Ronald M. Martino

Quantitative stepper metrology using the focus monitor test mask

A new lithographic test pattern, the focus monitor, is introduced. Through the use of phase shift techniques, focus errors translate into easily measurable overlay shifts in the printed pattern. Each individual focus monitor pattern can be directly read for the sign and magnitude of the focus error. This paper presents a detailed verification of the validity of this approach, along with several preliminary applications.

Application of the aerial image measurement system (AIMS)TM to the analysis of binary mask imaging and resolution enhancement techniques

The newly developed Aerial Image Measurement System (AIMSTM*) was used to quantify the lithographic benefits of several resolution enhancement techniques as compared to standard binary mask imaging. This system, a microscope based stepper emulator, permits rapid characterization of mask images from both binary and phase shifted mask (PSM) patterns at multiple focal planes. The resultant images are captured digitally with a CCD camera and analyzed using an exposure-defocus tree technique to quantify the depth-of-focus as a function of exposure latitude. The AIMS is used to extract both phase and transmission errors from captured aerial images of all the masks evaluated. AIMS results are compared to wafer electrical linewidth data. A 0.5 numerical aperture (NA) DUV stepper was used with a partial coherence of 0.6 combined with IBM APEX-E resist process. Collected data were analyzed using techniques identical to the AIMS analysis, allowing for a high level of consistency. Comparative data focused on binary mask imaging for the verification of the AIMS results. Trends associated with feature sizes and types are discussed.

Application of an Aerial Image Measurement System to Mask Fabrication and Analysis

Application of an Aerial Image Measurement System (AIMSTM) to binary and phase- shift mask fabrication and evaluation is described. The AIMS tool, an optical system which measures the aerial image directly from a mask, provides rapid feedback on lithographic performance for a variety of stepper configurations through modifications of the wavelength, numerical aperture, and illuminator design. The AIMS tool has been applied during the implementation of an alternating phase-shift mask (PSM) fabrication process in order to understand the impact of the etched-quartz sidewall on lithographic performance. AIMS measurements were used to extract the effective phase and transmission as a function of phase- etch depth as well as post-etch treatment condition. A set of basic test structures are proposed which can be used in conjunction with the AIMS tool to automate the extraction of transmission, phase, and second-level overlay for phase-shifting processes such as alternating and attenuating PSM.

Some image modeling issues for I-line, 5X phase-shifting masks

The current image-theoretical basis for phase shifting masks (PSMs) relies on the scalar and Kirchhoff approximations, which neglect vector wave and edge diffraction effects around the mask. In this paper we use EMFlex finite element modeling to quantify vector diffraction effects, and show a method for modeling broadband illumination using the codes transient (optical pulse) capability and the Fourier transform in time. Simulations indicate that: the Kirchhoff approximation applied to etched quartz PSMs can lead to unacceptable errors due to a dark boundary layer on the quartz sidewall; diffraction produces relatively strong vector wave fields near feature edges but their contribution to the lithographic image is negligible; and the paraxial partial coherence approximation is generally valid for 4x or 5x projection systems. We discuss examples illustrating needs for better PSM metrology and phase measurements.

Data analysis methods for evaluating lithographic performance

Lithographic improvements obtained from process modifications or application of optical enhancement techniques can often be obscured within experimental noise and inconsistent data analysis techniques. At IBM, the need for a common platform for the accurate analysis of lithographic data led to the development of a new software analysis package entitled LEOPOLD. In this article, the exposure-defocus methodology taken by LEOPOLD for accurate calculation of the total lithographic process window in the presence of experimental noise is outlined; the extension of this method to the evaluation of the common process window through applications in the areas of optical proximity effects, analysis of optical enhancement techniques, and across-field exposure tool characterization is also demonstrated.

Lithographic evaluation of the hydrogenated amorphous carbon film

Electrical measurement techniques and aerial image simulation were used to quantify the lithographic benefits of an IBM hydrogenated amorphous carbon attenuated phase shifting mask for 0.25 and 0.2 μm features. Both a chrome and attenuated amorphous carbon blank were patterned with electrically testable structures consisting of varying dimensions and pitch. Detailed characterization was performed on the blanks to confirm phase, transmission, and dimensional accuracy. Process window characterization as a function of image biasing and pitch were evaluated using SPLAT simulations and electrical measurements for SVGL Micrascan II exposures combined with exposure‐defocus analysis techniques. The electrical linewidth data showed a 12.5% reduction of depth‐of‐focus corresponding to a 25 nm offset from optimum bias, demonstrating the importance of accurate mask biasing. Electrical measurements structures were patterned on doped polysilicon films using APEX‐E resist. Exposure latitude enhancement was achieved for ...

The Application of Deep UV Phase Shifted-Single Layer Halftone Reticles to 256 Mbit Dynamic Random Access Memory Cell Patterns

We have applied deep UV (DUV) halftone reticles, with a single layer absorptive shifter consisting of silicon nitride, to 256 Mbit dynamic random access memory (DRAM) critical levels, and have evaluated the resolution in those cell patterns. For the periodic line levels, halftone reticles were combined with off-axis illumination (OAI) techniques. Resolution capabilities were characterized not only with stepper exposures but also with direct aerial image measurement. For hole levels such the Storage Node and Bitline Contact, halftone reticles offered clear improvement with standard illumination as compared to conventional reticles. For line levels such the Isolation, Wordline and Bitline, dramatic improvement was obtained with the combination of halftone reticles and off-axis illumination.

Negative DUV photoresist for 16Mb-DRAM production and future generations

This paper discusses a new negative tone aqueous base developable photoresist that has demonstrated excellent sub-half micron resolution with commercially available DUV (deep ultraviolet) exposure systems. This system which consists of a phenolic resin (pHOST), a glycoluril crosslinker (TMMGU), and a triflic acid generating material is currently in use for the manufacturing of 16 M b-DRAM and related CMOS logic technology. We provide supporting manufacturing data relating to our experiences with this program, along with the benefits realized by the implementation of a negative tone photoresist system.

Impact of attenuated mask topography on lithographic performance

Experimental evaluations were used in conjunction with rigorous electromagnetic simulations to evaluate the affect of attenuated phase-shifting mask (PSM) fabrication processes on lithographic performance. Three attenuated PSMs were fabricated including a normal leaky- chrome reticle and two novel approaches: a recessed leaky-chrome reticle for reduction of edge scattering and a single-layer reticle employing a hydrogenated amorphous carbon film. Direct aerial image measurements with the Aerial Image Measurement System (AIMSTM), exposures on an SVGL Micrascan 92 deep-UV stepper, and TEMPEST simulations were used to explore the effects of edge-scattering phenomena for the different mask topographies. For each reticle, the process window at a feature size of 0.25 micrometers was evaluated for four basic feature types: nested lines, isolated lines, isolated spaces, and contact holes. Further evaluation of the sidewall profiles and the image size on the mask are required to address these discrepancies.

Negative tone aqueous developable resist for photon, electron, and x-ray lithography

The use of negative acting photoresists has become a integral part of device fabrication strategy. In this paper we will. discuss a phenolic based photoresist which incorporates a crosslinkable resin and an acid generating sensitizer. When exposed and thermally treated, the resist forms a negative tone image which is developable in an alkaline medium. We will discuss the materials, processes and results from photon, electron and X-ray lithographic evaluations.