Specialty Gases for III-V Semiconductors

The compound semiconductor sector has been gaining importance in the worldwide market. In 1990, it accounted for 5% of the $50B semiconductor industry, rising to ~10% of the overall market ($168B) in 2001.¹ A wide range of electronic and photonic (optoelectronic) systems use compound semiconductor devices (see "Applications of III-V Devices"). Based on compounds of Group II and Group VI elements, Group III and Group V elements and dissimilar Group IV elements, compound semiconductor devices typically use gallium arsenide, gallium nitride and indium phosphide base materials. In heterostructural devices, alloys of these base materials with aluminum and/or indium generate the desirable thin-film properties (optical and/or electrical). The II-VI (e.g. zinc selenide and cadmium telluride on silicon or GaAs) and IV-IV (e.g. SiGe on silicon) devices are also gaining popularity.

The compound semiconductor market is well established in Japan (35%) and Taiwan (25%), with significant recent growth in China and Korea (Fig. 1). North America constituted 18% of the worldwide III-V market in 2001: GaAs manufacturers include Anadigics, Skyworks, RFMD, Triquint, Kopin and Vitesse; GaN manufacturers include Cree Research, Lumileds, AXT, United Epitaxy and Nitronics; and InP manufacturers include Velocium and Vitesse. The more prominent players in Europe are Osram for GaN production and IQE for GaAs production.

In 1999, Japan's photonics equipment and devices industry was $55.8B (¥6357B at ¥114/US$), which grew to $65.5B in 2000 (¥7069B at ¥108/US$) and plummeted to $34.9B (¥4218B at ¥121/US$) in 2001.^2-5 Exchange rate changes played an important part in the market fluctuation. The value of the yen recovered slightly in 2000 but dropped again in 2001 — a change that would have caused an 11% contraction in the optical storage market in dollars even if unit volumes and yen-based pricing had held steady.⁶

An NH3 delivery system must be able to supply energy promptly and efficiently to sustain high flow rates. The connection between reactors and gas cabinets must accommodate the cyclical NH3 consumption pattern.

1. III-V semiconductor production is dominated by Japanese and Taiwanese device manufacturers. (Sources: ITRI, Commerzbank, DrKW, Compound Semiconductor)

In addition to Taiwan's global dominance in the GaAs market, companies such as United Epitaxy, Procomp, Visual and South Epitaxy are also investing in AlGaInP, GaP, GaAsP and AlGaAs devices. Taiwan has ~70% of the merchant market for AlGaInP and InGaN epi wafers. Taiwan is a much smaller player than Japan in semiconductor lasers, accounting for <1% of the world market.⁹ In December 2001, the OES Laboratories of ITRI announced successful development of Taiwan's first GaN laser.¹⁰

Korea is also rapidly emerging as a compound semiconductor powerhouse, as the industrial landscape becomes less dominated by companies such as Samsung, LG Semicon and Hyundai. The industry now boasts a large number of startups making a broad range of products, with much activity focused on InP-based lasers (Opto*on) and GaAs-based vertical-cavity, surface-emitting lasers (VCSELS) for fiber-optic applications (Opto*on, Opticis, Optowell, Optoway, Vichel). There are also epi wafer foundries (Epiplus, EpiValley, Nanotron Technologies, PROWTech), packaging and assembly companies (Knowledge*on Semiconductor and Seoul Opto Device), and 6 in. GaAs foundries (NeosemiTech and HiQTech).¹¹

China took rank as a III-V-producing country in early 2002 and has progressed toward InGaN and AlGaInP/GaAs LED epi wafer production. Startups such as Xiamen Sanan, Qingdong Aulong, Shanghai Beida Blue Light and Shandong Huaguang are targeting late 2002 or early 2003 production.

2. Compound epitaxial wafers account for the bulk of the consumables market, followed by back-end materials, resists, slurries and masks, then organometallics and process gases. (Sources: Commerzbank, DrKW, Cahners/Reed Business Information, Epichem, MRC)

Metalorganic growth (MOCVD) of III-V materials requires ultrapure specialty gases such as arsine, phosphine and ammonia. GaAs can be doped using silane (n-type) or p-type dopants such as dimethylzinc, diethylzinc or bis(cyclopentadienyl) magnesium. Hydrogen chloride or chlorine usually serve as etchant while argon, hydrogen or nitrogen act as carrier gases for less volatile organometallic precursors.

Epitaxial thin-film deposition plays a major role in the growth of many compound semiconductor devices, including LEDs, LDs, solar cells and GaAs/InP electronic devices (e.g. heterobipolar transistors [HBTs] and high-electron-mobility transistors [HEMTs] in wireless communication). Deposition methods include MOCVD, molecular beam epitaxy (MBE), vapor phase epitaxy (VPE) and liquid phase epitaxy (LPE). Due to their abilities to deposit thin-film structures with abrupt interfaces, MOCVD and MBE techniques dominate.¹²

The MBE process is characterized by its vacuum-based growth chamber, elaborate effusion cells, and the capability to precisely control deposition rate and composition profile. For these reasons, MBE is ideal for HBT and HEMT fabrication where interface control is critical. Recent advances in technology have made volume production using MBE possible. On the other hand, MOCVD is characterized by its relative high pressure, high growth temperature and fast deposition rate.¹³ It is the prevailing method for optoelectronic and solar cell production because of demands for high-volume throughput. MOCVD can also be used in the formation of GaP, GaAs, GaAlAs, GaAsP, InP, InGaAs, InGaAsP, GaN, InGaN, etc., using phosphine or ammonia as source chemical.

For the growth of GaAs epitaxial layers, substrates of GaAs single crystals are commercially available. GaAs ingots are prepared from high-temperature, high-pressure melts of elemental gallium and arsenide with a pure, single-crystal GaAs seed. It is very difficult, however, to keep the GaAs substrates clean and free from oxidation or particle deposition during transportation. A 1-2 µm buffer layer is grown via VPE or MOCVD before device layer growth begins. By fine-tuning the growth parameters and with the addition of dopants, the GaAs epi layer can be grown with a particular surface termination, surface composition and charge carrier concentration.

The growth of GaN semiconductors is different because, unlike GaAs and InP, single crystals of GaN are difficult to form and are not available commercially. Current GaN-based devices are grown epitaxially on top of a substrate such as sapphire (Al₂O₃) or silicon carbide. SiC offers the best lattice match to GaN, but its use is limited due to high manufacturing cost. Al₂O₃ is a more economical substrate but offers a poor lattice match.

Regardless of the choice of substrate, the growth of a buffer layer (~10 nm) of GaN or aluminum nitride is necessary to reduce threading dislocations in the overlayers. Epitaxial lateral overgrowth utilizing patterned silicon dioxide also helps reduce total threading dislocations and boost LED wafer luminescence output. Small-area GaN substrates obtained by hydride VPE have recently become available, but economical production is still under development.

Epitaxial deposition involves reactions between precursor gases (metalorganics and hydrides) on the growth surface, thus requiring sophisticated gas delivery systems. Recent advances in gas manufacturing and purification offer great improvements in contamination and defect control for MOCVD systems using gaseous sources. For example, Praxair now produces AsH₃ and PH₃ in Grade 6.0 (99.9999%) and NH₃ in Grade 6.5 (99.99995%).

Eliminating possible sources of contamination and degradation in the gas delivery and storage systems is one way to maintain the purity of gases in MOCVD systems. Compounds such as H₂O, O₂ and CO₂ can incorporate oxygen atoms into the thin-film structure. The oxygen impurities are well known electron donors in GaAs and GaN systems, while metallic impurities may manifest themselves as deep levels in GaN.

A uniform, highly controlled gas flow rate as well as controlled maintenance of gas temperatures in the source chamber, gas lines and process chambers are critical to efficient and uniform epi layer growth. Standard flow rates of AsH₃ and PH₃ for GaAs and InP growth are generally low (~1.5-2.0 L/min). Peak flow rates could be 2-3× higher when multiple MOCVD tools are tied to the same gas cabinet.

The demand for high-purity AsH₃ increases as the GaAs market expands. The emerging business in compound semiconductor electronics and communications components based on InP and GaN also calls for more high-purity PH₃ and NH₃. In addition to increasing supply capacity, many gas suppliers have formed joint ventures with Asian companies to provide semiconductor gases in the area.

Meeting the higher volumes of specialty gas usage is no easy task. Manufacturing facilities can either increase the number of gas cylinders or install bulk delivery systems on site. Traditionally, multiple cylinders can be connected to the delivery system to increase the gas flow rate for various processes. Not only is this approach cumbersome, time-consuming and expensive, but the cylinders occupy precious floor space. Moreover, the many connections and frequent cylinder exchanges present additional contamination risk.

A just-in-time inventory approach helps alleviate contamination and quality degradation as a result of idle cylinders in storage. Bulk delivery systems provide another alternative.

Bulk delivery systems for NH₃, HCl and Cl₂ offer the flexibility of gas volume and flow rate control as well as customization for contamination control and safety.

The standard flow rate of NH₃ for GaN growth has been 20-60 L/min, depending on MOCVD reactor type. The nucleation of nitrogen into a lattice structure is very inefficient, and MOCVD of nitride layers relies on overpressure of ammonia (or high V/III ratio up to 5000) on a high-temperature substrate.¹⁴ NH₃ source chemical is stored as a liquified gas that requires energy for delivery (heat of evaporation). High NH₃ flow leads to sub-cooling of the cylinder because the heat absorbed from the ambient and cylinder wall is generally insufficient to provide the heat of evaporation (Joule-Thomson effect).

Therefore, a satisfactory NH₃ delivery system must be able to supply energy promptly and efficiently to sustain high flow rates. System design and configuration is also important — the connection between reactors and gas cabinets must be able to accommodate the cyclical ammonia consumption pattern.

Most conventional bulk delivery systems can only deliver up to 235 L/min flow rates. The only way to reach higher flow rates would be by connecting many cylinders to these systems. High-throughput MOCVD tools that require NH₃ flows of up to 60-80 L/min have become available, and many fabs are tying multiple reactors to a single ammonia source requiring flow rates near 500 L/min. The current demand for bulk NH₃ system is unprecedented. The latest delivery system includes an enclosed heating system that can control sufficient NH₃ evaporation, provide high sustained flow rates (480 L/min) and peak flows (900 L/min), while maintaining controlled external cylinder or container temperatures. Together with weatherproof gas cabinets, the latter capability allows fabs to place bulk gas systems indoors and outdoors.

On-site gas management programs have become very popular in silicon fabs. They are, however, not as easily supported by specially trained service personnel due to III-V's much smaller fabs. Instead, satellite programs that combine the needs of III-V fab clusters with similar requirements allow resourceful sharing of gas handling skills and safety knowledge. Intranet databases of safety and maintenance issues are critical to the success of these programs.

The growing III-V market is posing more stringent and increasing supply demands on specialty electronic gas suppliers. In addition to continued development of ultrahigh-purity gas sources for epi growth of GaAs, InP and GaN-based semiconductors, efficient and reliable gas supply and management is key to III-V development and production.