Transcript Lithography - Chemical Engineering IIT Madras

Analytical Techniques
 Used in production and R&D
 Crude view: High magnification microscope and high
sensitivity titration (composition analysis)
 Basic Principles, capabilities, limitations
 Recent advances
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Index
 Microscopy
 Scanning Probe Microscopy (SPM)
 e.g. STM, AFM and their variations
 Secondary Electron Microscopy (SEM)
 Analytical
 XPS(ESCA), AES, AAS, FTIR,
 SIMS,ICP-MS
 ...
 Others
 Thickness measurements
 Ellipsometry, Interferometry, Four point Probe
 FIB
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SPM
 Scanning Probe Microscopy
 Atomic Force Microscopy (more commonly used now)
 Scanning Tunneling Microscopy (First)
 A 3 dimensional picture, at the best with atomic resolution
DVD 10 um x 10 um image
(color by software)
©Photometrics
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Graphite
X and Y scales 30 A, each bump is an atom
Images for STM are from ©Leiden Univ, Netherlands
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STM: Principle
 Basics
 Quantum Physics
 Probability of locating an electron in any region can be calculated
 Note
 The probability of finding an
electron at a particular ‘point’ is zero
 To find the probability in a small
region DxDyDz
 integrate the PDF
 The probability to find it rapidly
decreases, as one moves from the
‘center’ point
Prob
Location
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STM: Principle
 Basics
 If two conductors are brought close together
 close means 1 nm (approx 3 atomic distances)
 and a DC bias is applied
 prob of finding an electron on the ‘other’ side is non-zero
 Some of the electron ‘tunnels’ through the barrier
 Not the same as AC voltage
 ac current can pass through, because of the varying field
 Classical physics sufficient for AC voltage
 Not the same as breakdown (arcing) in DC
 If the field is strong, electrons overcome the barrier, not
tunneling
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STM: Principle
 Metal with electrons
in conduction band.
Electrons are at Ef and
need work function F
to escape from the
metal
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 When probe and tip
are brought together,
they still need F to
‘jump’ above the barrier
 If the distance is
small and a voltage is
applied, electron can
‘tunnel’ through the
barrier
©Leiden Univ, Netherlands
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STM: Principle
 Tunneling depends very
strongly (exponential) on the
distance
 3 A distance corresponds
to 1000 times change in
current
 Directly proportional to
voltage (approx)
 Work function etc remain
constant for a material
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©Leiden Univ, Netherlands
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Scanning Tunneling Microscopy
 History
 In IBM, Binnig & Rohrer invented STM and were awarded
Nobel Prize
 Design/Working
 Similar to Record player
 Sharp tip (probe)
 not touching the sample
 Applied voltage (few mV to
few V)
 Piezo element for precise
movement of tip (up/down)
 Precision galvanometer
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©Leiden Univ, Netherlands
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Scanning Tunneling Microscopy
 If tip touches the sample (conducting sample), there will be
(significant) current
 If tip is far away from sample, zero current
 At very small (1 nm or less) distances, tunneling current
 pico amp or nano amp
 Very strong dependence on distance
 e.g. If the distance increases by about 3 Angstroms, the current
decreases by 1000 times!
 Feed back loop with piezo electric element
 Apply voltage to piezo electric element to change the length (and
hence the distance between the probe and the sample)
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STM: Operation
 Operation:
Apply a voltage and bring the tip down, until a certain amount of
current is measured (eg 1nA)
 Move the tip in X direction (horizontal)
 (actually the sample is moving in horizontal plane)
 using piezo electric material
 for very precise movement
z
 If the sample is closer
y
 tunneling current increases
 a pre-amplifier shows increasing current
x
 converts it to a voltage
 A circuit checks it with a reference voltage (corresponding to
some preset current, for example 1 nA)
 Difference in voltage is amplified , recorded and supplied to piezo
tube
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STM: Operation
 Electronics
 Also uses filters, PID controls in the circuits
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©Leiden Univ, Netherlands
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STM : Operation
 Another piezo tube moves the sample in the x direction
 the operation is repeated
 moving the sample is better than moving the whole assembly of
tip+piezo and electronic
 however, moving sample, in a precise manner is not very easy
 x and y resolutions are not as good as z resolution
 normal sample size is about an inch
 At the end of one x scan, the tip is brought back to the beginning,
stepped a bit in y direction (again using piezo) and another ‘line scan’
begins
 The voltage applied to each piezo element (for x, y and z) are
recorded
 used to create the final map
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STM: Miscellaneous
 Graphite in atomic resolution
 What is meant by atomic resolution?
 Electronic Fermi level map
 Works for conductors
 Practically, for non conductors, can coat a thin film of gold
 one will not get atomic level resolution of original surface,
but still very good resolution
 Easier to use in air/vacuum, more difficult in liquid medium
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STM: Summary
 Atomic level resolution
 nm level resolution achieved repeatedly
 Better suited for conductors
 Instrumentation requirements
 Capability to detect very small current precisely
 Capability to move the tip in a very controlled and
precise manner ( 0.1 A is the current advertised
capability)
 Robust feed back electronics
 Sharp tip
 need not be exactly conical
 High aspect ratios are preferred
 Blunt tip will ‘convolute’ the image
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AFM: Principle
 When a probe attached to a very fine spring is brought
near a surface
 if the probe is far, there is not much force
 when the probe comes near, there is an attractive
force
 when the probe comes very near, there is repulsion
 If one can monitor the force and keep it constant...
 Similar to keeping the
‘tunneling current’ constant Force
Repulsion
in STM...
 And record the voltage
Distance
applied to piezo, then one
can get the topography
Attraction
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AFM: Construction Detail
 A very flexible spring puts very little load on the
tip
 AFM spring constant is 0.1 N/m
 achieved used cantilever
 If spring constant is low, it is flexible
 If spring constant is high, then the tip will
not ‘respond’ quickly if it is dragged over a
surface with topography
 Hence tip with high spring constant will
slow down the data acquisition (if one wants to
do it correctly)
 Tip with low spring constant and mass is
preferred
 corresponds to high resonant frequency
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© US Navy Res Lab
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AFM: Construction Detail
© US Navy Res Lab
 Sample Tip images
Normal
SEM images of tips
‘super’ tip
‘ultra’ lever
(home made)
 The end radius (~ 30 nm for normal & super tips and 10 nm for
ultra)
 Limits the resolution (similar to wavelength in optics)
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AFM: Construction Detail
 AFM: Tip structure
 Cantilever
 Note: STM had a ‘rigid’ probe structure
 Measurement of force
 optical lever
©Leiden Univ, Netherlands
©Muller Institute, Swiss
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© US Navy Res Lab
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AFM: Construction Detail
 Position Sensitive Detector (usually segmented
detector)
 4 or 2 segments
Detector Schematic
 Intensity (i.e. Detected current) must be equal in
all the four segments
 Laser, cantilever and detector are put together in
a rigid mount
 If cantilever position moves, reflected beam will
move
 Four segment (quadrant) can be used to detect
lateral forces (measure torsion) also
 Lateral Force Microscopy (LFM)
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AFM: Construction Detail
 Piezo construction
 Original construction with 3 orthogonal bars
 Newer construction with a tube
 Inner cylinder is one electrode,
outer cylinder is segmented (typically
4)
 Constant potential between inner
and outer cylinders will move the
piezo in z direction (compression or
tension)
 Potential difference between outer
plates will move the piezo in (mainly)
x or y direction
©Leiden Univ, Netherlands
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AFM: Operation
 Sample is moved in x and y direction
 Cantilever moves up or down, depending on sample topography
 Lever movement is detected by
segmented photo detector
 signal is amplified and compared
with ‘reference’ or ‘pre-set’ value
 difference is amplified and fed
back to the piezo
 cantilever (tip) is pulled back (for
example)
© US Navy Res Lab
 *PID controls included, filters to prevent ‘oscillation’
 *When the tip is first brought close to the sample, one can see the
tip getting pulled towards the sample (using a microscope)
 (attractive force) before repulsion
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AFM: Operation Modes
1. Constant Force Mode: (with feed back control). a.k.a. Height
mode
 (a) Contact Mode:
 The probe is in ‘contact’ with the sample
 working in repulsion regime
 better (more accurate) reproduction of surface topography
 tip wears off quickly, (tip is dragged over suface and lateral forces are
significant)
 soft surface may get damaged
 (b) Non contact mode
 probe is bit farther from the sample (attraction regime?)
 resolution is worse than that in contact mode
 sample not damaged and tip life is longer (soft samples)
 however, tip may ‘jump’ to contact mode (e.g.water on the surface etc)

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AFM: Operation Modes
1. Constant Force Mode: (with feed back control). a.k.a. Height
mode
(c) Tapping mode (®Digital Instruments)
 Probe oscillates at a particular frequency
 AC signal at the detector (in each segment) detected
 probe is probably in both the regimes (attractive and repulsion)
 better resolution than non contact mode, but longer tip life etc...
2. Constant Height Mode:(with out feed back control). a.k.a.
Deflection mode
 Works for surface with out too much topography
 Tip is brought close to the sample and once a preset force is reached, tip is
stopped
 sample scanned (line by line) and tip position is recorded (from the laser
position). Tip is not moved up or down
 Tip can crash if the sample is rough!
 Relatively lower resolution
 Faster image acquisition (since tip need not move in z direction)
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AFM: Tip Effects
 Compression effect
 If the sample is soft, it may get compressed when the tip comes close
 (compare the elasticity of probe vs surface). Probe has low spring
constant
 Can actually be used (with slight modification) to measure surface
hardness
 Note: The force is low , in nano Newton
 However, pressure can be MPa!
 Image broadening
 Due to large tip
(Tip convolution)
© US Navy Res Lab
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© Univ. Bristol
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AFM: Tip Effects
 Interaction Forces
 If other forces come into play
 e.g. Chemical , magnetic, etc
 If one is aware of it, more information
can be obtained
 If not, image obtained is incorrectly
interpreted!
 Aspect Ratio
 If sample has vertical features
 At the best, features with ‘probe aspect
ratio’ can be measured accurately
 sharper features will still show ‘probe
aspect ratio’
© Univ. Bristol
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AFM: Miscellaneous
 STM not used regularly in the fab; more in R&D
 AFM applicable for insulators and conductors; used in fab and R&D
labs
 Used after dep and CMP to measure ‘roughness’
 RMS of the height
 Or any other feature (e.g. Poly after etch), to find side-wall slope
 aspect ratio issue can be overcome on one side by tilting the
sample
 Tips are easy to change and are ‘reasonably’ expensive
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