IBM Advanced Research Articles

From the Editor's Desk

This issue features articles on software in the context of the history of intellectual property, the first in a two-part series on the challenge of writing contemporary history by James Cortada, an exploration of the transition from analog to digital methods in the modeling of electrical power distribution networks, an article on computing prime numbers on the ENIAC and EDSAC, and a firsthand account of the IBM Advanced Computer Systems (ACS) project of the mid-to-late 1960s by Brian Randell.

Crystallographic Texture and Phase Metrology During Damascene Copper Processing

AbstractThe rapid adoption of damascene copper technology has brought about an increased need to understand and control microstructure in the barrier and metal layers during processing. We have developed and implemented a methodology for rapidly characterizing thin film polycrystalline microstructures on 200 mm Si substrates using an X-ray diffraction (XRD) based metrology tool to measure crystallographic texture and phase in a time frame suitable for in-line applications. The acquired data can be used as a direct measure of the deposition process in terms of film quality, reproducibility and stability over time. The spatial distribution of crystallographic texture and phase can be measured on a single wafer in order to check within wafer uniformity. These same measurements can also be carried out at predetermined intervals on wafers from single deposition tools, with the results used to create a database that can be applied to process control trend charting and to the establishment of acceptance criteria. The methodology developed makes use of several novel data analysis and collection techniques, such as the use of a direct matrix transformation method to determine the orientation distribution function (ODF) from a group of truncated pole figures. Useful quantitative outputs of the ODF, such as volumetric fractions of crystallographic texture components, can be used in quantifying texture evolution within and between wafers. On-the-fly texture compensation can also be used to generate useful quantitative phase and film thickness measurements. We present the principles of operation of this metrology tool and selected examples collected in the IBM's Advanced Semiconductor Technology Center (ASTC), East Fishkill, NY.

A probabilistic estimation framework for predictive modeling analytics

IBM ProbE (for probabilistic estimation) is an extensible, embeddable, and scalable modeling engine, particularly well-suited for implementing segmentation-based modeling techniques, wherein data records are partitioned into segments and separate predictive models are developed for each segment. We describe the ProbE framework and discuss two key business solutions that have been built using ProbE: the IBM Underwriting Profitability Analysis for insurance risk management, and the IBM Advanced Targeted Marketing for Single Events for direct mail database marketing.

Advanced DUV photolithography in a pilot line environment

As the critical path to increasing circuit density, deep-ultraviolet (DUV) lithography has played a key role in the development of new semiconductor products. At present, DUV refers to imagery at the 248-nm wavelength, with the introduction of 193-nm photolithographic systems anticipated in the next few years. This paper presents an overview of DUV lithography applications in the IBM Advanced Semiconductor Technology Center (ASTC). Since 1990, we have used DUV lithography for critical levels of advanced generations of DRAM (64Mb, 256Mb, and 1Gb) and associated families of logic products. We describe the means by which DUV capability and productivity have increased in a decreasing process window environment. Tooling, processes, and process control systems have undergone continuous improvement to accommodate increasing wafer starts and the rapid introduction of new products.

Si-Ge heterojunction bipolar technology for high-speed integrated circuits

This review discusses the fundamentals of SiGe epitaxial base heterojunction bipolar transistor (HBT) technology that have been developed for use in analog and mixed-signal applications in the 1–20 GHz range. The basic principles of operation of the graded base SiGe HBT are reviewed. These principles are then used to explore the design optimization for analog applications. Device results are presented that illustrate some important trade-offs in device design. A discussion of the use of UHV/CVD for the deposition of the epitaxial base profile is followed by an overview of the integrated process. This process, which has been installed on 200 mm wafers in IBM's Advanced Semiconductor Technology Center in Hopewell Junction, N.Y., also includes a full range of support devices. The process has demonstrated SiGe HBT performance, reliability, and yield in a CMOS fabrication with the addition of only one tool for UHV/CVD deposition of the epi-base and, with minimal additional process steps, can be used to fabricate full BiCMOS designs. This paper concludes with a discussion of high-performance circuits fabricated to date, including ECL ring'oscillators, power amplifiers, low-noise amplifiers, voltage-controlled oscillators, and finally a 12-bit DAC that features nearly 3000 SiGe HBT devices demonstrating medium-scale integration.

Effect of absorber thickness on image shortening in x-ray lithography

Image shortening effects have been shown to be a problem for pattern replication using current optical tools. However, a previous study [R. DellaGuardia, J. R. Maldonado, and H. Oertel, J. Vac. Sci. Technol. B 12, 3936 (1994)] indicated that image shortening is less pronounced when pattern replication is performed using x-ray lithography. This article describes the effect of absorber thickness on the image shortening observed in x-ray lithography. The goal is to determine the optimum absorber thickness that minimizes image shortening when replicating complex patterns with the x-ray spectrum from the HELIOS storage ring installed at the IBM Advanced Lithography Facility. To study these effects, an x-ray mask with four quadrants, each having different gold thickness, was fabricated using the IBM vector scan (VS-5) electron-beam system. The mask contains challenging patterns used in IBM devices with ground rules from 0.4 μm down to 0.15 μm. The image shortening effects for different mask/wafer gaps, various line shapes, and feature sizes will be presented in this article. In addition, experimental results with conventional and chemically amplified resists will be presented to shed light on the role of resist in image shortening.

Determination of Helium presence in the mask/wafer gap of X-ray lithography steppers

This paper describes a technique to determine the presence and uniformity of Helium in the mask/wafer gap of x-ray lithography steppers by utilizing the oxygen sensitive resist polychlorostyrene (PSC). Results obtained at the IBM Advanced Lithography Facility using a SUSS stepper and the HELIOS superconducting synchrotron storage ring are presented.

Comparison of image shortening effects in x-ray and optical lithography

Data on image shortening effects with patterns replicated with x-ray and optical lithography are presented. The x-ray exposures were performed at the IBM Advanced Lithography Facility using the Helios superconducting storage ring and a SUSS stepper. The optical exposures were performed using SVGL Micrascan 1 and 2 tools and biased optical masks. The results indicate that the image shortening effects using x-ray lithography (XRL) are considerably less pronounced than the effects observed with the optical tools. In addition, modeling of the image shortening effects for XRL using the xmas three-dimensional program for resist patterns is presented and compared with experimental results.

Experimental determination of the effective lithographic contrast for x-ray masks

A new technique for measuring effective lithographic contrast based on its definition is presented. Results obtained at the IBM Advanced Lithography Facility using the Helios storage ring are compared with conventional ways of determining mask contrast. In addition, a method to decrease the effects of multiple exposures in adjacent fields during the step and repeat operation of an x-ray lithography stepper is presented.

The HELIOS series of advanced, synchrotron based, X-ray sources

The compact synchrotron X-ray source, HELIOS built by Oxford Instruments, was commissioned at IBM's Advanced Lithography Facility at Fishkill New York in 1991. Key features and specifications for HELIOS are described, emphasising the unique features of this storage ring X-ray source. The performance of the first machine is outlined, based on almost two years'routine operation, identifying key factors such as output flux, beam lifetime and reliability. A description of the second HELIOS and enhancements to its performance is included

Operating experience with the Helios compact storage ring at IBM

Helios is a compact, superconducting storage ring designed and manufactured by Oxford Instruments specifically for use as an X-ray source for lithography. The ring, which runs at 686 MeV and has a critical wavelength of λ c = 9 A ̊ , uses a 200 MeV linac for injection. The ring was delivered to the IBM Advanced Lithography Facility in East Fishkill, New York in the spring of 1991 and has been in routine use since the beginning of 1992. Its performance has been characterized from both technical (beam size, lifetime, etc.) and functional (reliability, ease of use, etc.) standpoints. The specifications for the storage ring are reviewed with an emphasis on their relation to lithographic requirements. The ring has met and in some cases significantly exceeded the original specifications, having an average stored current of ∼ 165 mA with a 1/e lifetime of ∼ 20 hours at a current of 200 mA. Re-injection typically takes 15–20 min. Beam has been available more than 95% of the scheduled time since January 1992, which is particularly significant for a tool considered for use in a manufacturing application. This paper discusses the operating history of the ring, describing the improvements that have been achieved in such areas as beam lifetime, reliability, and injection, including the commissioning of a low energy (90 MeV) injection mode. The present performance of the ring is also summarized.

Optimizing synchroton-based x-ray lithography for 0.1 μm lithography

Abstract Of the many factors affecting the image intensity distribution, the variables that can be controlled are source spectrum, source spatial coherence, proximity gap, mask linewidth bias, and absorber thickness. To obtain the highest quality aerial image, all of these parameters must be optimized simultaneously. We describe an optimization of the spectrum of the Helios synchrotron, located at IBM's Advanced Lithography Facility (ALF) [1]. To obtain a spectrum better suited to 0.1 μm-linewidth x-ray lithography, the vacuum window should be changed from Be to 2 μm of Si. We present experimental results on the strength of SiN x windows for use as vacuum windows and propose a fabrication procedure for making beamline windows. Using the determined optimum spectrum, we summarize the results of a previous optimization study [2] of the x-ray aerial image. This optimization is performed using a rigorous electromagnetic model that accounts for diffraction in the absorber, source partial coherence, and diffraction in the proximity gap. The aerial image of 0.1 μm features at a gap of 10 μm is optimized.

Performance of a wide-field flux delivery system for synchrotron x-ray lithography

An x-ray flux delivery system consisting of a compact storage ring and dedicated beamlines is now in routine use in the IBM Advanced Lithography Facility in East Fishkill, New York. The beamlines have been designed for use with a stepper that scans the mask and wafer vertically through a stationary beam. The performance of the system from a lithographic standpoint is reported. The measured size and stability of the synchrotron x-ray source will be presented, and the design and performance of the beamlines will be discussed. The beamlines have been designed to provide high intensity, uniform illumination over a wide field. The overall performance of the system has been evaluated by measuring the flux at the mask plane in the stepper. A horizontal uniformity of better than ±2% over a field more than 50 mm wide has been achieved; the vertical uniformity is limited by the uniformity of the stepper scanning mechanism. The delivered mask-incident flux per milliampere of stored beam current has been found to be 1.5 mW/(mA cm), approximately four times greater than obtained with a simple planar mirror. A 100 mJ/cm2 resist with a mask transmission of 50% therefore requires a less than 2 s exposure (plus overscan overhead) for a 2.5 cm high field at a typical stored current of 200 mA, independent of the field width (up to the maximum of more than 50 mm). In addition, the source has proved itself to be highly reliable, with beam available for 99% of the scheduled time over the most recent seven month period.

Overlay performance of x-ray steppers in IBM Advance Lithography Facility

This article describes the overlay performance of the Suss XRS200/3 synchrotron compatible x-ray steppers installed in the IBM Advanced Lithography Facility. The Suss XRS200/3 uses a video-based brightfield microscope alignment system. The overall performance of the stepper was described previously. This article will specifically focus on the test, optimization, and performance of the alignment system. In the Suss alignment system, the quality of optics and test methods of the optical assembly was a critically important factor in achieving the 110 nm overlay quoted previously. A detailed discussion is given on the selection and characterization of these alignment optics, along with the subsystem level tests used to ensure performance of the complete imaging assembly prior to and after installation on the main stepper body. The overlay test methodology and data obtained during the acceptance testing is also discussed in detail. The exposure testing was designed to measure a full range of performance of the stepper. Indeed, overlay ranged from 50 nm (mean plus three sigma) on the very best levels to in excess of 150 nm on the most difficult. Sensitivity of the alignment system to small film thickness changes was tested along with alignment precision as a function of alignment mark depth or signal contrast.

The Helios 1 compact superconducting storage ring X-ray source

The basic properties of synchrotron radiation are described, the design of storage rings to produce synchrotron radiation is outlined, and the criteria for matching storage ring design to the needs of X-ray lithography are discussed. Simple scaling laws are presented showing the benefits for a storage ring of using the higher fields which superconducting magnets are able to provide. Helios 1 is a compact superconducting storage ring built by Oxford Instruments for installation at the IBM Advanced Lithography Facility (ALF). Design choices for superconducting rings are discussed, and the design and construction of Helios are described. Test results from the initial commissioning of Helios at Oxford are presented, but the main data on its performance when installed at ALF are given in another paper in this issue.

X-ray lithography in IBM, 1980–1992, the development years

The IBM X-ray lithography research and development program is outlined, from a personal perspective, covering the period from the inception of the program in 1980 through the development of IBM's own storage ring for X-ray production in 1992. The following aspects, among others, are discussed: origins of the program; acquisition of an X-ray port at Brookhaven National Laboratory; masks for X-ray lithography; development of special tooling for X-ray lithography, including a wafer stepper, a precision e-beam X-ray mask writing system, and a superconducting (dipole) electron synchrotron installed in the IBM Advanced Lithography Facility (ALF) in East Fishkill, New York. Key device programs were conducted to increase understanding of the X-ray lithography process and confirm its utility.

Performance of the IBM synchrotron X-ray source for lithography

The compact superconducting synchrotron X-ray source at the IBM Advanced Lithography Facility in East Fishkill, New York has been in service to customers since the start of 1992. Its availability during scheduled time is greater than 90%, with recent months frequently surpassing 95%. Data on the long-term behavior of the X-ray source properties and subsystem performance are now available. The full system continues to meet all specifications and even to surpass them in key areas. Measured electron beam properties such as beam size, short- and long-term positional stability, and beam lifetime are presented. Lifetimes greater than 20 hours for typical stored beams have significantly simplified operations and increased availability compared to projections. This paper also describes some unique features of this X-ray source and goes beyond a discussion of downtime to describe the efforts behind the scenes to maintain and operate it.

X-ray lithography beamlines in the IBM Advanced Lithography Facility

In 1991 a storage ring designed as a source of X-rays for X-ray lithography was delivered, installed, and commissioned in the IBM Advanced Lithography Facility (ALF) in East Fishkill, New York. Beamlines of two different designs have been constructed and installed on the ring to deliver the X-rays to the exposure stations. One design is intended for use with a stepper for the fabrication of integrated circuits. The second design is for a general-purpose research and development beamline which is used for unaligned exposures as well as for characterization of beamline components. The design and performance of both are described. Special attention is given to a paraboloid mirror optical system which is used to collimate the radiation from the storage ring. Both the theoretical and the measured performance of the mirror are presented and shown to be in excellent agreement. An exposure nonuniformity of less than ±3%, including contributions from both the mirror and the beryllium exit window, has been achieved.

Early operating experience with the helios compact synchrotron x-ray source

The Helios compact synchrotron installed at IBM's Advanced Lithography Facility provides a unique opportunity to obtain compact synchrotron reliability data in an industrial environment. This paper presents data from the first eight months of user oriented operation of the machine. Running and uptime statistics are tabulated, as well as achieved performance parameters. Unscheduled downtime in the various subsystems is described.

Measurement of the effective wavelength of x-ray lithography sources

This paper describes techniques to determine the effective wavelength of x-ray lithography sources. The experimental results give information on the actual x-ray absorption of the resist materials for the x-ray source under test. Results for two beam lines of the HELIOS storage ring installed at the IBM Advanced Lithography Facility, and for one beam line at the VUV ring at the Brookhaven National Laboratory are presented, and compared to calculations.