Industrial aspects of silicon molecular beam epitaxy

Vacuum/volume 41/numbers 4-6/pages 929 to 932/1990 Printed in Great Britain

0042-207X/9053.00 + .00 O 1990 Pergamon Press plc

Industrial aspects of silicon molecular beam epitaxy H K i b b e l a n d E K a s p e r , Daimler-Benz Research Institute, Sedanstr 10, D-7900 U/m, FRG

Molecular beam epitaxy of silicon based materials (Si-MBE) is a versatile method for science, and development of industry-relevant electronic materials. Future electronics will benefit from the marriage of high performance heterostructure devices with high complexity conventional integrated circuits on a silicon substrate. This paper will focus on development of equipment, growth of advanced device structures, preparation of artificial semiconductor material from superlattices, and will discuss possible routes for monofithic integration based on a low temperature process.

1. Introduction Silicon molecular beam epitaxy (Si-MBE) is a powerful tool for growing semiconductor homo and heterostructures for high speed 1, microwave 2 and optoelectronic device3 applications. Low process temperature, precise control of thickness and doping profile on a submicron scale, and flexibility in the choice of material combinations and layer structures increase the potential for sophisticated devices. Completely new material systems can be grown and bandstructures can be tailored according to specific applications4, 5. Si-MBE is an uhv process. The improved reliability of uhv components, an increased number of commercial Si-MBE-subsystems such as doping sources and electron beam evaporators, as well as increased cooperaton between research groups and equipment manufacturers, have advanced Si-MBE machines from research apparatus to small-scale production equipment 6. For an in-depth view of the Si-MBE method the reader is referred to the Proceedings of the biannual Symposia 7 and to a recently published handbook s .

2. Apparatus Si-MBE equipment for small-scale industrial production requires at least two-chamber systems: growth chamber and load lock chamber. An example of an industrial single-slice Si-MBE apparatus is shown in Figure 1: the system is described in ref 9. Two stainless steel uhv chambers are connected by a 200 mm diameter gate valve. The storage chamber accommodates the wafer transfer mechanism and a cassette-type magazine for 25 silicon wafers up to 150 mm diameter. Essential features of growth chambers are the subsystems sources, wafer heater and in situ monitoring. The elements silicon and germanium are evaporated by electron beam evaporators (EBEs), while the n-type dopant antimony (Sb) and the p-type dopant gallium (Ga) are evaporated by modified Knudsen cells. Boron (B) is the most useful p-type dopant in Si-MBE. Elemental boron has a low vapor pressure and demands high temperature effusion cells. Boron compounds B203

and HBO/dissociate at low temperature and can be evaporated from conventional effusion cells. Low energy ion implanters for As and B doping, and internal ion sources for Sb are also used. Industrial Si-MBE production machines have to fulfill the following conditions: (I) high wafer throughput, (II) uniform layer thickness and doping profiles, (III) sufficient reproducibility from batch to batch, (IV) automatic process, and (V) adaptability to clean room conditions. These requirements are only partly fulfilled by existing equipment. Future production equipment development should follow the design principle of greatest simplicitys. This design will be achieved if only absolutely necessary process steps, subsystems and components are used. A Si-MBE design based upon simplicity will result in an elegant, flexible, reliable, and cost-effective solution for industrial applications. Recalling that the principle of MBE is simpler than that of any other comparable growth method for semiconductor structures with arbitrary doping and compositional profiles, such a design philosophy would reflect the principle of MBE. 2.1. UHV. Si-MBE as a physical vapor deposition (PVD) process refines the evaporation method by condensing molecular beams on heated substrates within an uhv environment. The cleanliness of the uhv equipment and the wafer surface during all steps of MBE process are essential. Base pressure and residual gas composition of the MBE chamber during growth are dependent on the design of the equipment and subsystems, the materials, employed conditioning, and operating procedures. Modern uhv systems are manufactured from stainless steel with all-metal sealed flanges. Base pressures in the 10-x~ mbar range are routinely obtained after baking out. The vacuum pressure is limited by the outgassing of equipment materials and surfaces; permeation through the walls is nearly negligible. Hydrogen is the main detectable residual species in clean uhv systems (Figure 2). Working MBE systems are partially coated with matrix materials and dopants. The coverage depends on the source design and the evaporated material. Amorphous 929

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Figure 2. Residual gas spectrum of a well conditioned MBE chamber during MBE process. silicon films on the inside of the MBE chamber are highly hygroscopic and therefore the primary source of water vapor pressure after opening and bake out process. The water vapor pressure is reduced stepwise by combination of thermal desorption, electron and ion-stimulated desorption by EBE and gettering by silicon flux. The sensitivity of the semiconductor growth to impurity species is a very suitable measure for estimating the purity of the constituent materials and sources, and of the quality of the vacuum environment. Crystal quality and defect density are drastically limited by insufficient partial pressure, water vapor (H20) and methane (CH4) for example l°. Although Si-MBE is a low temperature deposition process, high temperature sources are necessary components. Currentheated substrata heaters, typically tantalum or pyrolytically coated graphite for wafers up to 150 mm diameter, electron beam evaporators (EBE) for the matrix element silicon and p-type doping sources (Ga, B), are thermal components requi-

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2.2. Electron beam evaporator (EBE). Generation of flux densities in the range of l0 ts to 1016 atoms c m - 2 s -1 for typical growth rates of the order of nanometers per second requires Si temperatures of approximately 2000 K. At such temperatures, the reactivity of molten Si would cause excessive contamination from a resistivity-heated crucible. Electron beam heating of the Si source surmounts this problem. A high purity monocrystalline Si ingot, mechanically formed and placed in a watercooled Cu-crucible is partially melted by a focused electron beam. The molten region is restricted to the area directly heated by the electron beam due to the thermal properties of Si. Therefore Si is effectively evaporated from a silicon-crucible. Several problems result, however, from the EBE application, the most important of which are as follows: (I) Electron and ion beam stimulated desorption of gas molecules adsorbed on the system walls and cold shrouds, (2) damage of the grown film by impinging electrons and ions, (3) interference with in situ monitoring instruments (ion gauge, mass spectrometer, high and low energy diffraction apparatus). Care must be taken to avoid sputtered Si ions being accelerated to the EBE filament region. In order to minimize these problems the EBEs are screened by cold walls with defined aperture which limit the beam cone and reduce the thermal load of the chamber walls. Covering of all metal surfaces with high purity Si panels reduces the risk of mutual effects with electrons and ions. Commerical uhv compatible EBEs are modified for MBE systems. In situ flux monitoring is performed by mass specific instruments, in particular quadrupole mass spectrometers (QMS) and electron emission spectrometers (EIES). 2.3. Doping. N-type and p-type doping over a wide range of concentrations and with abrupt profiles are necessary for novel device structures. Several doping techniques are used in SiMBE.

H Kibbel and E Kasper: Industrial aspects of silicon MBE

The simplest method is to coevaporate dopants, e.g. Sb or Ga from controlled sources during silicon evaporation. The kinetics of spontaneous incorporation has first been studied by Iyer et al ~. The sticking coefficient S, defined by the ratio of incorporated to incident dopant, is usually low and very temperature dependent. Growth of layers with different doping levels is possible by pre-build-up and flash-off techniques I1. However, the extreme temperature sensitivity and the very low absolute values of the sticking coefficient for both Ga and Sb make accurate doping control difficult. There are a number of techniques to overcome the above problem. Doping by secondary implantation (DSI) l~- has demonstrated the enhancement of Sb incorporation, which can be increased by more than two orders of magnitude. In addition very abrupt doping profiles can be grown by switching the substrate voltage. Boron is favoured in p-type doping because of its high solubility and very low segregation. Additional in situ doping techniques in Si-MBE are coevaporation of neutral dopant during deposition of amorphous films in the solid phase epitaxy (SPE) mode, and low energy ion implantation. 2.4. Wafer transfer. Wafer transfer systems in MBE techniques are an engineering challenge. The reliable function is a necessary but not sufficient postulate. Van Gorkum and coworkers 7 have demonstrated the influence of the transport process referring to the wafer defect density. Approximately 90% of the defects are generated by wafer transfer into the growth chamber and back to the magazine. Transport of wafer or wafer platen into the substrate heater position is demonstrated by several techniques. Table 1 gives a short overview of proved systems. An example for a commercial solution of a transfer system is shown in Figure 3.

Table 1. Proved wafer transfer systems

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3. Devices

Devices from Si-MBE material have been available for several years. An excellent example is found in the IMPATT diodes offered by A E G for the microwave market 13. IMPATT diodes drive the most powerful ram-wave oscillators. At 100 GHz more than 1 W output power has been obtained with Si-IMPATT's from MBE material. This is one order of magnitude greater than lnP Gunn diode output powers, and more than two orders of magnitude greater than output powers of existing three terminal devices. Further increase is expected with improved thermal design. Pulsed operation has delivered more than 40 W output power 14 in the 100 GHz frequency regime. The success of Si-IMPATT devices is due to the inherent advantages of the MBE fabrication process (multilayer structures, abrupt junctions, low temperature processing). Perhaps the most exciting development in the above frequency regime is the integration of all active and passive components (including the antennae) required for complete transmitter and receiver modules 15 (Silicon Monolithic Mm-Wave Integrated Circuits--SIMMWIC). The workhorse of modern microelectronics is the integrated circuit (IC) based on MOS and bipolar transistors. S i - S i G e

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