Kenneth P. Bishop

Grating line shape characterization using scatterometry

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.

Use of scatterometry for resist process control

The formation of resist lines having submicron critical dimensions (CDs) is a complex multistep process, requiring precise control of each processing step. Optimization of parameters for each processing step may be accomplished through theoretical modeling techniques and/or the use of send-ahead wafers followed by scanning electron microscope measurements. Once the optimum parameters for any process having been selected, (e.g., time duration and temperature for post-exposure bake process), no in-situ CD measurements are made. In this paper we describe the use of scatterometry to provide this essential metrology capability. It involves focusing a laser beam on a periodic grating and predicting the shape of the grating lines from a measurement of the scattered power in the diffraction orders. The inverse prediction of lineshape from a measurement of the scatter power is based on a vector diffraction analysis used in conjunction with photolithography simulation tools to provide an accurate scatter model for latent image gratings. This diffraction technique has previously been applied to looking at latent image grating formation, as exposure is taking place. We have broadened the scope of the application and consider the problem of determination of optimal focus.

Latent image exposure monitor using scatterometry

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.

Results of a new polarization simulation

Including polarization signatures of material samples in passive sensing may enhance target detection capabilities. To obtain more information on this potential improvement, a simulation is being developed to aid in interpreting IR polarization measurements in a complex environment. The simulation accounts for the background, or incident illumination, and the scattering and emission from the target into the sensor. MODTRAN, in combination with a dipole approximation to singly scattered radiance, is used to polarimetrically model the background, or sky conditions. The scattering and emission from rough surfaces are calculated using an energy conserving polarimetric Torrance and Sparrow BRDF model. The simulation can be used to examine the surface properties of materials in a laboratory environment, to investigate IR polarization signatures in the field, or a complex environment, and to predict trends in LWIR polarization data. In this paper we discuss the simulation architecture, the process for determining and roughness as a function of wavelength, which involves making polarization measurements of flat glass plates at various angles and temperatures in the laboratory at Kirtland AF Base, and the comparison of the simulation with field dat taken at Elgin Air Force Base. The later process entails using the extrapolated index of refraction and surface roughness, and a polarimetric incident sky dome generated by MODTRAN. We also present some parametric studies in which the sky condition, the sky temperature and the sensor declination angle were all varied.

Multispectral polarimeter imaging in the visible to near IR

Polarization imaging can provide significant improvements in contrast in a number of target detection and discrimination applications. A multi-spectral imaging polarimeter has been constructed for the development of discrimination methods that exploit the polarization properties of a scene. The Stokes vector of a given scene is computed from a sequence of retardance measurements made with the instrument. A significant effort has been made to create a fast polarimeter which can make the necessary retardance measurements to produce a set of Stokes images in a minimum amount of time before the scene changes significantly, which shows up as errors in the resultant Stokes images. A number of wavebands spanning from 600 to 850 nm are considered to determine the dependence of polarization on the wavelength of light both emitted and reflected from various scene topologies. The retardance measurements are made using a rotating quarter- waveplate as opposed to using liquid crystal technology and benefits and drawbacks are discussed for this type of device. Extensive calibration of the instrument is performed to ensure the accuracy of the retardance measurements. This paper will discuss calibration methods, general operation, and results characterizing the polarization properties of numerous targets.

Two Long-Wave Infrared Spectral Polarimeters for Use in Understanding Polarization Phenomenology

Abstract : Abstract. Spectrally varying long-wave infrared (LWIR) polarization measurements can be used to identify materials and to discriminate samples from a cluttered background. Two LWIR instruments have been built and fielded by the Air Force Research Laboratory: a multispectral LWIR imaging polarimeter (LIP) and a full-Stokes Fourier transform infrared (FTIR) spectral polarimeter (FSP), constructed for higher spectral resolution measurements of materials. These two instruments have been built to gain an understanding of the polarization signatures expected from different types of materials in a controlled laboratory and in varying field environments. We discuss the instruments, calibration methods, general operation, and measurements characterizing the emitted polarization properties of materials as a function of wavelength. The results show that we are able to make polarization measurements with a relative accuracy of 0.5% degree of polarization (DOP) between two different instruments that are calibrated with the same techniques, and that these measurements can improve the understanding of polarization phenomenology.

Modeling and measurement of optical turbulence by tomographic imaging of a heated air flow

An eight-view tomographic system based on one-dimensional Hartmann sensors is currently in use to image heated air flows. The system produces two-dimensional maps of refractive index at rates of several kilohertz. The high rate and good resolution enable comparison of measured results to expected results from fluid flow models. However, the novel nature of the application complicates validation of the systems performance. This paper describes computer simulations and experimental results quantifying the performance of the tomographic system at resolving spatial and temporal structures within the flow. The computer simulations model the physics and noise sources inherent in the wavefront sensing and tomographic imaging systems. An error budget for the system and an effective resolution metric allow quantitative comparison of performance against a given model of the flow. However, the accuracy of the simulations predictions depends upon the accuracy of the disturbance model. By combining computer models of fluid flow, time averaged measurements such as temperature across the flow, and intrusive measurements such as smoke visualizations, we refine our flow model and improve the simulation. Experimental results show good agreement with this model, and the model allows us to discriminate and remove reconstruction artifacts from the imaged flow.

One-dimensional wavefront sensor development for tomographic flow measurements

Optical diagnostics are extremely useful in fluid mechanics because they generally have high inherent bandwidth, and are nonintrusive. However, since optical probe measurements inherently integrate all information along the optical path, it is often difficult to isolate out-of-plane components in 3D flow events. It is also hard to make independent measurements of internal flow structure. Using an arrangement of 1D wavefront sensor, we have developed a system that uses tomographic reconstruction to make 2D measurements in arbitrary flow. These measurements provide complete information in a plane normal to the flow. We have applied this system to the subsonic free jet because of the wide range of flow scales available. These measurements rely on the development of a series of 1D wavefront sensors that are used to measure line-integral density variations in the flow of interest. These sensors have been constructed using linear CCD cameras and binary optics lenslet arrays. In designing these arrays, we have considered the coherent coupling between adjacent lenses and have made comparisons between theory and experimental noise measurements. This paper will present examples of the wavefront sensor development, line-integral measurements as a function of various experimental parameters, and sample tomographic reconstructions.

Spectral polarization signatures of materials in the LWIR

The emitted polarization signature of materials is of interest for use in discriminating targets from cluttered backgrounds. In addition, spectrally varying polarization signatures might be used for material identification or to separate target and environment radiance contributions. A spectrally filtered LWIR Imaging Polarimeter (LIP) has been constructed and used in the lab and in the field to make signature measurements of controlled targets. In addition, a full-stokes FTIR Polarization Spectrometer (FSP) has been constructed for higher spectral resolution measurements of materials. This paper will discuss the instruments, calibration methods, general operation, and results characterizing the emitted polarization properties of materials as a function of wavelength.

Use of diffracted light from latent images to improve lithography control

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.