Lithography Lithography

• Lithography in the MEMS context is typically Lithography in the MEMS context is typically the transfer of a pattern to a photosensitive the transfer of a pattern to a photosensitive material by selective exposure to a material by selective exposure to a radiation source such as light. radiation source such as light.

• A photosensitive material is a material that A photosensitive material is a material that experiences a change in its physical experiences a change in its physical properties when exposed to a radiation properties when exposed to a radiation source. If we selectively expose a source. If we selectively expose a photosensitive material to radiation (e.g. by photosensitive material to radiation (e.g. by masking some of the radiation) the pattern masking some of the radiation) the pattern of the radiation on the material is of the radiation on the material is transferred to the material exposed, as the transferred to the material exposed, as the properties of the exposed and unexposed properties of the exposed and unexposed regions differs regions differs

STEPS IN LITHOGRAPHYSTEPS IN LITHOGRAPHY• COATING THE SUBSTRATE WITH COATING THE SUBSTRATE WITH

PHOTO SENSITIVE MATERIAL (PHOTO PHOTO SENSITIVE MATERIAL (PHOTO RESIST)RESIST)

• FIXING THE MASK WITH THE FEATURES FIXING THE MASK WITH THE FEATURES ON THE COATON THE COAT

• EXPOSURE TO RADIATIONEXPOSURE TO RADIATION• SPRAY OF DEVELOPER TO OBTAIN SPRAY OF DEVELOPER TO OBTAIN

EITHER ‘POSITIVE’ OR ‘NEGATIVEEITHER ‘POSITIVE’ OR ‘NEGATIVE• ETCH OR DEPOSITETCH OR DEPOSIT• STRIP THE PHOTO RESISTSTRIP THE PHOTO RESIST

MASK, EXPOSURE & DEVELOPMASK, EXPOSURE & DEVELOP

ETCH(SUBTRACT) or ETCH(SUBTRACT) or DEPOSIT(ADD)DEPOSIT(ADD)

CARE TO BE TAKENCARE TO BE TAKEN

• AlignmentAlignment

• ExposureExposure

AlignmentAlignment• In order to make useful devices the patterns for different In order to make useful devices the patterns for different

lithography steps that belong to a single structure must be lithography steps that belong to a single structure must be aligned to one another. aligned to one another.

• The first pattern transferred to a wafer usually includes a The first pattern transferred to a wafer usually includes a set of alignment marks, which are high precision features set of alignment marks, which are high precision features that are used as the reference when positioning subsequent that are used as the reference when positioning subsequent patterns, to the first pattern patterns, to the first pattern

• Often alignment marks are included in other patterns, as Often alignment marks are included in other patterns, as the original alignment marks may be obliterated as the original alignment marks may be obliterated as processing progresses. processing progresses.

• It is important for each alignment mark on the wafer to be It is important for each alignment mark on the wafer to be labeled so it may be identified, and for each pattern to labeled so it may be identified, and for each pattern to specify the alignment mark (and the location thereof) to specify the alignment mark (and the location thereof) to which it should be aligned. which it should be aligned.

• By providing the location of the alignment mark it is easy By providing the location of the alignment mark it is easy for the operator to locate the correct feature in a short for the operator to locate the correct feature in a short time. Each pattern layer should have an alignment feature time. Each pattern layer should have an alignment feature so that it may be registered to the rest of the layersso that it may be registered to the rest of the layers

Exposure Exposure • The exposure parameters required in order to The exposure parameters required in order to

achieve accurate pattern transfer from the achieve accurate pattern transfer from the mask to the photosensitive layer depend mask to the photosensitive layer depend primarily on the wavelength of the radiation primarily on the wavelength of the radiation source and the dose required to achieve the source and the dose required to achieve the desired properties change of the photoresist. desired properties change of the photoresist.

• Different photoresists exhibit different Different photoresists exhibit different sensitivities to different wavelengths. sensitivities to different wavelengths.

• The dose required per unit volume of The dose required per unit volume of photoresist for good pattern transfer is photoresist for good pattern transfer is somewhat constant somewhat constant

EFEECTS OF OVER EFEECTS OF OVER EXPOSUREEXPOSURE• if an image is overexposed, the dose received by if an image is overexposed, the dose received by

photoresist at the edge that shouldn't be exposed photoresist at the edge that shouldn't be exposed may become significant. may become significant.

• If we are using positive photoresist, this will result in If we are using positive photoresist, this will result in the photoresist image being eroded along the edges, the photoresist image being eroded along the edges, resulting in a decrease in feature size and a loss of resulting in a decrease in feature size and a loss of sharpness or corners sharpness or corners

• If we are using a negative resist, the photoresist If we are using a negative resist, the photoresist image is dilated, causing the features to be larger image is dilated, causing the features to be larger than desired, again accompanied by a loss of than desired, again accompanied by a loss of sharpness of corners. sharpness of corners.

• If an image is severely underexposed, the pattern If an image is severely underexposed, the pattern may not be transferred at all, and in less sever cases may not be transferred at all, and in less sever cases the results will be similar to those for overexposure the results will be similar to those for overexposure with the results reversed for the different polarities with the results reversed for the different polarities of resist of resist

Industrial Process steps Industrial Process steps • Dehydration bake - dehydrate the wafer to Dehydration bake - dehydrate the wafer to

aid resist adhesion.aid resist adhesion. • prime - coating of wafer surface with prime - coating of wafer surface with

adhesion promoter. Not necessary for all adhesion promoter. Not necessary for all surfaces.surfaces.

• Resist spin/spray - coating of the wafer with Resist spin/spray - coating of the wafer with resist either by spinning or spraying. resist either by spinning or spraying. Typically desire a uniform coat.Typically desire a uniform coat.

• Soft bake - drive off some of the solvent in Soft bake - drive off some of the solvent in the resist, may result in a significant loss of the resist, may result in a significant loss of mass of resist (and thickness). Makes resist mass of resist (and thickness). Makes resist more viscous.more viscous.

• Alignment - align pattern on mask to Alignment - align pattern on mask to features on wafers.features on wafers.

• Exposure - projection of mask image on resist Exposure - projection of mask image on resist to cause selective chemical property change.to cause selective chemical property change.

• Post exposure bake - baking of resist to drive Post exposure bake - baking of resist to drive off further solvent content. Makes resist more off further solvent content. Makes resist more resistant to etchants (other than developer).resistant to etchants (other than developer).

• Develop - selective removal of resist after Develop - selective removal of resist after exposure (exposed resist if resist is positive, exposure (exposed resist if resist is positive, unexposed resist if resist is positive). Usually unexposed resist if resist is positive). Usually a wet process (although dry processes exist).a wet process (although dry processes exist).

• Hard bake - drive off most of the remaining Hard bake - drive off most of the remaining solvent from the resist.solvent from the resist.

• Descum - removal of thin layer of resist scum Descum - removal of thin layer of resist scum that may occlude open regions in pattern, that may occlude open regions in pattern, helps to open up corners.helps to open up corners.

ResolutionResolution• Resolution or the critical dimension is the Resolution or the critical dimension is the

minimum feature size that could be printedminimum feature size that could be printed• The ability to project a clear image of a The ability to project a clear image of a

small feature onto the wafer is limited by small feature onto the wafer is limited by the wavelength of the light that is used. the wavelength of the light that is used.

• The minimum feature size that a projection The minimum feature size that a projection system can print is given approximately bysystem can print is given approximately by

• λλ = wave length, = wave length, NA = numerical aperture, K = constant (0.4)NA = numerical aperture, K = constant (0.4)

NUMERICAL APERTURENUMERICAL APERTURE• In most areas of optics, and In most areas of optics, and

especially in microscopy, especially in microscopy, the numerical aperture of an the numerical aperture of an optical system such as an optical system such as an objective lens is defined byobjective lens is defined by

• NA = n SinNA = n Sinθθ• where where nn is the index of is the index of

refraction of the medium in refraction of the medium in which the lens is working which the lens is working (1.0 for air, 1.33 for pure (1.0 for air, 1.33 for pure water, and up to 1.56 for water, and up to 1.56 for oils), and oils), and θθ is the half-angle is the half-angle of the maximum cone of of the maximum cone of light that can enter or exit light that can enter or exit the lensthe lens

• Photolithography has used ultraviolet Photolithography has used ultraviolet light from gas-discharge lamps using light from gas-discharge lamps using mercury, sometimes in combination mercury, sometimes in combination with noble gases such as xenon. with noble gases such as xenon. These lamps produce light across a These lamps produce light across a broad spectrum with several strong broad spectrum with several strong peaks in the ultraviolet range. This peaks in the ultraviolet range. This spectrum is filtered to select a single spectrum is filtered to select a single spectral line, usually the "g-line" spectral line, usually the "g-line" (436 nm) or "i-line" (365 nm). (436 nm) or "i-line" (365 nm).

• CD is 200 to 150nmCD is 200 to 150nm

• Current state-of-the-art Current state-of-the-art photolithography tools use deep photolithography tools use deep ultraviolet (DUV) light with ultraviolet (DUV) light with wavelengths of 248 and 193 nmwavelengths of 248 and 193 nm

• which allow minimum feature sizes which allow minimum feature sizes down to down to 100 nm100 nm

Immersion lithographyImmersion lithography• Immersion lithographyImmersion lithography

is a photolithography is a photolithography resolution enhancement resolution enhancement technique that replaces technique that replaces the usual air gap between the usual air gap between the final lens and the the final lens and the wafer surface with a wafer surface with a liquid medium that has a liquid medium that has a refractive index greater refractive index greater than one. The resolution than one. The resolution is increased by a factor is increased by a factor equal to the refractive equal to the refractive index of the liquid. (CD = index of the liquid. (CD = 60nm)60nm)

Other issues in photo Other issues in photo lithographylithography

• Low depth of field and depth of focusLow depth of field and depth of focus

• Depth of field is a measurement of Depth of field is a measurement of depth of acceptable sharpness in the depth of acceptable sharpness in the object space, or subject space.object space, or subject space.

• Depth of focus is a measurement of Depth of focus is a measurement of how much the film / substrate can be how much the film / substrate can be displaced while an object remains in displaced while an object remains in acceptably sharp focusacceptably sharp focus

                      

Depth of field diagram Depth of field diagram

Depth of field and depth of Depth of field and depth of focusfocus

Depth of focusDepth of focus

Depth of FocusDepth of Focus

• t is the depth of focus,•N is the f-number of the optical system•C is the circle of confusion•v is the distance of the object from lens•f is the focal length

maskless lithographymaskless lithography• In In maskless lithographymaskless lithography, the , the

radiation that is used to expose a radiation that is used to expose a photosensitive emulsion (or photosensitive emulsion (or photoresist) is not projected from, or photoresist) is not projected from, or transmitted through, a photomask. transmitted through, a photomask. Instead, most commonly, the radiation Instead, most commonly, the radiation is focused to a narrow beam. The beam is focused to a narrow beam. The beam is then used to directly write the image is then used to directly write the image into the photoresist, one or more pixels into the photoresist, one or more pixels at a time at a time

FORMS OF MASKLESS FORMS OF MASKLESS LITHOGRAPHYLITHOGRAPHY

• Laser (Optical)Laser (Optical)

• Focused ion beamFocused ion beam

• Electron beam Electron beam

Multiphoton lithographyMultiphoton lithography• Multiphoton lithographyMultiphoton lithography (also known as (also known as

direct laser writingdirect laser writing) is a technique for ) is a technique for creating small features in a photosensitive creating small features in a photosensitive material, without the use of complex material, without the use of complex optical systems or photomasks.optical systems or photomasks.

• By scanning and properly modulating the By scanning and properly modulating the laser, a chemical change (usually laser, a chemical change (usually polymerization) occurs at the focal spot of polymerization) occurs at the focal spot of the laser and can be controlled to create the laser and can be controlled to create an arbitrary two or three-dimensional an arbitrary two or three-dimensional periodic or non-periodic pattern.periodic or non-periodic pattern.

• This method could also be used for rapid This method could also be used for rapid prototyping of structures with fine features prototyping of structures with fine features

Multiphoton lithographyMultiphoton lithography

• In laser physics the numerical aperture is In laser physics the numerical aperture is defined slightly differently defined slightly differently

• The NA of a Gaussian laser beam is related The NA of a Gaussian laser beam is related to its minimum spot size by to its minimum spot size by

• D = beam diaD = beam dia

Focused ion beamFocused ion beam• Focused ion beam (FIB) systems operate Focused ion beam (FIB) systems operate

in a similar fashion to a scanning in a similar fashion to a scanning electron microscope (SEM) except, electron microscope (SEM) except, rather than a beam of electrons and as rather than a beam of electrons and as the name implies, FIB systems use a the name implies, FIB systems use a finely focused beam of ions (usually finely focused beam of ions (usually gallium) that can be operated at low gallium) that can be operated at low beam currents for imaging or high beam beam currents for imaging or high beam currents for site specific sputtering or currents for site specific sputtering or milling milling

Why Ions ? Why Ions ? • ions are larger than electronsions are larger than electrons

• they cannot easily penetrate within they cannot easily penetrate within individual atoms of the sample. Interaction individual atoms of the sample. Interaction mainly involves outer shell interaction mainly involves outer shell interaction resulting in atomic ionization and breaking resulting in atomic ionization and breaking of chemical bonds of the substrate atoms. of chemical bonds of the substrate atoms.

• The penetration depth of the ions is much The penetration depth of the ions is much lower than the penetration of electrons of lower than the penetration of electrons of the same energy. the same energy.

Why Ions ?Why Ions ?• ions are heavier than electronsions are heavier than electrons• ions can gain a high momentum. For the ions can gain a high momentum. For the

same energy, the momentum of the ion is same energy, the momentum of the ion is about 370 times larger. about 370 times larger.

• For the same energy ions move a lot slower For the same energy ions move a lot slower than electrons. However, they are still fast than electrons. However, they are still fast compared to the image collection mode and compared to the image collection mode and in practice this has no real consequences. in practice this has no real consequences.

• The magnetic lenses are less effective on The magnetic lenses are less effective on ions than they would be on electrons with ions than they would be on electrons with the same energy. As a consequence the the same energy. As a consequence the focused ion beam system is equipped with focused ion beam system is equipped with electro-static lenses and not with magnetic electro-static lenses and not with magnetic lenseslenses

http://upload.wikimedia.org/wikipedia/commons/0/0a/Sch%C3%A9ma_FIB.jpg

USE of FIBUSE of FIB

• Unlike an electron microscope, FIB is Unlike an electron microscope, FIB is inherently destructive to the specimen. inherently destructive to the specimen. When the high-energy gallium ions When the high-energy gallium ions strike the sample, they will sputter strike the sample, they will sputter atoms from the surface. atoms from the surface.

• Gallium atoms will also be implanted Gallium atoms will also be implanted into the top few nanometers of the into the top few nanometers of the surface surface

• FIB assisted depositionFIB assisted deposition

• the surface will be made amorphousthe surface will be made amorphous

FIBFIB

• Because of the sputtering capability, Because of the sputtering capability, the FIB is used as a micro-machining the FIB is used as a micro-machining tool, to modify or machine materials tool, to modify or machine materials at the micro- and nanoscale.at the micro- and nanoscale.

• nano machining with FIB is a field nano machining with FIB is a field that still needs developing. that still needs developing.

• The common smallest beam size is The common smallest beam size is 2.5-6 nm 2.5-6 nm

ion beam induced ion beam induced depositiondeposition

• FIB-assisted chemical vapor deposition occurs FIB-assisted chemical vapor deposition occurs when a gas, such as tungsten hexacarbonyl when a gas, such as tungsten hexacarbonyl (W(CO)6) is introduced to the vacuum chamber (W(CO)6) is introduced to the vacuum chamber and allowed to chemisorb onto the sample.and allowed to chemisorb onto the sample.

• By scanning an area with the beam, the By scanning an area with the beam, the precursor gas will be decomposed into volatile precursor gas will be decomposed into volatile and non-volatile components; the non-volatile and non-volatile components; the non-volatile component, such as tungsten, remains on the component, such as tungsten, remains on the surface as a deposition.surface as a deposition.

• From nanometers to hundred of micrometers in From nanometers to hundred of micrometers in length, tungsten metal deposition allows to put length, tungsten metal deposition allows to put metal lines right where needed. metal lines right where needed.

• Other materials such as platinum, cobalt, carbon, Other materials such as platinum, cobalt, carbon, gold, etc., can also be locally deposited gold, etc., can also be locally deposited