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INDUSTRIAL PERSPECTIVES 5 1 Industrial Perspectives INTRODUCTION The main purpose of this report is to highlight the opportunities for more rapid and effective development of plasma process modeling and simulation as an engineering tool in the semiconductor manufacturing and plasma equipment supplier industries. It is intended for a variety of audiences: academic and government laboratory researchers, industrial engineers and scientists, and program managers at federal funding agencies. The scope of the report is substantial, coveting the industrial needs for better plasma process engineering, the current state of the art in plasma modeling, and the various supporting databases and diagnostic techniques that underlie and complement modeling and simulation. The report begins with this chapter on industrial perspectives to emphasize the primary purpose of this activity: to serve the needs of industrial suppliers and users of plasma process equipment. The need to maintain this industrial perspective is a recurring theme of this report. The potential for using modeling and simulation to benefit industrial users of plasma processes and equipment has never been greater. Computational costs continue to decrease steadily, and in the last several years, considerable progress has been achieved in establishing the major modeling strategies that are necessary to achieve practical industrial objectives. Nevertheless, low-temperature plasma processing science is a relatively young field, and has not therefore received the in-depth, sustained attention that is required to have a significant, timely impact in industry. This situation is perhaps most evident in the area of the database for physical and chemical processes in plasma materials processing. The data that are currently available are often scattered throughout the scientific literature, and assessments of their reliability are usually unavailable. The goals of this report include identifying strategies to add data to the existing database, to improve access to the database, and to assess the reliability of the available data. In addition to identifying the most important needs, this report assesses the experimental and theoretical/computational techniques that can be used, or must be developed, in order to begin to satisfy these needs. A major complication in this process is the fact that industry uses a large variety of gases and materials in plasma processes and equipment. Since time and resources are always limited, one must make choices regarding which chemical systems to examine carefully. Experiments are expensive and time-consuming, and therefore it may be necessary to augment these measurements of fundamental data with theory. Computational techniques are useful, but may require careful testing since the methodologies are sometimes not fully mature. The panel has attempted to develop a compromise between the competing needs for breadth and depth in the database, recognizing that needs change as industry evolves. The recommendations presented here will therefore require updating, probably within 3 to 5 years of the publication of this report. PLASMA PROCESSING FOR SEMICONDUCTOR MANUFACTURING The semiconductor industry and related industries such as flat panel display manufacturing are growing rapidly. In the semiconductor industry, worldwide revenues grew from $50 billion to $140 billion from 1990 to 1995.1 Projections are that a similar rate of growth will persist at least through the end of the decade, and probably longer. Plasma processing is one of the key technologies in this industry, accounting for Ë 30% of all process equipment in a typical wafer fabrication facility and a similar percentage of process steps. These include etching, deposition, cleaning, and stripping. The current level of annual sales for plasma equipment is on the order of $3 billion to $4 billion, with increases projected to match the rate
INDUSTRIAL PERSPECTIVES 6 of growth of the industries it serves.2 Projections are that by the end of the decade, sales of equipment for plasma etching, cleaning, physical vapor deposition (PVD), and chemical vapor deposition (CVD) will reach $10 billion.3 An important trend in the industry is the rapidly escalating cost of capital equipment, and the growing share of equipment costs (now about 60%) in the total cost of a state-of-the-art fabrication facility (currently about $1.5 billion).4 The projected changes in integrated circuit technology anticipated in the 1994 roadmap of the Semiconductor Industry Association (SIA) are given in Table 1.1. The historic trend of a doubling in device density every 1.5 years or so will continue. New generations of technology are introduced every 3 years, fueled by information age demands that continue to explode. Notably, the introduction of 0.25 Âµm technology overlaps the introduction of the next jump in wafer size to 300 mm diameter in 1999. Part of the reason for the exceptional rate of growth in the semiconductor industry has been the dramatic and steady rise in performance per unit cost. In order that this trend in the performance/cost ratio continue, the next-generation developments listed above must be accomplished with a corresponding increase in manufacturing efficiency. TABLE 1.1 Changes in Silicon Integrated Circuit Technology Projected to 1999 1990 1993 1996 1999 Critical dimension (Âµm) 0.8 0.5 0.35 0.25 DRAM capacity (Mbits)a 4 16 64 256 MPU/logic clock speed (MHz)b 25-40 50-200 140-350 240-500 Wafer size (mm) 150 150-200 200 200-300 Defect density (no./cm2) 0.2 0.1 0.05 0.03 Interconnect levels 2-3 34 4-5 5-6 a DRAM = dynamic random access memory. b MPU = microprocessor unit. SOURCE: A. Voshchenkov, Workshop on Database Needs in Plasma Processing, Washington, D.C., April 1-2, 1995. Increasing manufacturing efficiency will require a significant increase in the sophistication and effectiveness of process control, among other changes. For example, as critical dimension (CD) decreases, the control of the CD must be to within 0.03 Âµm. Even though the wafer diameter will increase, etching rate and selectivity nonuniformity across the wafer must in some cases be kept to less than 2%. During etching, an important control variable is the angle of the microfeature sidewall with respect to the surface. Control of this profile angle is sought to within 3 degrees. Another important processing variable in MOSFET (metal oxide semiconductor field effect transistor) manufacturing is related to the thin (< 80 Ã ) gate oxide between the gate electrode and the active device region below it. During the etching step in which the gate electrode is defined, and when gate oxide may be exposed to the plasma, selectivity must be high enough to keep gate oxide loss to less than 15 Ã . This is only 4 to 5 atomic layers. From the industrial perspective, plasma processing, especially plasma etching, is often seen as being unusually difficult to understand and control.5 Although some of the general mechanisms of the plasma are knownâsuch as the role of chemical interactions with radicals such as F or C1 atoms, the role of sidewall passivation in preserving etch anisotropy, and the fact that positive ion bombardment of surfaces has a mechanical sputtering roleâmany details remain obscure. Interactions between plasma species and the walls bounding the discharge are complex and depend on surface temperature, surface and bulk composition, and other variables that are empirically observed to change with time, but are not well understood. The goals of plasma etching, including high rate, selectivity, uniformity, minimal damage to the underlying nascent electrical devices, minimal chemical residue contamination, minimal particulate deposition, and microfeature critical dimension control, sometimes depend in subtle ways on the plasma quantities. Moreover, these processing objectives are commonly difficult to achieve simultaneously. For example, there is often a conflict between etch anisotropy (enhanced by more energetic ion bombardment)
INDUSTRIAL PERSPECTIVES 7 and selectivity (degraded by more energetic ion bombardment, since most solid materials sputter at about the same rate). Measuring fundamental plasma quantities is challenging, especially in industrial plasma tools with limited access for diagnostic instruments and rudimentary sensors. Most often, one must develop empirical correlations between observed process characteristics such as etching rate, uniformity, and selectivity, and design and operating variables such as chamber shape and materials, applied power, gas pressure, composition of the inlet gases, and substrate temperature. There are many of these parameters in a plasma etching tool. The tendency to view plasma etching as '' an art rather than a science, more black magic than engineering,''6 can be attributed largely to the murky relationship between the output of a plasma tool and its operating and design characteristics. In order to develop a manufacturable plasma process, engineers must carefully balance the competing process objectives, selecting process operating conditions (and tool type) so that the trade-offs are reasonably satisfied. In addition, this optimization must occur in a region of parameter space that is as large as possible. That is, if some set of operating conditions yields acceptable process results, but a very small change in gas pressure or wall temperature (for example) results in unacceptable process characteristics, the "operating window" is too narrow and the process will be ill suited to an industrial operation. Finding the right combination of operating conditions usually involves a tedious search of a large parameter space, unaided by theory or computation and guided only by intuition and/or experience. Partly in response to the challenges of etching high-aspect-ratio features, plasma equipment manufacturers have introduced in the last several years new plasma tools capable of operating at relatively low gas pressures, usually between several millitorr and several tens of millitorr. It is thought that the lower operating pressures improve etched feature profilesâfor example, anisotropyâby minimizing collisions between bombarding ions and neutral molecules in the sheaths. However, lower operating pressures tend to reduce etching rates, lowering wafer throughput and increasing the cost of operating the equipment (part of the cost of ownership). In order to raise rates in spite of the reduced gas pressure, the newer tools have been designed to sustain a higher plasma charged-particle number densityâtypically on the order of 1011 to 1012 cm-3. Perhaps more importantly, these newer tools (high density, low pressure) have at least partly separated power deposition into electrons to maintain plasma density (the source power) from power deposition into ions bombarding the substrate (the bias power). It should be noted that in addition to decreasing the frequency of ion-neutral collisions in the sheath and therefore increasing ion velocity anisotropy at the processed surface, these newer tools operate with considerably different ratios of ion flux to neutral flux bombarding the substrate, and with different neutral species impacting the surfaces. The latter effect is due to higher plasma densitiesâboth neutral and ionic species in the high-density plasmas tend to be more highly dissociated, no doubt affecting the plasma chemistry. For example, fluorocarbon gases commonly used in dielectric film etching are considerably more dissociated into atomic F than is the case in more conventional tools. This has caused problems with selectivity between silicon dioxide and silicon and has necessitated other changes in tool operation (or design) to counter this effect. Some observers have noted that the chemistry in high-density plasmas, originally thought to be simpler and "cleaner" than in conventional tools, is perhaps simply different, still requiting careful balancing to meet all objectives. Other effects are observed in high-density tools that are not observed in conventional tools. For example, in high-density plasmas, the flux of positive ions to the walls, and the return flux of neutral species, represent an internal recirculating mass flow of a magnitude similar to that of the gas flow entering and leaving the chamber due to pumping. The effects of this internal recirculation on processing are not known at present. In sum, because of the relatively new conditions experienced in high-density plasma tools, it is fair to say that modifications are needed to some of the established wisdom gleaned from years of experience with conditions of lower plasma density and higher gas pressure. The combination of rapidly changing process objectives (see, e.g., Table 1.1) and a rapidly changing technology (e.g. the shift to low-pressure, high-density plasma tools) compounds the challenges for