Low Temperature Epitaxy

Silicon epitaxy by CVD at lower temperatures is now' of significant interest. Silicon epitaxy has been effectively demonstrated as:

• • In the 800-1000°C range in a conventional reactor using SiH₄ or SiH,Cb after first initiating growth at a higher temperature.
• • In the 800-1000°C range using SiH₄ and substituting He for H-, as the main carrier gas.
• • In the 850-1000°C range in a conventional reactor using SiH₄ with wafers that were etched in HF just prior to placing in the reactor.
• • In the 850-900°C range in a conventional reactor using SiH-,CL, at reduced pressures of 10-20 Torr.
• • In the 800-900°C range using photo dissociation of silane and other silicon compounds.
• • In the 750-950°C range in a cold wall reactor after removing the native oxide with a plasma etch.
• • In the 700-900°C range by operating at veiy low' pressure in a load locked, hot w'all reactor.

All these low' temperature epitaxy processes have limited application because they require very low' growth rates to achieve even partially satisfactoiy ciystal quality. As the required epitaxy layer thicknesses go below' 0.6 pm, such techniques must be further developed and characterized for commercial production.

Physical Vapor Deposition (PVD)

PVD is a combined set of processes used to deposit thin layers of material, typically in the range of few nanometers to several micrometers. PVD is basically unlimited choice of coating materials: metals, alloys, semiconductors, metal oxides, carbides, nitrides, cermets, sulfides, selenides, tellurides etc. PVD process containing of three fundamental steps:

• • Vaporization of the material from a solid source supported by high temperature vacuum or gaseous plasma.
• • Transportation of the vapor in vacuum or partial vacuum towards the substrate surface
• • Condensation onto the substrate to generate thin films

Physical Vapor Deposition (PVD) comprises of different methods, such as evaporation, sputtering, and molecular beam epitaxy (MBE)

• • Evaporation: Material is heated to get gas phase below its melting temperature, where it then diffuses by high vacuum to the substrate.
• • Sputtering: Plasma is created first which contains ions and electrons. Next, atoms from the target are ejected after being struck by ions. The atoms from the target then travel across the plasma and form a thin layer on the substrate.
• • Molecular beam epitaxy (MBE): The substrate is cleaned then loaded into a chamber that is evacuated and heated to drive off surface contaminants and to roughen the surface of the substrate. The molecular beams emit a small amount of source material through a shutter that is collects on the substrate.

PVD is used in a variety of applications, including fabrication of interconnects, microelectronic devices, diffusion banders, optical and conductive coatings, surface modification, battery and fuel cell electrodes. We will discuss detailed PVD application in Metallization chapter (Chapter 8). Here we will focus on MBE process of epitaxy.

Molecular Beam Epitaxy (MBE)

MBE, a technique of vacuum evaporation is one of the easiest and oldest techniques of depositing solid films. Although vacuum evaporation was used as early as in the 1950s for preparing semiconductors, epitaxial growth conditions were not realized until improvements occurred in Ultra-High Vacuum (UHV) technology, in the design and control of the sources and substrate cleaning procedures. MBE has now become a versatile technique for growing epitaxial thin films of semiconductors, metals and superconductors.

A functional schematic diagram of a MBE system is shown in figure 2.20. It consists of a growth chamber and auxiliary chamber (not present with first generation systems), diffusion pumps and a load-lock. Each chamber has an associated pumping system. The load-lock facilitates the introduction and removal of samples or wafers without significantly influencing the growth chamber vacuum. The auxiliary chamber may contain supplementary surface analytical tools not contained in the growth chamber, additional deposition equipment and other processing equipment. Separating equipment in this manner allows for more efficacious use of the growth chamber and enhances the quality of operations in both the auxiliary and growth chambers.

Fig. 2.20 MBE System

The growth chamber is shown in greater detail in figure. 2.21. Its main elements are: sources of molecular beams; a manipulator for heating, translating and rotating the sample; a cryoshroud surrounding the growth region; shutters to occlude the molecular beams; a nude Bayard Alpert gauge to measure chamber base pressure and molecular beam fluxes; a RHEED (Reflection Electron Diffraction) gun and screen to monitor film surface structure and a quadmpole mass analyzer to monitor specific background gas species or molecular beam flux compositions. The auxiliary chamber may be host to a wide variety of process and analytical equipment. Typical surface analytical equipment would be: an Auger electron spectrometer, or equipment for Secondary Ion Mass Spectrometry (SIMS), ESCA (Electron Spectroscopy for Chemical Analysis) or XPS (X-ray Photoelectron Spectroscopy). There may be a heated sample station and an ion bombardment gun for surface cleaning associated with this equipment. Process equipment may include sources for deposition or ion beam etching.

Fig. 2.21 Schematic cross-section of a typical MBE growth chamber.

Fig. 2.22 Effusion Cell

The principle underlying MBE growth is quite simple: it basically consists of atoms or clusters of atoms, which are produced through heating up a solid source. They then move in an UHV environment and impose on a hot substrate surface, where they can diffuse and eventually incorporate into the glowing film. The MBE growth process involves controlling to achieve epitaxial growth via shutters, source temperature, molecular and/or atomic beams directed at a single crystal sample (suitably in-situ heated). The beams are thermally generated in Knudsen-type effusion cells (shown in figure 2.22) that contain the essential elements or compounds of the desired epitaxial films. The temperatures of the cells are precisely controlled to give the thermal beams of appropriate intensity. The beam fluxes emerging from these nonequilibrium effusion cells are generally determined experimentally in most cases using movable nude ionization gauge placed in the substrate position. The cells are made from Pyrolytic Boron Nitride (PBN) or high purity graphite materials, those are non-reactive, refractory materials that can withstand high temperatures and strictly do not contribute to the molecular beams.

The cell consists of an inner crucible and an outside tube which is wound with Та or Mo wires for resistive heating. The various cells are all placed and angled in such a way that their beams converge on the substrate for epitaxial growth. A chemically stable W-Re thermocouple facilitates precise contr ol of the cell temperature which is very essential for achieving constant growth rates since small temperature fluctuations of the order of ±1°C can result in ±2 to 4 percent fluctuations in molecular beam intensity. Individual shutters provided for each cell and the cell temperature can be computer contr olled to achieve high reproducibility with little human interventions. Tire cells are individually surrounded by a liquid nitr ogen covering to prevent cross heating and cross contamination. For group V elements, a high temperature cracker which dissociates the tetramers to dimers, with internal buffer is incorporated at the exit end of effusion cell. The gas background necessary to minimize unintentional contamination is predicated by the relatively slow' film growth rate of approximately lpm/h and is usually in the 1СГ¹¹ torr range. At this pressure, the mean free path of gases in the beams themselves is several orders of magnitude gr eater than the normal source-to-sarnple distance that of about 15 cm. Hence, the beams impinge unreacted on the sample wdth a cryo-shroud cooled by liquid nitrogen. Reactions take place predominantly at the substrate surface where the source beams are incorporated into the developing film. Proper initial preparation of the substr ate will present a clean, single crystal surface upon which the developing film can deposit epitaxially. Actuation properly and timely of the source shutters allows film growth to be controlled to the monolayer level. Monolayers level formation ability and precisely control epitaxial film growth and composition has attracted the attention of material and device scientists towards MBE.

Silicon MBE is performed under UHY conditions of 10'⁸ to 10“¹⁰ ton. The mean free path of the atom is given by 5x 10^_3/P where P is the system pressure in Toit. Transport velocity is dominated by thermal energy effects at a typical pressure of 10“⁹ ton and L is 5 x 10⁶ cm. The lack of inteimediate reactions and diffusion effects, coupled with relatively high thermal velocities, results in film properties changing rapidly with any change of the source. The typical growth temperature in order to reduce autodoping and out-diffusion is between 400°C and 800°C. Growth rates are in the range of 0.01 to 0.3 pm/minute.

Fig. 2.22 MBE Chamber

Molecular Beam Epitaxial growth technique has a number of advantages over other techniques. A particular advantage is that it pennits growth of ciystalline layers at temperatures where solid-state diffusion is negligible. Since chemical decomposition is not required for growth, deposition species need require only enough energy to migrate along the substrate surface to crystalline bonding site. The impurity dopant incorporation during molecular beam epitaxial growth is possible by having an additional source of the dopant. As a result, MBE has rapidly established itself as a versatile technique for growing elemental and compound semiconductor films. Thus using MBE, it is possible to produce multilayered structures including superlattices with layer thickness as low' as 10 A for DH lasers and waveguide applications.

How'ever, there are few' limitations in the epitaxial growth of compound semiconductors by MBE technique. The ultra-high vacuum apparatus is very expensive. Frequent shutdowns are required to replenish the source materials and opening the UHY apparatus. A major problem is the difficulty in glowing phosphorus-containing materials.