Upload
kriticka-sharma
View
181
Download
9
Embed Size (px)
Citation preview
IC TECHNOLOGY
PATTERN TRANSFER & ETCHING
By:Kritica Sharma
Assistant Professor (ECE)
CONTENTS
2
Lithography Introduction to photo/optical lithography Mask generation Contact/ proximity printers Projection printers Photo resists Etching Dry & Wet etching Methods for anisotropic etching Plasma etching Reaction ion etching (RIE).
FABRICATION PROCESSES FOR VLSI DEVICES
Chip Fabrication ProcessesSilicon Wafer Manufacture
Packaging
Epitaxial
Growth
Photo-lithography
oxidation
Etching
Diffusion (Ion Implantation)
Metalization
4
LITHOGRAPHY A light sensitive photoresist is spun onto the wafer forming a thin
layer on the surface. The resist is then selectively exposed by shining light through a mask which contains the pattern information for the particular being fabricated. The resist is then developed which completes the pattern transfer from the mask to the wafer.
Lithography comes from two Greek words, “lithos” which means stone and graphein which means write.“ writing a pattern on stone”
LITHOGRAPHY Lithography is the most complicated, expensive, and
critical process of modern IC manufacturing.
Lithography transforms complex circuit diagrams into pattern which are define on the wafer in a succession of exposure and processing steps to form a number of superimposed layers of insulator, conductor, and semiconductors materials.
Typically 8-25 lithography steps and several hundred processing steps between exposure are required to fabricate a packed IC.
The minimum feature size i.e., the minimum line width or line to line separation that can be printed on the surface, control the number of circuits that can be placed on the chip and has a direct impact on circuit speed. The evolution of IC is therefore closely linked to the evolution of lithographic tools.
6
review
PHOTOLITHOGRAPHY Temporarily coat photoresist on wafer. Transfers designed pattern to photoresist. Most important process in IC fabrication. 40 to 50% total wafer process time. Determines the minimum feature size.
APPLICATIONS OF PHOTOLITHOGRAPHY
Main application: IC patterning process
Other applications: Printed electronic board, nameplate and printer plate.
BASIC STEPS OF PHOTOLITHOGRAPHY
Photoresist coating Alignment and exposure Development
BASIC STEPS - OLD TECHNOLOGY Wafer clean Dehydration bake Spin coating primer and PR Soft bake Alignment and exposure Development Pattern inspection Hard bake
PR coating
Development
BASIC STEPS - ADVANCED TECHNOLOGY
Wafer clean Pre-bake and primer coating Photoresist spin coating Soft bake Alignment and exposure Post exposure bake Development Hard bake Pattern inspection
PR coating
Development
Track-stepper integrated system
Hard bake
Strip PR
Etch
Previous Process
Ion Impla
nt
Rejected
Surface preparation PR
coatingSoft bake Alignme
nt&
Exposure
Development
Inspection
PEB
Approved
Clean
Track system
Photo Bay
Photo cell
WAFER CLEAN
P-WellUSGSTI
Polysilicon
Gate Oxide
WAFER CLEAN Remove contaminants Remove particulate Reduce pinholes and other defects Improve photoresist adhesion Basic steps
Chemical clean Rinse Dry
SURFACE CLEANING Typical contaminants that must be removed prior to
photoresist coating: dust from scribing or cleaving (minimized by laser scribing)
Photoresist residue from previous photolithography (minimized by performing oxygen plasma ashing)
atmospheric dust (minimized by good clean room practice)
bacteria (minimized by good DI water system)
films from other sources:-solvent residue-H2 O residue-photoresist or developer residue-silicone
For particularly troublesome grease, oil, or wax stains: Start with 2-5 min. soak in 1,1,1-trichloroethane (TCA) or trichloroethylene (TCE) with ultrasonic agitation prior to acetone
PRE-BAKE AND PRIMER VAPOR
P-WellUSGSTI
Polysilicon
Primer
Primer Vapor CoatingDehydration Bake
Wafer
Prep Chamber Primer Layer
Pre-bake and Primer Vapor Coating
Wafer
Hot Plate Hot Plate
HMDS Vapor
Dehydration bake Remove moisture from wafer surface Promote adhesion between PR and surface Usually around 100 °C Integration with primer coating
PHOTOLITHOGRAPHY PROCESS, PREBAKE
Promotes adhesion of PR to wafer surface Wildly used: Hexamethyldisilazane (HMDS) HMDS vapor coating prior to PR spin coating Usually performed in-situ with pre-bake Chill plate to cool down wafer before PR coating
PHOTOLITHOGRAPHY PROCESS, PRIMER
WAFER COOLING Wafer need to cool down Water-cooled chill plate Temperature can affect PR viscosity
Affect PR spin coating thickness
PHOTORESIST COATING
P-WellUSGSTI
PolysiliconPhotoresist
Primer
SPIN COATING Wafer sit on a vacuum chuck Slow spin ~ 500 rpm Liquid photoresist applied at center of wafer Ramp up to ~ 3000 - 7000 rpm Photoresist spread by centrifugal force Evenly coat on wafer surface
SPIN COATING WITH PHOTORESIST Wafer is held on a spinner chuck by vacuum and resist
is coated to uniform thickness by spin coating.
Typically 3000 - 6000 rpm for 15-30 seconds.
Resist thickness is set by: primarily resist viscosity secondarily spinner rotational speed
Most resist thicknesses are 1-2 μm for commercial Si processes.
Resist thickness is given by t =square of( kp)/root of(w1), where,
k = spinner constant, typically 80-100p = resist solids content in percentw = spinner rotational speed in rpm/1000
SPIN COATER Automatic wafer loading system from robot of track system Vacuum chuck to hold wafer Resist containment and drain Exhaust features Controllable spin motor Dispenser and dispenser pump Edge bead removal
PHOTORESIST SPIN COATER
PR
Vacuum
EBR
Wafer
Chuck
Water Sleeve
Drain Exhaust
PHOTORESIST APPLYING
Spindle
PR dispenser nozzle
Chuck
Wafer
To vacuum pump
PHOTORESIST SUCK BACK
Spindle
To vacuum pump
PR dispenser nozzle
Chuck
PR suck backWafer
PHOTORESIST SPIN COATING
Spindle
To vacuum pump
PR dispenser nozzle
Chuck
PR suck backWafer
PHOTORESIST SPIN COATING
Spindle
To vacuum pump
PR dispenser nozzle
Chuck
PR suck backWafer
PHOTORESIST SPIN COATING
Spindle
To vacuum pump
PR dispenser nozzle
Chuck
PR suck backWafer
PHOTORESIST SPIN COATING
PR suck back
Spindle
To vacuum pump
PR dispenser nozzle
Chuck
Wafer
PHOTORESIST SPIN COATING
PR suck back
Spindle
To vacuum pump
PR dispenser nozzle
Chuck
Wafer
PHOTORESIST SPIN COATING
Spindle
To vacuum pump
PR dispenser nozzle
Chuck
PR suck backWafer
PHOTORESIST SPIN COATING
Spindle
To vacuum pump
PR dispenser nozzle
Chuck
PR suck backWafer
PHOTORESIST SPIN COATING
Spindle
To vacuum pump
PR dispenser nozzle
Chuck
PR suck backWafer
PHOTORESIST SPIN COATING
Spindle
To vacuum pump
PR dispenser nozzle
Chuck
PR suck backWafer
EDGE BEAD REMOVAL
Spindle
To vacuum pump
Chuck
WaferSolvent
EDGE BEAD REMOVAL
Spindle
To vacuum pump
Chuck
WaferSolvent
OPTICAL EDGE BEAD REMOVAL EXPOSURE
SpindleChuck
Wafer
PhotoresistLight sourceLight beam
Exposed Photoresist
OPTICAL EDGE BEAD REMOVAL After alignment and exposure Wafer edge expose (WEE) Exposed photoresist at edge dissolves during
development
READY FOR SOFT BAKE
Spindle
To vacuum pump
Chuck
Wafer
EDGE BEAD REMOVAL (EBR)
PR spread to the edges and backside PR could flakes off during mechanical handling
and causes particles Front and back chemical EBR Front optical EBR
SOFT BAKING Used to evaporate the coating
solvent.
Typical thermal cycles:90-100°C for 20 min. in a convection oven 75-85°C for 45 sec. on a hot plate
Microwave heating or IR lamps are also used
Optimizes light absorbance characteristics of photoresist
Spindle
To vacuum pump
Chuck
Wafer
SOFT BAKE
P-WellUSGSTI
PolysiliconPhotoresist
PURPOSE OF SOFT BAKE Evaporating most of solvents in PR Solvents help to make a thin PR but absorb radiation and affect
adhesion Soft baking time and temperature are determined by the
matrix evaluations Over bake: polymerized, less photo-sensitivity Under bake: affect adhesion and exposure
METHODS OF SOFT BAKE Hot plates Convection oven Infrared oven Microwave oven
BAKING SYSTEMS
Heater
Vacuum
Wafer
Heater
Heated N2
Wafers
MW Source
VacuumWafer
Photoresist
Chuck
Hot plate Convection oven
Microwave oven
HOT PLATES
Widely used in the industry Back side heating, no surface
“crust” In-line track system
Heater
Wafer
WAFER COOLING BEFORE EXPOSURE Need to cool down to ambient temperature Water-cooled chill plate Silicon thermal expansion rate: 2.510-6/C For 8 inch (200 mm) wafer, 1 C change causes 0.5 mm
difference in diameter PR thermal expansion effect
ALIGNMENT
P-WellUSGSTI
PolysiliconPhotoresist
Gate Mask
EXPOSUREGate Mask
P-WellUSGSTI
PolysiliconPhotoresist
READY FOR POST EXPOSURE BAKE
P-WellUSGSTI
PolysiliconPhotoresist
ALIGNMENT AND EXPOSURE Most critical process for IC fabrication Most expensive tool (stepper) in an IC fab. Most challenging technology Determines the minimum feature size Currently 0.18 mm and pushing to 0.13 mm
POST EXPOSURE BAKE PEB normally uses hot plate at 110 to 130 C for about 1
minute. For the same kind of PR, PEB usually requires a higher
temperature than soft bake. Insufficient PEB will not completely eliminate the standing
wave pattern. Over-baking will cause polymerization and affects
photoresist development
PURPOSE OF POST EXPOSURE BAKE Baking temperature higher than the Photoresist glass
transition temperature Tg. Thermal movement of photoresist molecules. Rearrangement of the overexposed and underexposed PR
molecules. Average out standing wave effect. Smooth PR sidewall and improve resolution.
WAFER COOLING BEFORE DEVELOPMENT After PEB the wafer is put on a chill plate to cool down
to the ambient temperature before sent to the development process.
High temperature can accelerate chemical reaction and cause over-development and PR CD loss.
DEVELOPMENT
P-WellUSGSTI
PolysiliconPR
Development: Immersion
Spin DryDevelop
Rinse
Schematic of a Spin Developer
Vacuum
DeveloperWafer
Chuck
Water sleeve
Drain
DI water
APPLYING DEVELOPMENT SOLUTION
SpindleChuck
Wafer
Exposed Photoresist
Development solution dispenser nozzle
To vacuum pump
APPLYING DEVELOPMENT SOLUTION
Spindle
To vacuum pump
Chuck
Wafer
Exposed Photoresist
Developer Spin Off
Spindle
To vacuum
pump
Chuck
Wafer
Patterned photoresist
Edge PR removed
DI WATER RINSE
Spindle
To vacuum pump
Chuck
Wafer
DI water dispenser nozzle
SPIN DRY
Spindle
To vacuum pump
Chuck
Wafer
READY FOR HARD BAKE
SpindleChuck
Wafer
DEVELOPMENT• Developer solvent dissolves the softened part
of photoresist• Transfer the pattern from mask or reticle to
photoresist• Three basic steps:
– Development– Rinse– Dry
DEVELOPMENT
PR
PR PR
PR
Substrate Substrate
Substrate Substrate
Film Film
FilmFilm
Mask
Exposure
DevelopmentEtching
PR Coating
DEVELOPMENT PROFILES
PR PR
Substrate Substrate
PR
Substrate
PR
Substrate
Normal Development
Under Development Over Development
Incomplete Development
DEVELOPER SOLUTION +PR normally uses weak base solution The most commonly used one is the tetramethyl
ammonium hydride, or TMAH ((CH3)4NOH).
HARD BAKING Used to stabilize and harden the developed
photoresist prior to processing steps that the resist will mask.
Postbake removes any remaining traces of the coating solvent or developer.
Higher temperature than soft bake (120-150 degree)
HARD BAKE
P-WellUSGSTI
PolysiliconPR
PURPOSE OF HARD BAKE
• Evaporating all solvents in PR• Improving etch and implantation resistance• Improve PR adhesion with surface• Polymerize and stabilize photoresist• PR flow to fill pinhole
PR PINHOLE FILL BY THERMAL FLOW
PR
Substrate Substrate
PR
Pinhole
HARD BAKE CONDITIONS• Hot plate is commonly used• Can be performed in a oven after inspection• Hard bake temperature: 100 to 130 C • Baking time is about 1 to 2 minutes• Hard bake temperature normally is higher than the soft bake
temperature for the same kind of photoresist
EFFECTS OF IMPROPER HARD BAKE• Under-bake
– Photoresist is not filly polymerized– High photoresist etch rate – Poor adhesion
• Over-baking – PR flow and bad resolution
Photoresist Flow• Over baking can causes too much PR flow, which affects
photolithography resolution.
PR Substrate SubstrateNormal Baking
Over Baking
PR
PATTERN INSPECTION
P-WellUSGSTI
PolysiliconPR
PATTERN INSPECTION Surface irregularities such as scratches, pin holes,
stains, contamination, etc. Critical dimension (CD) Overlay or alignment
run-out, run-in, reticle rotation, wafer rotation, misplacement in X-direction, and misplacement in Y-direction
PATTERN INSPECTION Fail inspection, stripped PR and rework
Photoresist pattern is temporary Etch or ion implantation pattern is permanent.
Photolithography process can rework Can’t rework after etch or implantation. Scanning electron microscope (SEM) Optical microscope
CRITICAL DIMENSION
Good CD CD Loss Sloped Edge
PR PRSubstrate
PRSubstrate Substrate
MASK ALIGNMENT & EXPOSURETransfers the mask image to the resist-coated wafer
Activates photo-sensitive components of photoresist
Three types of masking(1) Contact printing (2) Proximity printing(3) Projection printing
84
Contact printing capable of high resolution but has unacceptable defect densities. May be used in Development but not manufacturing.
Proximity printing cannot easily print features below a few mm in line width. Used in nano-technology.
Projection printing provides high resolution and low defect densities and dominates today. They print » 50 wafers/hour.
CONTACT PRINTER Simple equipment Use before mid-70s Resolution: capable for sub-micron Direct mask-wafer contact, limited mask lifetime Particles
CONTACT PRINTERLight Source
Lenses
Mask
PhotoresistWafer
CONTACT PRINTING
N-SiliconPR
UV Light Mask
PROXIMITY PRINTER ~ 10 mm from wafer surface No direct contact Longer mask lifetime Resolution: > 3 mm
PROXIMITY PRINTERLight Source
Lenses
Mask
PhotoresistWafer
~10 mm
PROXIMITY PRINTING
N-SiliconPR
UV Light~10 mm Mask
PROJECTION PRINTER Works like an overhead projector Mask to wafer, 1:1 Resolution to about 1 mm
Projection System
Light Source
Lenses
Mask
PhotoresistWafer
Scanning Projection SystemLight Source
Lens
Mask
Photoresist
Wafer
Synchronized mask and wafer movement
Slit
Lens
PHOTOLITHOGRAPHY REQUIREMENTS High Resolution PR High PR Sensitivity PR Precision Alignment Machine Precise Process Parameters Control Low Defect Density ultra-clean room
PHOTORESIST (PR) Photo sensitive material Temporarily coated on wafer surface Transfer design image on it through exposure Very similar to the photo sensitive coating on the
film for camera
REQUIREMENT OF PHOTORESIST High resolution
Thinner PR film has higher the resolution Thinner PR film, the lower the etching and ion
implantation resistance High etch resistance Good adhesion Wider process latitude
Higher tolerance to process condition change
REQUIREMENT OF PHOTORESIST-2 Ion implantation blocking Expose rate, Sensitivity and Exposure Source Pinholes Particle and Contamination Levels Step Coverage Thermal Flow
Types of Photoresist
Negative Photoresist• Becomes insoluble after
exposure• When developed, the
unexposed parts dissolved.
• Cheaper
Positive Photoresist• Becomes soluble after
exposure• When developed, the
exposed parts dissolved• Better resolution
Negative and Positive Photoresists
Mask/reticle
Exposure
After Development
Negative Photoresist
UV light
Positive Photoresist
Substrate
Substrate
Substrate
Photoresist
SubstratePhotoresist
NEGATIVE PHOTORESIST
Mask
Expose
Development
Negative Photoresist
NEGATIVE RESIST Most negative PR are polyisoprene type Exposed PR becomes cross-linked polymer Cross-linked polymer has higher chemical etch
resistance. Unexposed part will be dissolved in development
solution.
POSITIVE PHOTORESIST Novolac resin polymer. Acetate type solvents. Sensitizer cross-linked within the resin. Energy from the light dissociates the sensitizer and
breaks down the cross-links. Exposed part dissolve in developer solution. Image the same that on the mask. Higher resolution. Commonly used in IC fabrication.
Positive Photoresists
Mask/reticle
Exposure
After Development
UV light
Positive Photoresist
Substrate
Substrate
Substrate
Photoresist
SubstratePhotoresist
DISADVANTAGES OF NEGATIVE PHOTORESIST
• Polymer absorbs the development solvent• Poor resolution due to PR swelling • Environmental and safety issues due to the main
solvents xylene.
COMPARISON OF PHOTORESISTS
- PR
Film+ PR
Film
Substrate Substrate
Negative photoresist Positive photoresist
SUMMARY Photolithography: temporary patterning process Most critical process steps in IC processing Requirement: high resolution, low defect density Photoresist, positive and negative Process steps: Pre-bake and Primer coating, PR spin coating, soft
bake, exposure, PEB, development, hard bake, and inspection NGL: EUV and e-beam lithography
After a thin film is deposited, it is usually etched to remove unwanted materials and leave only the desired pattern on the wafer
The process is done many times In addition to deposited films, sometimes we also
need to etch the Si wafer to create trenches (especially in MEMS)
The masking layer may be photoresist, SiO2 or Si3N4 The etch is usually done until another layer of a
different material is reached
ETCHING
INTRODUCTION
Etching can be done “wet” or “dry” Wet etching
uses liquid etchants Wafer is immersed in the liquid Process is mostly chemical
Wet etching is not used much in VLSI wafer fabrication.
INTRODUCTION
Dry etching◦ Uses gas phase etchants in a plasma.◦ The process is a combination of chemical and
physical action.◦ Process is often called “plasma etching”.
This is the normal process used in most VLSI fabrication.
The ideal etch produces vertical sidewalls as shown in Fig.10-1.
In reality, the etch occurs both vertically and laterally (Figure 10-2).
INTRODUCTION
INTRODUCTION
Note that There is undercutting, non-vertical sidewalls, and
some etching of the Si. The photoresist may have rounded tops and non-
vertical sidewalls. The etch rate of the photoresist is not zero and the
mask is etched to some extent. This leads to more undercutting.
INTRODUCTION
Etch selectivity is the ratio of the etch rates of different materials in the process.
If the etch rate of the mask and of the underlying substrate is near zero, and the etch rate of the film is high, we get high selectivity.
This is the normally desired situation If the etch rate of the mask or the substrate is high,
the selectivity is poor Selectivity of 25 – 50 are reasonable. Materials usually have differing etch rates due to
chemical processes rather than physical processes.
INTRODUCTION
Etch directionality is a measure of the etch rate in different directions (usually vertical versus lateral)
INTRODUCTION
In isotropic etching, the etch rates are the same in all directions.
Perfectly anisotropic etching occurs in only one direction.
Etch directionality is often related to physical processes, such as ion bombardment and sputtering.
In general, the more physical a process is, the more anisotropic the etch is and the less selective it is.
Directionality is often desired in order to maintain the lithographically defined features.
INTRODUCTION
Note, however, that very anisotropic structures can lead to step coverage problems in subsequent steps
Selectivity is very desirable The etch rate of the material to be removed should
be fast compared to that of the mask and of the substrate layer
It is hard to get good directionality and good selectivity at the same time
INTRODUCTION
Other system requirements include: Ease of transporting gases/liquids to the wafer
surface Ease of transporting reaction products away from
wafer surface Process must be reproducible, uniform, safe, clean,
cost effective
INTRODUCTION
We consider two processes “wet” etching “dry” etching
Wet process is well-established, simple, and inexpensive
The need for smaller feature sizes could only be met with plasma etching
Plasma etching is used almost exclusively today
TYPES OF ETCHING
In wet process by immersing the wafer in these chemicals, exposed areas could be etched and washed away
For SiO2, HF was used. Wet etches work through chemical processes to
produce a water soluble byproduct
WET ETCHING
O2HSiFH6HFSiO 2622
In some cases, the etch works by first oxidizing the surface and then dissolving the oxide
An etch for Si involves a mixture of nitric acid and HF The nitric acid (HNO3) decomposes to form nitrogen
dioxide (NO2)
The SiO2 is removed by the previous reaction The overall reaction is
WET ETCHING
22222 2HNOHSiOO2H2NOSi
222623 HOHHNOSiFH6HFHNOSi
Buffers are often added to keep the etchants at maximum strength over use and time
Ammonium fluoride (NH4F) is often used with HF to help prevent depletion of the F ions
This is called Basic Oxide Etch (BOE) or Buffered HF (BHF)
The ammonium fluoride reduces the etch rate of photoresist and helps eliminate the lifting of the resist during oxide etching
Acetic acid (CH3COOH) is often added to the nitric acid/HF Si etch to limit the dissociation of the nitric acid
CONTD..
Wet etches can be very selective because they depend on chemistry
The selectivity is given by
Material “1” is the film being etched and material”2” is either the mask or the material below the film being etched
If S>>1, we say the etch has good selectivity for material 1 over material 2
CONTD..
2
1
rrS
Most wet etches etch isotropically The exception is an etch that depends on the
crystallographic orientation Example—some etches etch <111> Si slower than
<100> Si Etch bias is the amount of undercutting of the mask If we assume that the selectivity for the oxide over
both the mask and the substrate is infinite, we can define the etch depth as “d” and the bias as “b”
CONTD..
CONTD..
We often deliberately build in some overetching into the process
This is to account for the fact that the films are not perfectly uniform the etch is not perfectly uniform
The over etch time is usually calculated from the known uncertainties in film thickness and etch rates
It is important to be sure that no area is under-etched; we can tolerate some over-etching
CONTD..
This means that it is important to have as high a selectivity as possible to eliminate etching of the substrate
However, if the selectivity is too high, over-etching may produce unwanted undercutting
If the etch rate of the mask is not zero, what happens? If m is the amount of mask removed, we get
unexpected lateral etching
CONTD..
CONTD..
m is called “mask erosion” Etching is usually neither perfectly anisotropic nor
perfectly isotropic We can define the degree of anisotropy by
Isotropic etching has an Af = 0 while anisotropic etching has Af = 1
CONTD..
vert
latf r
rA -1
Plasma etching has (for the most part) replaced wet etching
There are two reasons: Very reactive ion species are created in the plasma
that give rise to very active etching Plasma etching can be very anisotropic (because
the electric field directs the ions)
PLASMA ETCHING(DRY ETCHING)
Plasma systems can be designed so that either reactive chemical components dominate or ionic components dominate
Often, systems that mix the two are used The etch rate of the mixed system may be much
faster than the sum of the individual etch rates A basic plasma system is shown in the next slide
CONTD..
PLASMA ETCHING
Features of this system Low gas pressure (1mtorr – 1 torr) High electric field ionizes some of the gas
(produces positive ions and free electrons) Energy is supplied by 13.56 MHz RF generator A bias develops between the plasma and the
electrodes because the electrons are much more mobile than the ions (the plasma is biased positive with respect to the electrodes)
CONTD..
CONTD..
If the area of the electrodes is the same (symmetric system) we get the solid curve of 10-8
The sheaths are the regions near each electrode where the voltage drops occur (the dark regions of the plasma)
The sheaths form to slow down the electron loss so that it equals the ion loss per RF cycle
In this case, the average RF current is zero
CONTD..
The heavy ions respond to the average voltage The light electrons respond to the instantaneous
voltage The electrons cross the sheath only during a short
period in the cycle when the sheath thickness is minimum
During most of the cycle, most of the electrons are turned back at the sheath edge
The sheaths are thus deficient in electrons They are thus dark because of a lack of light-emitting
electron-ion collisions
PLASMA ETCHING
For etching photoresist, we use O2 For other materials we use species containing halides
such as Cl2, CF4, and HBr Sometimes H2, O2, and Ar may be added The high-energy electrons cause a variety of reactions The plasma contains
free electrons ionized molecules neutral molecules ionized fragments Free radicals
PLASMA ETCHING
PLASMA ETCHING
In CF4 plasmas, there are Free electrons CF4 CF3 CF3
+
F CF and F are free radicals and are very reactive Typically, there will be 1015 /cc neutral species and
108-1012 /cc ions and electrons
PLASMA ETCHING
The main species involved in etching are Reactive neutral chemical species Ions
The reactive neutral species (free radicals in many cases) are primarily responsible for the chemical component
The ions are responsible for the physical component The two can work independently or synergistically
PLASMA ETCHING MECHANISMS
When the reactive neutral species act alone, we have chemical etching
Ions acting by themselves give physical etching When they work together, we have ion-enhanced
etching
PLASMA ETCHING MECHANISMS
Chemical etching is done by free radicals Free radicals are neutral molecules that have
incomplete bonding (unpaired electrons) For example
Both F and CF3 are free radicals Both are highly reactive F wants 8 electrons rather than 7 and reacts quickly to
find a shared electron
CHEMICAL ETCHING
--- eFCFCFe 34
The idea is to get the free radical to react with the material to be etched (Si, SiO2).
The byproduct should be gaseous so that it can be transported away (next slide).
The reaction below is such a reaction
Thus, we can etch Si with CF4 There are often several more complex intermediate
states.
CHEMICAL ETCHING
4SiFSi4F
CHEMICAL ETCHING
Gas additives can be used to increase the production of the reactive species (O2 in CF4)
The chemical component of plasma etching occurs isotropically
This is because The arrival angles of the species is isotropic There is a low sticking coefficient (which is more
important) The arrival angle follows what we did in deposition
and there is a cosn dependence where n=1 is isotropic
CHEMICAL ETCHING
The sticking coefficient is
A high sticking coefficient means that the reaction takes place the first time the ion strikes the surface.
For lower sticking coefficients, the ion can leave the surface (usually at random angles) and strikes the surface somewhere else.
CHEMICAL ETCHING
incident
reactedc F
FS
One would guess that the sticking coefficient for reactive ions is high
However, there are often complex reactions chained together. This complexity often means low sticking coefficients
Sc for O2/CF4 on Si is about 0.01 This additional “bouncing around” of the ions leads to
isotropic etching Since free radicals etch by chemically reacting with
the material to be etched, the process can be highly selective
CHEMICAL ETCHING
CHEMICAL ETCHING
Due to the voltage drop between the plasma and the electrodes and the resulting electric field across the sheaths, positive ions are accelerated towards each electrode
The wafers are on one electrode Therefore, ionic species (Cl+ or Ar+) will be accelerated
towards the wafer surface These ions striking the surface result in the physical
process The process is much more directional because the
ions follow the field lines
PHYSICAL ETCHING
PHYSICAL ETCHING
This means n is very large in the cosn distribution But, because the process is more physical than
chemical, the selectivity will not be as good as in the more chemical processes
We also assume that the ion only strikes the surface once (which implies that the sticking coefficient is near 1)
Ions can also etch by physical sputtering (Chapter 9)
PHYSICAL ETCHING
The ions and the reactive neutral species do not always act independently (the observed etch rate is not the sum of the two independent etch rates)
The classic example is etching of Si with XeF2 and Ar+ ions are introduced.
ION-ENHANCED ETCHING
ION-ENHANCED ETCHING
The shape of the etch profiles are interesting The profiles are not the linear sum of the profiles from
the two processes The profile is much more like the physical etch alone
(c)
ION-ENHANCED ETCHING
If the chemical component is increased, the vertical etching is increased, but not the lateral etching
The etch rate is also increased The mechanisms for these effects are poorly
understood Whatever the mechanism, the enhancement only
occurs where the ions hit the surface Since the ions strike normal to the surface, the
enhancement is in this direction This increases the directionality
ION-ENHANCED ETCHING
ION-ENHANCED ETCHING
Possible models include Enhancement of the etch reaction Inhibitor removal
The reaction takes place only where the ions strike the surface
Since the ions strike normal to the surface, removal is thus only at the bottom of the well
It is assumed that etching by radicals (chemical etching) is negligible
Note that even under these assumptions, the side walls may not be perfectly vertical
ION-ENHANCED ETCHING
Note that an inhibitor can be removed on the bottom, but not on the sidewalls
If inhibitors are deliberately deposited, we can make very anisotropic etches
If the inhibitor formation rate is large compared to the etch rate, one can get non-vertical walls (next slide)
ION-ENHANCED ETCHING
ION-ENHANCED ETCHING
ANISOTROPY Etchant can not distinguishes b/w vertical or horizontal
dimensions (isotropic). Anisotropy = 1 – dH/dV Wet etching is isotropic and dry etching is anisotropic.
SELECTIVITY Etchant should distinguish b/w SiO2 and Si wafer. Wet Etching is Selective than Dry Etching.
ADVANTAGES OF PLASMA ETCHING OVER WET ETCHING Eliminates handling of dangerous acids and solvents. Uses small amounts of chemicals. Anisotropic etch profiles. High resolution and cleanliness. Less undercutting. Better process control.
THANK YOU