The phase-shifting mask consists of a normal transmission mask that has been coated with a transparent layer patterned to ensure that the optical phases of nearest apertures are opposite. Destructive interference between waves from adjacent apertures cancels some diffraction effects and increases the spatial resolution with which such patterns can be projected. A simple theory predicts a near doubling of resolution for illumination with partial incoherence σ < 0.3, and substantial improvements in resolution for σ < 0.7. Initial results obtained with a phase-shifting mask patterned with typical device structures by electron-beam lithography and exposed using a Mann 4800 10× tool reveals a 40-percent increase in usuable resolution with some structures printed at a resolution of 1000 lines/mm. Phase-shifting mask structures can be used to facilitate proximity printing with larger gaps between mask and wafer. Theory indicates that the increase in resolution is accompanied by a minimal decrease in depth of focus. Thus the phase-shifting mask may be the most desirable device for enhancing optical lithography resolution in the VLSI/VHSIC era.

Improving resolution in photolithography with a phase-shifting mask

Before the diffraction from a diffracting structure on a semiconductor wafer is measured, where necessary, the film thickness and index of refraction of the films underneath the structure are first measured using spectroscopic reflectometry or spectroscopic ellipsometry. A rigorous model is then used to calculate intensity or ellipsometric signatures of the diffracting structure. The diffracting structure is then measured using a spectroscopic scatterometer using polarized and broadband radiation to obtain an intensity or ellipsometric signature of the diffracting structure. Such signature is then matched with the signatures in the database to determine the grating shape parameters of the structure.

Spectroscopic scatterometer system

An overlay target for spectroscopic measurement includes at least two diffraction gratings, one grating overlying the other. The diffraction gratings may include an asymmetry relative to each other in order to improve resolution of the presence as well as the direction of any mis-registration. For example, the asymmetry between the two diffraction gratings may be a phase offset, a difference in pitch, line width, etc. The overlay target may be spectroscopically measuring, for example, using an optical model and a best fit analysis. Moreover, the overlay target may be optimized by modeling the overlay target and adjusting the variable parameters and calculating the sensitivity of the overlay target to changes in variable parameters.

Spectroscopically measured overlay target

A system for monitoring a latent image exposed in a photo resist (300, 310, 675, 790) during semiconductor manufacture is provided. The system includes one or more light sources (110, 635, 740), each light source (110, 635, 740) directing light (760, 1602, 1712, 1840, 1940, 2040) to the latent image and/or one or more gratings exposed on one or more portions of a wafer (160, 400, 540, 665, 910, 1204, 1820, 1920, 2020). Light reflected (770, 1604, 1714, 1842, 1942, 2042) from the latent image and/or the gratings is collected by a signature system (630, 720), which processes the collected light. Light passing through the latent image and/or gratings may similarly be collected by the signature system (630, 720), which processes the collected light. The collected light is analyzed and can be employed to generate feedback information to control the exposure. The collect light is further analyzed and can be employed to generate feed forward information that can be employed to control post exposure processes including development and baking processes.

Critical dimension monitoring from latent image

This past decade has seen the development of a non-imaging optical dimensional metrology technique known as scatterometry. Scatterometry is a nondestructive optical technique that records and analyzes changes in the intensity of light reflected from a periodic scattering surface. By measuring and analyzing the light diffracted from a patterned periodic sample, the dimensions of the sample itself can be measured. Scatterometry exploits the sensitivity of diffraction from a sample to changes in the line-shape of the sample. The use of an elementary sample illumination system, combined with a rigorous scattered signal analysis procedure presents a significant advantage over conventional metrology techniques. These are discussed in detail in this paper.

Scatterometry applied to microelectronics processing

We discuss the use of light scattered from a latent image to control photoresist exposure dose and focus conditions which results in improved control of the critical dimension (CD) of the developed photoresist. A laser at a nonexposing wavelength is used to illuminate a latent image grating. The light diffracted from the grating is directly related to the exposure dose and focus and thus to the resultant CD in the developed resist. Modeling has been done using rigorous coupled wave analysis to predict the diffraction from a latent image as a function of the substrate optical properties and the photoactive compound (PAC) concentration distribution inside the photoresist. It is possible to use the model to solve the inverse problem: given the diffraction, to predict the parameters of the latent image and hence the developed pattern. This latent image monitor can be implemented in a stepper to monitor exposure in situ, or prior to development to predict the developed CD of a wafer for early detection of bad devices. Experimentation has been conducted using various photoresists and substrates with excellent agreement between theoretical and experimental results. The technique has been used to characterize a test pattern with a focused spot as small as 36 micrometers in diameter. Using diffracted light from a simulated closed-loop control of exposure dose, CD control was improved by as much as four times for substrates with variations in underlying film thickness, compared to using fixed exposure time. The latent image monitor has also been applied to wafers with rough metal substrates and focus optimization.

Latent image exposure monitor using scatterometry

A novel laser scatterometer linewidth measurement tool has been developed for the CD metrology of
photomasks. Calculation of the linewidth is based on a rigorous theoretical model, thus eliminating the need
for any calibrations. In addition, the effect of the glass slab on which the grating is placed, is explicitly taken
into account. The experimental arrangement consists of a chrome-on-glass diffraction grating illuminated with
a converging spherical wave from a He-Ne laser. A photodiode mounted in the Fourier plane of the scatterer
measures the scattered power in each diffracted order. A rigorous theoretical model is used to provide a
lookup table giving the 0-order transmitted power as a function of the linewidth for a fixed pitch of the grating.
This table is then used to associate a linewidth with the experimentally measured value of the power in
the 0 transmitted order.
A local company manufactured various photomask gratings having a 2 micron pitch and varying
linewidths. The 0-order transmitted power for each of these gratings was measured by the scatterometer, and
a prediction of the linewidth was made based on the theoretical model. The linewidth measured by the scatterometer
system represents an average of the linewidths over the total lines illuminated by the laser. All
present CD measurement systems however, measure the linewidth of a single line. If the variation of
linewidth is assumed to be small, comparable results should be obtained from the two procedures. The
predicted linewidth values were compared to those obtained using commercial optical linewidth measurement
systems and excellent agreement was obtained.

Simple technique for linewidth measurement of gratings on photomasks

Metrology of etched quartz and chrome embedded phase shift gratingsusing scatterometrySusan M. Gaspar Wilson, S. Sohail H. Naqvi, John R. McNeilCenter for High Technology MaterialsThe University of New MexicoAlbuquerque, New Mexico 87131Herschel M. MarchmanAT&T Bell LabsMurray Hills, New Jersey 07974Blame JohsJ.A. Woollam Co., Inc.650 J. Street, Suite 39Lincoln, Nebraska 68508Roger H. FrenchDuPont Central Science and EngineeringWilmington, Delaware 19880Franklin D. KalkDuPont Photomasks, Inc.100 Texas AvenueRound Rock, Texas 78664

Metrology of etched quartz and chrome embedded phase shift gratings using scatterometry

Genome wide association studies (GWAS) have been used to search for associations between genetic variants and a phenotypic trait of interest. New technologies, such as next-generation sequencing, hold the potential to revolutionize GWAS. However, millions of polymorphisms are identified with next-generation sequencing technology. Consequently, researchers must be careful when performing such a large number of statistical tests, and corrections are typically made to account for multiple testing. Additionally, for typical GWAS, the p value cutoff is set quite low (approximately <10−8). As a result of this p value stringency, it is likely that there are many true associations that do not meet this threshold. To account for this we have incorporated a priori biological knowledge to help identify true associations that may not have reached statistical significance. We propose the application of a pipelined series of statistical and bioinformatic methods, to enable the assessment of the association of genetic polymorphisms with a disease phenotype--here, hypertension--as well as the identification of statistically significant pathways of genes that may play a role in the disease process.

/pdf/integrated-statistical-and-pathway-approach-to-next-270y3qrkq1.pdf

Integrated statistical and pathway approach to next-generation sequencing analysis: a family-based study of hypertension.

Scatterometry is presented as an optical metrology technique potentially capable of determining the critical parameters of a phase etched diffraction grating test structure (sidewall profile, etch depth, and linewidth). The technique is noncontact, rapid and nondestructive. The test grating structure is illuminated by a laser beam and the intensities in the different diffracted orders are measured as the angle of incidence of the laser beam is varied over a certain range. A phase shift mask consisting of an array of chrome and chromeless phase etched gratings was fabricated at AT&T Bell Labs using e-beam techniques. The grating linewidths varied from nominal 0.5 micrometers to 5.0 micrometers , while the etch depths varied from a nominal 190 nm to 400 nm depths. Both the chrome and the quartz gratings were measured, although only data for the quartz gratings is presented here. The measurements of the diffracted orders were made using the two theta scatterometer located at the University of New Mexico. The shape of the diffraction curves obtained in this manner has been shown to be sensitive to the grating structure parameters (sidewall profile, etch depth, linewidth, etc.). An estimate of the quarts phase etched structure parameters was obtained through a combination of rigorous coupled wave theory (CWT) and minimum mean square error (MMSE) analysis. Additionally, each grating was measured using an AFM located at AT&T. Comparison of the scatterometer and AFM measurements are presented along with their absolute differences. Finally, the long term and short term repeatabilities of the scatterometer measurements are shown to be excellent.

Susan M. Wilson

Papers

Latent image exposure monitor using scatterometry

Simple technique for linewidth measurement of gratings on photomasks

Metrology of etched quartz and chrome embedded phase shift gratings using scatterometry

Integrated statistical and pathway approach to next-generation sequencing analysis: a family-based study of hypertension.

Phase-shift mask metrology using scatterometry