Large area masks (LAMs) and 1X lithography are being used in the manufacturing of large area electronic devices. Some of these devices include, but are not limited to: flat panel displays (FPDs), active matrix liquid crystal displays (AMLCDs), electroluminescent flat panel displays (ELFPD), plasma display panels (PDPs), multichip modules (MCMs), and advanced interconnect systems. The use of LAMs along with 1X lithography reduces the cost of manufacturing these types of devices. However, there are several challenges in the manufacturing of LAMs. The first challenge is the lack of large area, quality substrates with quality coatings. The different types of substrates and the coatings presently available for large area photomasks are reviewed. The second challenge comes in dealing with the limitations of commercially available exposure systems used in writing LAMs. In this paper, we present an overview of several exposure systems, now available, and their resolution capabilities. The third challenge is the lack of availability of systems for the processing of photomasks larger than 14' by 14'. Processing systems for manual and automatic plate loading are also discussed. The final challenge to manufacture LAMs is inspection and quality certification. Presently, there is a void of commercially available metrology tools that can be used to measure and quantify defects on LAMs. Metrology techniques available for certifying finished LAMs are discussed along with their quality assurance capabilities. Current LAMs availability and specifications are also presented including LAMs with active areas as large as 20' by 18' (508 by 450 mm) and CDs of 2.0 micrometer.
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.
Laser scatterometry is a noncontact, rapid method of collecting and analyzing light scattered from a structure. We have applied optical scatter techniques to measure the surface roughness as well as the etch depth of phase shifting masks (PSMs). Experimental results and theoretical modeling are discussed.
We have applied optical scatter techniques to improve several aspects of microelectronic manufacturing. One technique involves characterizing light scattered from two dimensional device structures, such as those from VLSI circuitry etched on a wafer, using a frosted dome which is imaged by a CCD camera. Previously, limited dynamic range available from affordable digital imaging systems has prevented the study of two dimensional scatter patterns. We have demonstrated a simple technique to increase the dynamic range by combining multiple images taken at different intensities. After the images have been acquired, image processing techniques are used to find and catalog the diffraction orders. Techniques such as inverse least squares, principal component analysis, and neural networks are then used to evaluate the dependence of the light scatter on a particular wafer characteristic under examination. Characterization of surface planarization over a VLSI structure and measurement of line edge roughness of diffraction gratings are presented as examples.
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.
Identification of dimensional parameters of an arbitrarily shaped grating using scatter characteristics is presented. A rigorous diffraction model is used to predict the scatter from a known grating structure, and utilizing this information we perform the inverse problem of predicting line shape from a measurement of the scatter.
As the microelectronics industry strives to achieve smaller device design geometries, control of linewidth, or critical dimension (CD), becomes increasingly important. Currently, CD uniformity is controlled by exposing large numbers of samples for a fixed exposure time which is determined in advance by calibration techniques. This type of control does not accommodate variations in optical properties of the wafers that may occur during manufacturing. In this work, a relationship is demonstrated between the intensity of light diffracted from a latent image consisting of a periodic pattern in the undeveloped photoresist and the amount of energy absorbed by the resist material (the exposure dose). This relationship is used to simulate exposure dose control of photoresist on surfaces which have different optical properties chosen to represent surfaces typical of those found in operating process lines. Samples include a variety of photoresist materials and substrates with a wide variety of optical properties. The optical properties of the substrates were deliberately varied to determine the effect of these properties on CD (in the presence and absence of an exposure monitor) during lithography. It was observed that linewidth uniformity of the developed photoresist can be greatly improved when the intensity of diffracted light from the latent image is used to control the exposure dose. Diffraction from the latent image grating structures was modeled using rigorous coupled wave analysis. The modeling is used to predict the diffraction from a latent image as a function of the substrate optical properties and the parameters of the latent image (i.e., linewidth, sidewall angle). Good agreement is obtained between theoretical and experimental observations. Conversely, the inverse problem is solved in which the parameters of the diffracting structure (the latent image) are determined from a measurement of the diffracted power. Therefore, the diffracted power can be monitored for the purpose of determining when the latent image will produce the proper CD upon development.
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.