Beam Technologies for Integrated Processing (1992)

Semiconductor device manufacturing utilizes many beam technologies. The emphasis on the need to produce smaller and more complex microelectronic structures has led to the development of sophisticated beam processing techniques. Beam technologies are used extensively in integrated circuit manufacturing for materials deposition, to improve the thickness and compositional uniformity of metals, dielectrics, and semiconducting materials deposited on substrates. Over the past 10 years, in order to obtain product uniformity and quality while achieving size reduction, sputtering has replaced evaporation for metals deposition in integrated circuits, and low-pressure chemical vapor deposition has given significant improvements for dielectric materials, metals, and polysilicon deposits. For the most part, ion implantation has replaced diffusion for almost all semiconductor doping process steps since it gives more uniform control of dopant concentrations and impurity profiles.

Essentially all of the patterning of silicon integrated circuits (ICs) is done by optical lithography, with electron beam lithography used for defining very small features for some circuit designs in some small-volume applications. The apparent successor to optical lithography is x-ray lithography, where features of 0.1 µm have been demonstrated in test devices. Because of this capability, several industrial and university groups are using enhanced synchrotron x-ray sources to develop advanced IC fabrication processes.

Newer device structures, such as heterojunction field effect transistors (HFETs), which require close control of epitaxial structures (e.g., gallium arsenide [GaAs], aluminum gallium arsenide [AlGaAs]), depend heavily on metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) techniques. Similarly, fabrication of optoelectronic components, which in the past were fabricated by liquid-phase epitaxy or chloride-transport vapor-phase epitaxy, also depend on MBE and MOCVD techniques to fabricate higher-performance, yet more complex, devices incorporating superlattices and quantum-well confinement. Complementary beam technologies, involving plasma etching and ion milling for selective materials removal, along with rapid optical annealing to activate the implants of heterostructure devices, are also important in the IC fabrication process.

Beam technologies have been used extensively to produce new materials in areas other than microelectronics, such as coatings and surface modifications, formation of net shapes, composites, nanophases, and optical surfaces, including the treatment of polymeric substrates. CVD and physical vapor deposition (PVD) have been used extensively over the past 30 years to apply metal, alloy, and ceramic coatings on metal, ceramic, or polymer substrates of various sizes and shapes. These applications range from wear-resistant coatings on cutting tools and magnetic media on tape to optical coatings on polymeric materials.

The technologies have progressed in the area of PVD to plasma-assisted CVD (PACVD) and reactive evaporation (RE) processes to aid in the deposition of more refractory materials such as nitrides, carbides, and high-temperature oxides. Reactive evaporation and laser ablation, the modern version of flash evaporation, have both been successful for depositing films of high-temperature superconducting oxides such as YB₂Cu₃O_7-x. Electron impact ionization and excitation of reacting gases has led to development of the activated reactive evaporation (ARE) process. This has allowed the synthesis of refractory silicide and carbide films at much lower substrate temperatures. An interesting variation of the ARE process, in which a low melting compound of the element is used, led to the formation of cubic boron nitride by evaporation of boric acid in an ammonia plasma.

Enhanced CVD processes have taken the form of either thermally assisted CVD or PACVD for thermal and electron excitation deposition, particularly for selective-area deposition. The plasma-or photon-assisted CVD process is quite useful in depositing ceramic coatings (Si₃N₄, SiC, AlN, Al₂O₃, etc.) at lower temperatures than conventional CVD. This feature is important when the substrate is susceptible to temperature-induced irreversible structural or electrical changes. A further process improvement, known as remote PACVD, is used to protect both the substrate and the growing film from ion bombardment.

Recently, considerable interest has been centered on the growth of diamond, diamond-like, and cubic boron nitride coatings. Synthesis of these films, especially diamond, has been performed using a variety of chemical and PACVD techniques as well as PVD techniques. In particular, the PACVD technique (with direct current, radio frequency, or microwave excitation) has been used extensively for the synthesis of diamond and diamond-like coatings. Ion-beam-assisted and low-temperature ARE also have been used to prepare diamond films. In all these beam methods of deposition, it appears that the concentration of atomic hydrogen must be controlled to give good-quality diamond films. Unfortunately, it is still difficult to prepare single-crystal films of diamond; in most cases a polycrystalline film is formed.

The two beam techniques utilized for surface modification of materials are ion implantation and laser treatment. Ion implantation is used in a number of critical-use applications. It has flexibility in that any element can be implanted into a substrate without thermodynamic constraint (e.g., not requiring chemical reaction) at or near room temperature to maintain the dimensional integrity of the part. The main application has been surface treatment of metals. However, in recent years considerable interest has emerged in the treatment of ceramic and polymer substrates.

Laser surface treatment, since it can be employed in atmosphere or nonvacuum ambient environment, offers greater flexibility for modifying a surface. Hardening of metal surfaces, increasing corrosion resistance of materials, and laser welding are typical applications. The ease

of implementing lasers, compared to ion beams, makes this approach potentially more suitable for integrated manufacturing, unless other vacuum steps are also required.

In addition, lasers are being applied to materials forming. They are used for both deposition and removal of material in shaping operations. Lasers are used to form shapes through such processes as solidification, sintering, polymerization, and CVD. The advantages of a laser in an integrated processing system for forming shapes are the short interaction times, high flexibility, and ease of on-line inspection and process control. A commercial process already has been developed using lasers for hole drilling and sheet cutting; presently, sheet metal cutting accounts for the largest fraction of CO₂ laser sales. Two other processes worth mentioning are laser-assisted machining, which has been used in turning hard-to-machine metallic alloys, and laser-assisted chemical etching, which is used to shape hard ceramic materials.

Ceramic powders, whiskers, and fibers have been synthesized using beam forming technologies. These have been important developments for production of composites and generation of nanophase materials that exhibit unusual properties. The catalyzed growth of both carbon and silicon carbide whiskers has been achieved using hydrocarbon gases and hydrogen, and silicon carbide and silicon monoxide precursors. When combining these chemical beams with an iron catalyst, in the case of carbon whiskers, growth rates as high as several millimeters per minute have been achieved. Similarly, silicon carbide whiskers have been beam-grown with average tensile strengths of 8.4 GPa, strains of 1.74 percent, and elastic modulus of 578 GPa, properties that are superior to those of the best polymer-derived ceramic fiber yarns.

Continuous monofilaments of boron and silicon carbide are presently being produced on fine-wire substrates using CVD. The CVD-produced SiC fiber has properties superior to poly carbosilane-derived fibers, although, presently, its fiber diameter is too large for weaving processes. These fiber manufacturing developments are good examples of integrated manufacturing using beam technologies.

Lastly, matrices of metals or ceramics have been formed using chemical vapor infiltration (CVI). Prominent examples are SiC fiber/SiC matrix and carbon fiber/SiC matrix. Current commercial CVI processes have the advantages of a near-net-shape process and a relatively low temperature process that involves no high-pressure sintering, thereby minimizing fiber damage.

Nanophase powders, which have properties different from their bulk counterparts because of their high surface area-to-volume ratios, have been synthesized using beam technologies. Applications include ultrafine titania in paints and pigments, catalysts, sintered powder products, magnetic recording media, optical applications, and biomaterials. Argonne National Laboratory and Nanophase Technologies, Inc., an offspring firm, are generating equiaxed nanophase materials by a gas condensation process. It can produce pure metals as well as oxides (e.g., TiO₂, Al₂O₃, MgO) using oxygen in the low-pressure chamber rather than an inert gas. A high-pressure sputtering process (200 mT gas pressure) has been developed at UCLA for the generation of ultra-fine particles (10-20 nm in diameter) of Cu, Au, SnO₂, and Al₂O₃.

New opportunities are emerging for extending flexible machining systems (FMS) to include additional processing and control steps as related to beam technologies. Future processing of materials with microwave energy is an attractive energy-saving approach. Alternative cost-effective manufacturing technologies, such as beam technologies, warrant serious consideration to replace mechanical fabrication of some components. Great opportunity exists for fully integrating gear-making operations using energy beams.

The IC manufacturing process flow generally is still batch oriented, as it was before the widespread introduction of beam technologies. Wider process margins are required for batch-processed wafers than would be required for individually processed wafers if adequate control systems existed to repeatedly process individual wafers in a single-wafer system. Information between the process steps is rarely fed forward to modify the subsequent step based on the previous step data; therefore, measurements of results and modification of process steps on a wafer-by-wafer basis are not possible in the batch processing environment. Some systems have automatic control of the process variables, but little exists by way of real-time monitoring of actual process results.

No standardized interface exists for material flow from system to system. Several proposed standards are being developed at this time. High volume is required to justify the large capital expenditure for a continuous fabrication line. A major effort is required to interface the various process equipment that must transfer the material and the process information. Cluster tools that can perform multiple process steps are being introduced into the semiconductor industry. Significant problems must be overcome in throughput, cost, uptime, real-time control, process information flow, and process step development before there can be considerable use of integrated process tools.

Some important considerations for expanding integrated beam processing systems are as follows:

• more studies to better understand the process steps so that the proper experimental variables can be measured and process simulation models developed,

• development of computer algorithms based on simulation models coupled with expert system artificial intelligence techniques to develop smart control programs,

• education of students with cross-disciplinary skills to undertake the above items, and

• foster team building between equipment vendors and materials processors.

The committee concludes that all of the beam technologies treated in this report are capable of incorporation into fully integrated processing systems. The embodiment of each incorporation will be material-specific; however, there are common problems and deficiencies which must be overcome before any fully integrated beam processing system is realized in actual manufacture. Foremost among these in the need to monitor the process in situ and control it in real time. Achieving this ability and addressing other important issues requires the following actions:

• Conduct fundamental mechanism studies: Further studies are needed for the understanding of the fundamental mechanisms by which these techniques work. Understanding of the underlying physics and chemistry is necessary to establish the proper parameters to be monitored or controlled.

• Develop in-line sensors: Direct monitoring of the process requires the development of proper sensors. Having accurate real-time knowledge of each process step will be necessary for both machine control and elimination of inspections.

• Develop process models: An element required in machine control is the availability of accurate process models and simulations. These are essential to properly interpret sensor readings and thereby issue control commands.

• Improve equipment reliability: High equipment uptime is a prerequisite for an economical integrated process. Equally important are ease of maintenance and self-monitoring equipment.

• Develop vendor-user communication: Close communications must exist between the equipment maker and the equipment user to identify and correct weak points in the design and to incorporate modifications that improve machine performance in a timely and efficient manner.

• Adopt standards and interface specifications.

• Properly educate the work force: As process equipment becomes more sophisticated and more complex, highly skilled workers are required to maintain the equipment.