Machine-related writing errors are discussed in this paper. Butting errors are local writing errors which are manifested at the boundary of adjacent writing strips or fields. The overlay errors are concerned with the global-pattern registration errors on a mask set produced from the same machine. The absolute errors, often called machine-matching errors, are overlay errors of masks from different E-beam machines. The mechanisms of major error sources are described, and strategies to minimize them are also discussed. Finally a hypothetical design example is presented to demonstrate a highly accurate electron-beam litho-graphy system.
Optical lithography continues to improve resolution, further reducing minimum feature sizes in semiconductor device production. A critical step in this process is generation of masks with exceptional accuracy. The MEBES III electron beam lithography system was designed and built to fulfill the requirements of l to 2 micrometer technology and below. The results of overlay machine performance indicate that MEBES consistently meets and surpasses its overlay specifications. This paper discusses the philosophy behind the MEBES III design, the development of metrology methods to verify performance, and the measured accuracy of several machines in a production environment. Measurement tools internal to the MEBES system are compared to a Nikon model 21 X-Y measurement system to verify accuracy of the analysis.
As the dimensions of VLSI advanced process development devices increasingly become smaller, the need for an advanced inspection tool for semiconductor manufacturers increases as well. These dimensions, rapidly approaching the sub-micron level, make it difficult to perform accurate inspection and measurement with an optical microscope. The resolution and depth of focus limitations of the optical microscope present problems when attempting to measure or inspect the quality of contact holes. This study discusses the design considerations of a computerized low voltage electron beam. linewidth measuring tool which has been developed by Nanometrics Inc. of Sunnyvale, CA. This system, because of its non-penetrating electrons, permits the inspection and measure ment of contact holes, and alleviates the difficulties encountered with optical microscopes. Because the tool has been optimized to operate at low voltages (< 1KV), minimum charging on non-conducting materials, and the ability to look at the absolute surface of even the thin-nest films, is greatly. improved.
Recent work in retarding field optics for electron beam lithography has concentrated on the advantages to be gained for low electron landing energies (< 4 keV). We have examined the benefits obtained from the use of a retarding field when the electron landing energy is conventional (10-20 keV), eliminating the necessity for novel resist systems. The improved aberrations resulting from the use of a retarding field are discussed, and the reduction of space charge effects is simulated using a Monte Carlo calculation. Preliminary results indicate that a four-fold increase in current can be realized for a beam of given sharpness by using the retarding field. Practical considerations for implementing retarding field optics are examined.
Submicrometer focused ion beams have been used both for the maskless ion implantation of p-channel depletion-mode Si MOSFETs and for the gate lithography of n-channel enhancement-mode Si MOSFETs. B-Pt and Au-Si liquid-metal-alloy ion sources were utilized in a single-lens focusing column for the implantation and lithography steps, respectively. An 800-A-thick Al stopping layer was used at the target to separate the lighter ions from the heavier ion species in the beams. Reasonable dc electrical characteristics were measured for the chosen device process parameters.
One of the applications of high current density, focused ion beams (FIB) that has been made possible by the advent of the liquid metal ion source (LMIS) is milling of micron sized structures. In this study we examine the prospect of using a FIB system to selectively remove the passivation layer from IC's in order to carry out quantitative voltage contrast measurements on the conductors thus exposed.
Masked ion beam lithography has the potential to become a high resolution proximity printing technique which is capable of short exposure times. When used for proximity printing, the resolution of masked ion beam lithography is limited by the interaction of the transmitted ions with the ion beam mask. A straightforward approach to eliminating the mask induced scattering is to use a stencil mask where the transmission areas are simply holes in the mask. We have been developing an exposure process that uses a grid support mask which replaces completely open areas with a fine grid. The image of the grid can be eliminated by rocking the incident angle of the beam. We will discuss a fabrication procedure for such a mask as well as investigations into its uses and limitations.
For masked ion beam lithography (MIBL) beam-induced mask heating can cause deformations and image distortion. This can be avoided by the use of a resist ten times more sensitive than PMMA. Poly(2,2,2-trifluoroethyl -chloroacrylate), PTFECA, has been shown to be about ten times more sensitive than PMMA for proton beam exposures at 100 keV, and has demonstrated sub-half-micron resolution. The etch characteristics of PTFECA, however, are not as good as PMMA.
Negative tone images have been produced directly in glow discharge deposited amorphous silicon hydride by selective gallium-ion implantation. Like crystalline silicon, amorphous silicon exhibits a greatly reduced etch rate in aqueous caustic solutions when implanted with doses in excess of about 1013 Ga ions/cm2. The amorphous silicon behaves as a negative resist with a threshold sensitivity of about 1 pC/cm2. This material is particularly attractive as a possible masking material for ultra-violet light due to its high absorption properties in this wavelength region, low relative cost and the ease by which it can be deposited on a wide range of materials.
The increase in the beam energy spread <AE> of liquid metal ion sources with particle mass (rn) and current (I) is analyzed in terms of the pairwise coulomb interactions of particles emitted from a spherical emitter according to Poisson statistics. According to model calculations <AE> cc m1/4 11/2 which agrees with experimental results. In addition, a shift in the average energy of the particles due to the mutual interactions is predicted by the model calculations.
We describe the initial performance of the optical column and imaging instrumentation of a second-generation scanning ion microscope/microprobe. The instrument operates in the 20- 60 kV range and makes use of a Ga liquid metal ion source. High-quality secondary electron and ion images of conductors and insulators have been thus far obtained at 90 nm measured spot size. Ultimately the instrument is expected to perform secondary ion mass spectrometry, imaging, and microlithography functions in the 10 - 100 nm range of spot sizes.
X-ray lithography promises cost-effective integrated circuit production with submicron resolution. Achievement of this promise requires precision mask-to-wafer alignment over the whole wafer. This paper describes Perkin-Elmer's X-100 full-field X-ray lithography system with emphasis on the alignment subsystem. The alignment subsystem provides six degrees of freedom alignment between wafer and mask with an air gauge gap setting technique and a laser based physical optics lateral alignment system. These techniques were selected to be compatible with the subfield-stepper systems needed to pattern wafers over 100 mm in diameter. The physical optics technique has demonstrated alignment stability with signal-to-noise ratios representing less than 0.01 micron rms error for a 30 Hz bandwidth. The alignment system and its relation to the entire lithographic system is described in detail. X-ray exposed alignment overlays are shown.
An automatic, tri-field registration system is described, as used in an automatic wafer exposure system2, for wafers up to 100 mm diameter featuring a high power stationary anode X-ray source (Pd target). The system provides automatic registration and run out compensation based upon the use of Fresnel zone plate alignment targets. First, the basic concepts of the alignment method using circular Fresnel zone plates are presented. The complete alignment system is then described including machine hardware, software, and system operation. Finally, system results of overlay performance obtained with the registration system are given.
Current Bell Laboratories X-ray mask absorber pattern transfer technology consists of sputter-etching a 6000 - 7000A thick gold metallization using a 800A thin tantalum etch mask, resulting in features with a 70° wall slope. Since X-ray absorption is exponential with thickness, the resist under the sloped portion of the abosorber will receive various degrees of exposure. Therefore, as X-ray lithography approaches sub-micron dimensions, gold absorber walls must be made vertical in order to provide better resolution and linewidth control. We describe a method for obtaining vertical wall sub-micron gold patterns on X-ray masks by DC gold electroplating into a reactive ion etched trilevel stencil over a 200A gold plating base. A commercial gold sulphite solution is used in a fountain system to gold plate into the trilevel stencil. Gold features as small as 1000A wide and 1pm high have been obtained. The minimum gold feature size appears to be limited by stencil formation. Electroplated gold film stress, microstructure and thickness uniformity are within the prescribed limits for the manufacture of X-ray masks.
An electrical test structure and a methodology are proposed for equipment, process, and method characterization for submicron lithographic VLSI applications. An integrated approach of electrical test structures, optical test structures with manual or automatic optical inspection methods is described.
The penumbral shadow has been recognized as the resolution- limiting parameter in x-ray lithography. In the present study, the exposure of resist with monochromatic (2.84 keV) x-rays and an experimental palladium spectrum is simulated for various values of the penumbral shadow. Two mask structures with an absorber wall angle of 0 and 17 degrees and an absorber thickness of 1 um are compared. The effects of the penumbra and the finite absorber thickness on line acuity, linewidth control and uniformity are demonstrated. The simulation is compared to electrical linewidth measurements of resistors fabricated in 300 nm poly-silicon films. Negative acting resist was patterned with palladium radiation at a penumbra of 0.3 um through a boron nitride supported gold absorber mask with a thickness of about 0.8 um. The data indicate that the experimental linewidth control is considerably smaller than calculated, especially for small x-ray doses.
The first monochromatic resist exposures in the photon energy range 1 to 3.5 keV were completed at the in-vacuum lithogrpaphy beam line at the Stanford Synchrotron Radiation Laboratory (SSRL). Layered synthetic microstructures (LSM) were used as monochromators. The exposure times of 200 k Poly(chloromethylstyrene) (PCMS) through a boron nitride supported gold absorber mask were below 20 minutes. The sensitivity increase of PCMS at the chlorine edge was clearly observed. A considerable non-uniformity of the beam image in the 10 -20 um range was related to imperfections in the LSM substrate.
Low defect density masks are necessary to obtain the high chip yields required in VLSI technology. This paper will outline the boron nitride mask fabrication process that has been developed at AT&T Bell Laboratories, and discuss the nature and distribution of normally encountered defects. The advantages and disadvantages of various repair techniques will be addressed and specific examples of repairs made on silicon integrated circuit X-ray masks will be shown.