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19.2 Optical lithography

  Optical lithography is currently the dominant exposure process. In addition to the existence of mask aligners for research and development, where mask blanks are brought into proximity or direct contact with a resist-coated substrate and the structures of the mask are exposed directly onto the wafer ,steppers are use in high-volume production. in a stepper ,projection optics reduce the aerial image of a single chip onto the substrate ,which is scanned stepwise to define multiple chips on the wafer .the reduction factor of the projection lens is usually four to five ,which relaxes the specifications for the pattern mask, called the reticle .the high-precision stepping of the stepper is accomplished with a laser interferometer table ,which enables a placement accuracy of better than 50nm.This is necessary since the demands on overlay accuracy are currently smaller than 35nm and will decrease to less than 10nm by 2016.

  The key equations for the optical system of a stepper are given by R=K1.λ/NA(19.1)。DOF=K2.λ/NA2(19.2) ,where R is the feature size, DOF is the depth of focus ,λis the wavelength of the light used. NA is the numerical aperture of the lens system(aperture angle),and k1 and k2 are process parameters. Equation(19.1) follows from the Rayleigh criterion. For an aerial image k1=0.5,but the exact value is determined by the process conditions such as resist behavior and illumination mode . Equation(19.2) gives the depth of focus ,which can also be derived from the Rayleigh criterion, whereby the prefactor K2 is process dependent, similarly to k1.

 To achieve smaller line widths the direct approach is to reduce the wavelength λused in fig19.1it can be seen that wavelengths of 248nm (KrF excimer laser),197nm(ArF)and 157nm(F2) are used for optical lithography .A change of the wavelength means a new machine generation ,whose cost increases by a factor of 10per generation ,mainly as a result of the new optical system for the specific wavelength .Since an improvement of the wavelength by 20%is insufficient on its own,an increase in NA and a decrease in k1 are also aimed for .When NA is increased a trade-off has to be found because DOF depends quardratically on NA.At present NA typicaly ranges from 0.6 to 0.75.Finally ,k1 can also be decreased by process improvements, e.g. resist optimization, reticle engineering or off-axis illumination.

  Off-axis illumination is achieved by the introduction of annular or a quadrupole aperture into the illumination system .permitting higher diffraction orders to enter the objective,resulting in an increase of the resolution. theoretically K1can be reduced to a minimum value of 0.25 with off-axis illumination .but it is normally considerably larger.

  the reticle itself is a quartz blank with an electron-beam-patterned chromium absorber layer .Usually the chromium absorbs the light ,yielding an intensity gradient at the edge of the structure .Reticle engineering is applied to create steeper intensity edges in the structures in the aerial image by modifying the chromium-on-glass mask. One may differentiate between attenuated(ATN),alternating (ALT) and chromiumless(CRL) phase masks .IN all cases , an additional phase shift of the wave at the boundaries of structures is accomplished ,either by replacing the chromium by a 180 度 phase-shifting material (ATN) or by thinning the glass substrate in places where the chromium has been removed in order to obtain a phase shift (ALT).The option of creating the phase shift sollely by etching into the glass blank to generate a locally steep intensity profile leads to a CRL mask.

  Finally , the optical proximity effect manifests itself by a difference in the critical dimensions(CD ;the present target value is 3δ<5nm)depending on surrounding structures , end-of-line shortening depending on the feature size , and a corner-rounding effect. All these effects can be responded to with different optical-proximity-effect corrections(OPC),e.g. feature biasing (thinning of isolated lines),the addition of subresolution features (scatter bar mode), line length biasing (compensation for line shortening ,i.e line mode),and the additin of hammerheads .serifs or segments to corners of structures(see) Fig.19.2 Optical-proximity-correction methods used in state-of-the art optical reticles.In the upper picture the structure on the reticle can be seen (pale gray, uncorrected; dark gray, corrected);underneath ,the resulting structure after exposure is shown .The electron microscope pictures show a  comparison of an uncorrected mask with a segment-mode-corrected mask[19]

 Fig.19.2)Although simulation models are used to correct the mask data prior to the first exposure , iteration steps to obtain the correct mask exposure are normally necessary.

  All these methods have been introduced to decrease k1 further by about a factor of 2.In Fig.19.1,the implementation of these methods can be seen from the divergence of the curve with square points and the curve with circular points. in practice ,the value of k1 can currently reach values as small as 0.3.on the other hand ,these mask-correcting measures have significantly aggravated the problem of reticle fabrication :controlled chomium and glass etching are necessary ,and smaller line widths are required for the optical proximity  corrections. this has raised the cost of the masks, and often the time required for the generation a mask set is of the order of weeks.

(400-403) silicon evolution and future of a technology .