slit Experiment

36. Diffraction of Light

37. Diffraction of Light
The Huygens-Fresnel principle states that every unobstructed point of a wavefront at a given time acts as a point source of a secondary spherical wavelet at the same frequency
The amplitude of the optical field is the sum of the magnitudes and phases
For unobstructed waves, we propagate a plane wave
For light in the pin-hole, the ends propagate a spherical wave.

38. Diffraction of Light

39. Basic Optics

40. Basic Optics
Information about the shape of the pin hole is contained in all of the light; we must collect all of the light to fully reconstruct the pattern
If only part of the diffraction pattern is collected and focused on the substrate, the image created is not identical to the one on the mask.
The light diffracted at higher angles contains information about the finer details of the structure and are lost

41. Basic Optics
The image produced by this system is

42. Basic Optics The diameter of the central maximum is given by
Note that you get a point source only if d  

43. Basic Optics There are two types of diffraction
Fresnel, or near field diffraction
Fraunhofer, or far field diffraction
In Fresnel diffraction, the image plane is near the aperture and light travels directly from the aperture to the image plane.
In Fraunhofer diffraction, the image plane is far from the aperture, and there is a lens between the aperture and the image plane.
Fresnel diffraction applies to contact and proximity printing while Fraunhofer diffraction applies to projections systems

44. Fraunhofer Diffraction
We define the performance of the system in terms of
Resolution
Depth of focus
Field of view
Modulation Transfer Function (MTF)
Alignment accuracy
throughput

45. Fraunhofer Diffraction
Imagine two sources close together that we are trying to image (two features on a mask)
How close can these be together and we can still resolve the two points?
The two points will each produce an Airy disk.
Lord Rayleigh suggested that the minimum resolution be defined by placing the maximum from the second point source at the minimum of the first point source.

46. Fraunhofer Diffraction

47. Fraunhofer Diffraction
With this definition, the resolution becomes
For air, n=1
 is defined by the size of the lens, or by an aperture and is a measure of the ability of the lens to gather light

48. Fraunhofer Diffraction
This is usually defined as the numerical aperture, or NA
Defined only for point sources as the point source Airy function was used to develop the equation
A more generalized equation replaces 0.61 by a constant k1 which lies between 0.6 and 0.8 for practical systems.

49. Fraunhofer Diffraction
From this result, we see that we get better resolution (smaller R) with shorter wavelengths of light and lenses of higher numerical aperture
We now consider the depth of focus over which focus is maintained.
We define  as the on-axis path length difference from that of a ray at the limit of the aperture. These two lengths must not exceed /4 to meet the Rayleigh criterion

50. Depth of Focus

51. Depth of Focus
From this criterion, we have
For small 

52. Fraunhofer Diffraction
From this we note that the depth of focus decreases sharply with both decreasing wavelength and increasing NA.
The Modulation Transfer Function (MTF) is another important concept
This applies only to strictly coherent light, and is thus not really applicable to modern steppers, but the idea is useful

53. Fraunhofer Diffraction
Because of the finite aperture, diffraction effects and other non-idealities of the optical system, the image at the image plane does not have sharp boundaries, as desired
If the two features in the image are widely separated, we can have sharp patterns as shown
If the features are close together, we will get images that are smeared out.

54. Modulation Transfer Function

55. Fraunhofer Diffraction
The measure of the quality of the aerial image is given by
The MTF is really a measure of the contrast in the aerial image
The optical system needs to produce MTFs of 0.5 or more for a resist to properly resolve the features
The MTF depends on the feature size in the image; for large features MTF=1
As the feature size decreases, diffractions effects casue MTF to degrade

56. Change in MTF versus Wavelength

57. Contact and Proximity Systems
These systems operate in the Fresnel regime
If the mask and the resist are separated by some small distance “g” and a plane wave is incident on the mask, light is diffracted at the aperture edges.
As shown in next slide, there is
1. Small maximum at the edge from constructive interference
2. Ringing caused by constructive and destructive interference
To minimize effects, multiple wavelengths of light may be used to expose PR

58. Fresnel Diffraction

59. Fresnel Diffraction As g increases, the quality of the image decreases
The aerial image can be computed accurately when
where W is the feature size
Within this regime, the minimum resolvable feature size is:
Proximity aligner with a 10 m gap and an i-line source can resolve ~ 2 m features.

60. Resolution
A more exact solution for the theoretical resolution for proximity or contact aligners is given by:
Where l is the wavelength of light used to exposure the pattern, g is the distance between the bottom of the mask and the top of the photoresist, z is the thickness of the photoresist (typically mm).

61. Fresnel Number Fresnel diffraction when F ≥ 1
Fraunhofer diffraction when F << 1

62. Depth of Focus

63. Summary of the Three Systems

64. Photoresists
Parameters that determine the usefulness of the resist include:
Sensitivity: a measure of how much light is required to expose the resist - typically 100mJ/cm2
Resolution where the effects of exposure, baking, developing should not degrade the quality of the image
Chemical and physical properties: it must withstand chemical etching, mild temperature excursions, ion implantation

65. Photoresists Photoresists usually contain three components
Inactive resin (usually a hydrocarbon which forms the base material)
Photoactive compound (PAC)
Solvent which is used to adjust the viscosity
The most common g- and i-line resists use
Diazonaphthoquinones (DNQ) as the PAC
Novolac as the resin
Propylene glycol monomethyl ether acetate (PGMEA) as the solvent (this has replaced Cellosolve acetate, which is a toxic hazard)

66. Basic Structure of Novolac
Novolac is a polymer containing hydrocarbon rings with 2 methyl groups and 1 OH group
The basic ring structure is repeated to form a long chain polymer
Novolac readily dissolves in developer at about 15 nm/s

67. Diazoquinone
The photoactive part of the molecule is the part above the SO2

68. Diazoquinone
The function of the PAC is to inhibit the dissolution of the resin in the developer
DNQ is essentially insoluble in developer prior to exposure to light
When dissolved in the resin, DNQ reduce the resist dissolution rate from ~ 15nm/s to 1-2 nm/s
When the resist is exposed to light, the diazoquinone molecule changes chemically and increases the dissolution rate to ~100nm/s.

69. Properties and Characteristics of Resists
Two parameters are used to define the properties of photoresists
Contrast
Critical modulation transfer function (CMTF)

70. Contrast
The ability of the photoresist to distinguish between various levels of light intensities.
It is experimentally determined by exposing the resist to differing amounts of light, developed for a fixed time and measuring the thickness of resist remaining after developing.

71. Photoresist Contrast

72. Photoresist Contrast
For positive resists, material exposed to low light will not be attacked by the developer; material exposed to large doses will be completely removed
Intermediate doses will result in partial removal
The contrast is the slope of this curve and is given by
Typical g- and i-line resists will achieve a contrast of g = 2-3 and Qf values of 100 mJ/cm2

73. Photoresist Contrast
The contrast is not a constant, but depends on process variables such as
development chemistry,
bake times,
temperatures before and after exposure,
wavelength of light, and
underlying structure
It is desirable to have as high a contrast as possible in order to produce the sharpest edges in the developed pattern

74. Photoresist Contrast

75. Modulation Transfer Function (MFT)
Defined in two points of the lithographic system.
MTF: Measure of the dark versus light intensities in the aerial image produced by the projection system
CMTF: Measure of the exposed versus unexposed regions in the high contract image focused on the PR
The CMTF is the minimum optical transfer function necessary to resolve a pattern in the resist
For g- and i-line resists, CMTF  0.4

76. Effect of Resist Thickness
Resists usually do not have uniform thickness on the wafer
Edge bead: The build-up of resist along the circumference of the wafer
- There are edge bead removal systems
Step coverage
Centrifugal Force

77. Effect of Resist Thickness
The resist can be underexposed where it is thicker and overexposed where it is thinner
This can lead to linewidth variations
Light intensity varies with depth below the surface due to absorption where  is the optical absorption coefficient
Thus, the resist near the surface is exposed first
A process called bleaching in which the exposed material becomes almost transparent (i.e.,  decreases after exposure)
Therefore, more light goes to deeper layers after bleaching the near surface layer of PR

78. Photoresist Absorption
If the photoresist becomes transparent and if the underlying surface is reflective, reflected light from the wafer will expose the photoresist in areas we do not want it to.
This leads to the possibility of standing waves (due to interference), with resultant waviness of the developed resist
We can solve this by putting an antireflective coating on the surface of the substrate before spinning the photoresist  increases process complexity

79. Standing Waves Due to Reflections

80. Standing Waves Due to Reflections

81. Removal of Standing Wave Pattern
                                                                                                                       
(a)                                     (b)                                (c)
Diffusion during a post-exposure bake (PEB) is often used to reduce standing waves.
Photoresist profile simulations as a function of the PEB diffusion length: (a) 20nm, (b) 40nm, and (c) 60nm.

82. Mask Engineering
There are two ways to improve the quality of the image transferred to the photoresist
Optical Proximity Correction (OPC)
Phase Shift Masks (PSM)
We note that the lenses in projections systems are both finite and circular but most features on the mask are square.
The high frequency components of the pattern are lost and the “squareness” of the corners of the pattern disappear.
Can be taken into account by adjusting feature dimensions and shapes in the masks

83. Mask Engineering

84. Phase Shift Masks
In a projection system the amplitudes at the wafer add so that closely spaced lines interact; the intensity at the wafer is smeared
If we put a material of proper index of refraction on part of the mask, we can retard some of the light and change its phase by 180 degree and the two portions of light interfere and cancel out.
The thickness of the PS layer is n is the index of refraction of the phase shift material

85. Phase Shift Masks (PSM)
Intensity pattern is barely sufficient to resolve the two patterns.

86. Scanning Projection Aligners
Projection aligners have been industry standard for about 20 years
It is easier to correct for aberrations in small regions than in large
Scan a small slit across the mask while the wafer is simultaneously scanned
Scanning projection aligners must use 1:1 masks
Pattern on the mask is the same size as the one imaged on the wafer.

87. Scanning Projection Printer

88. Scanning Projection Systems
Cost effective and has high throughput
Linewidth control for smaller devices is difficult
As chips became larger, it is more difficult to produce good full wafer masks
With ULVI and WSI, this system could not scale and was replaced by systems that exposed only a single die at a time

89. Step-and-Repeat Projection Aligners
Exposed a limited portion of the wafer at a time
The image on the wafer is 4-5 times smaller than the image on the mask or reticule.
Masks thus are much larger, and thus repairable to some extent
Steppers also allow better alignment because they align on the exposure field rather than for the entire wafer
Wafer can be moved vertically to keep image plane at some location as the PR

90. Off-Axis Illumination
By changing the angle of incidence of the light on the mask, change the angle of the diffracted light
Although some of the diffracted light is lost in this scheme, much of the higher order diffraction is captured
As the resolution is decreased, it is harder to make these optics work

91. Off-Axis Illumination

92. Step and Scan
A hybrid has been developed called a “step-and-scan”, but is very complex and very expensive.

93. DNQ/Novolac Resist Process
The details of the process are more complex that described earlier

94. DNQ/Novolac Resist Process
We first must consider adhesion
There can be one or more operations depending on what is under the resist
The wafer must be clean before resist is applied
It may need to be heated to a few hundred degrees to drive off water
Adhesion to Si is not as good as to metals and silicon dioxide
Adhesion promoter, Hexamethyldisilane (HMDS), may be needed

95. DNQ/Novolac Resist Process
Dispensing the resist can be done either with a stationary or a slowly spinning wafer
The solvent evaporates rapidly after dispensing the resist and during the spin
Generally more uniform resist thicknesses are obtained the faster the wafer is accelerated.
The faster the final speed, the thinner the resist.

96. DNQ/Novolac Resist Process
Exposure times and source intensity are reciprocal—one can reduce exposure times with more intense sources
Exposure time is increase by increasing the bake temperature (due to decomposition of the PAC and thus decreased sensitivity)
Post-exposure bake is often done before development because the PAC can diffuse and this will eliminate the standing wave pattern
Post-development bake is done to remove standing wave pattern by flowing resist (90-100oC) or increase chemical/mechanical strength of resist ( oC)
Long UV exposure can also be used to cross-link the polymer chains in the remaining photoresist

98. Measurement Methods Measurement of Mask Features and Defects
Resist Patterns
Etched Features
Alignment
Measure resist pattern after development
The aerial image is not generally measurable
Because of the complexity of the masks, the inspection must be fully automated—manual observation under a microscope is not possible

99. Mask Inspection System

100. Measurement of Mask Features and Defects
Here, light is passed through the mask and collected by an image recognition system
Solid state detectors are used to collect the light
The information is compared against the database of the mask design or with an identical mask
The inspection process is more difficult if the mask contains OPC or is a PSM
Often, defects found in this process can be corrected
Lasers can burn off excess Cr or Fe oxide.
Adding absorber to clear areas is harder

101. SEM Measurement

102. State-of-the-Art Capable of exposing down to ~ 10nm E-beam lithography
X-ray lithography
Extreme UV lithography
E-beam and EUV are performed under vacuum
Throughput is very slow
New resist families are required
Most are very difficult to remove after use
Research needed on mask material for x-ray and EUV
Glass absorbs
Thickness of metal needed to block x-rays is very thick (20-50mm)