PLASMA PROCESSING REACTOR ON A BASE OF BEAM PLASMA DISCHARGE

Scientific supervisor: Evgeniy G. SHUSTIN, Dr. Sc., head of research group of Kotel'nikov IRE RAS (Fryazino branch),

Tel. (7-49656)52569� E-mail: [email protected]

 In plasma processing reactors used for deposition of thin films, etching and surface modification of materials for nano- and microelectronics, control of characteristics of the ions acting upon the processed material is highly important for the optimization of the quality of processing. In ion etching, an ion energy distribution function (FRIE) and an angular dispersion of ions reaching the surface of the material have critical influence on the speed and level of anisotropy of etching. Control of the spatial distribution of the bombarding particles is important for the deposition of films with a very homogeneous structure.

At IRE RAS the effect of ion beam emission from the region of beam-plasma discharge (BPD) with energy best suited to the tasks of etching and deposition of materials for microelectronics has been discovered. The mechanisms of ion flow acceleration have been investigated, ways have been determined and means have been created for control of energy and fluency of ions. A computer simulation has revealed the mechanism of the ion flux emission phenomenon and has determined qualitative characteristics of a beam-plasma reactor for different ways of controlling energy of the ion flux.

Our studies [1-7] showed that the source of a flow of ions with controllable energy and geometry could be created on a base of beam-plasma discharge (BPD). This discharge is excited by an electron beam with energy ~2 keV and current density >0.1 A/cm ² in a gas of low pressure (0.01-0.1 Pa) at a small magnetic field. Detected energy range of argon ions 20-70 eV is the optimal range for soft etching of AIIIBV compounds and heterostructures based on them with inert gases (without the involvement of reactive media): the ions with lower energy practically do not cause sputtering, the ions with greater energy create radiation structural defects of the semiconductor. Due to high efficiency of ionization in the BPD and to an ion escape on normal to the axis of the discharge problems of the hot cathode lifetime and contamination of the plasma with cathode erosion products are largely removed. Recent research showed the possibility to change average energy of ions bombarding the surface outside of the discharge in the range of 10-150 eV at small variation of the discharge parameters.

The most serious problem that occurs at structure etching and film deposition at electrically isolated surfaces is the surface charging with an uncompensated ion flow that decelerates ions and, consequently, leads to loss of productivity.

By analogy with the method used at the RIPT technology with an inductive RF plasma source, at early stages of this work it was suggested to apply the high frequency modulation of the substrate holder voltage. When the voltage U₀(t) at frequencies of 100 - 1000 kHz is applied to the holder, the voltage U_p(t)=f(U_o, C_p, C_s, Y_sh) appears on the surface contacting with plasma (where C_p is capacitance of plasma sheath, Y_sh - its (nonlinear) conductance, C_s is capacitance of substrate). If U_p(t) changes sign in the part of period of the oscillating voltage, electrons neutralizing the ion charge will be collected on the surface.

As shown by simulation, this way to compensate a charge of the surface contacting with plasma has limited use: in the case of large substrate thickness and its low dielectric constant, voltage drop on the substrate capacity is too large. Increase of the modulating voltage leads to an ion energy gain and the increase of portion of the period during which the surface is affected by ion current. However, there is a simultaneous unacceptable spreading of the distribution function and the corresponding increase in the concentration of radiation defects in the mode of etching. In the mode of deposition, this spreading leads to the loss of DLC films� quality.

An alternative way has been proposed to control the floating potential on the surface of electrically isolated structures - the modulation of the plasma potential due to collector supply with pulse voltage. In a DLC film deposition mode (see lower) this voltage feeds a special electrode at region of the discharge. A specific modulator has been designed and manufactured for this aim. The modulator provides the generation of pulsed voltage with amplitude of 50-150 V at load ≤1 A in the range of operating frequencies 100-400 kHz. The modulator can operate at off-duty factor ≥ 2.

 Accomplished technologies.  

1. Low energy etching of heterostructures

The test of etching technology with ion flows from BPD has been conducted for semiconductor heterostructures Al-GaAs/InGaAs/GaAs (P-HEMT), grown on GaAs substrates. These structures are promising in creating the microwave field transistors of millimeter band. The effect of such treatment on the concentration and mobility of the electrons was studied. These parameters are sensitive to defects caused by the etching process. We have shown the existence of the etching effect at a rate acceptable for industrial applications, without the heterostructure parameters� degrading indicating a low-density radiation-induced disorders and the possibility of using BPD in manufacturing technology of HEMT heterostructure microwave devices [3, 6].

Russian patent on �Method of plasma chemical etching of semiconductor and dielectric materials� ((�2316845, priority date 06.06.2006 g., entr. � 021244, reg. 10.02.2008). Applicant � Kotel'nikov Institute of Radio Engineering and Electronics of RAS, the authors of the invention � N.V. Isaev, Yu.V. Fedorov, E.G. Shustin.  

2. Production of graphene (the work is done in conjunction with the laboratory 184 � http: / / www.cplire.ru /nano /index.html) 

A monolayer of graphite - graphene, isolated recently in a free state under normal conditions, demonstrates unique physical properties of two-dimensional system containing massless carriers - Dirac fermions, as well as high promising application capabilities in creating new generation of field effect transistors, transparent conductive electrodes, etc. The first samples of graphene, obtained by mechanical cleavage of graphite, have a lateral size of the order of several microns and can be used only for laboratory research. Practical realization of nanostructures based on graphene in electronics and optoelectronics has raised the question of obtaining high quality, large area graphene samples.

Significant progress has recently been achieved by the method of chemical deposition of graphene from the gaseous phase (CVD method) on the Ni-substrate, followed by transferring it to arbitrary substrate. Although, thus obtained solid graphene films have a large area, they have microscopically large variations in thickness from ten to one graphite layers. Relevant domains of one-and two-layer graphene have a lateral size of about 5 microns.

An alternative method for obtaining graphene films is proposed [7] by means of etching of thin defect-free single graphite crystals in the reactor based on beam-plasma discharge. The perspective of this method is determined by the high structural perfection of initial single graphite crystals to be etched and their large area.

The graphite crystals with a thickness of 30-100 nm and lateral size of several hundred microns were obtained by cleavage of single crystals of natural graphite with adhesive tape. Tape adhesive agent was then dissolved in acetone, and thin crystal became free floating in the solvent. At the next stage the crystal was transferred to the firm substrate.

At the last stage the crystal was thinned by means of plasma etching which was carried out in the beam-plasma reactor. The argon-ion energy was 80 eV at the initial stage of etching and decreased to 60 eV at the final stage, thus ensuring flawless soft mode of etching at a rate about 10 nm/hour. The thickness of films was monitored by their resistance in situ in the plasma reactor chamber. Etching was stopped at the sheet resistance of the film ~ 1 kOhm, corresponding to conductivity of 1-2-layered graphene. Scanning the local Raman spectra with a step of 0.2 mm showed high structural perfection of thus obtained 1-2-layered graphene films (fig. 2) and its uniformity in thickness over a large area in excess of 100*100 mm².

The method of this production of one- or two-atomic layers can be extended to other layered materials. To make the process of thinning not too long, the thickness of the original crystal should not exceed 50-100 nm. Single crystal to be thinned can be put be put onto any substrate. Therefore, the proposed method can be implemented in any substrates, including flexible ones.

Application for Russian patent �Method of obtaining atomic-thin single-crystal films�, � 2009142861 dated 23.11.2009.  

3. Deposition of diamond-like carbon films (work done in conjunction with the laboratory 197 - http://fire.relarn.ru/index.htm?main=197/index.htm)

In the worldwide practice low-pressure plasma reactors based on RF and microwave discharges are widely used for the deposition of diamond-like carbon (DLC) films. The film characteristics are critically dependent on the composition of the plasma creating gas, the discharge regime parameters and the substrate surface quality. In [8] a special modification of the BPD, which we called �reflective BPD�, is proposed to be used for the deposition of DLC films. At this modification graphite disc with a diameter of 10 cm is used as a collector and a target, which is fed with cathode potential. Thus, the target is bombarded by the flow of ions of energy up to 2 keV from the discharge. In this mode, the electron density of plasma increases (1,5 - 3 times in relation to the usual scheme) and plasma potential relative to the chamber walls decreases. Oscillating voltage is applied to a special modulating electrode (see. figure 1) to control the ion flux density and energy. This modification of plasma-chemical reactor for deposition of DLC films differs from the known methods by simple control of the energy characteristics of the ion flux affecting the film during the deposition. Samples of DLC films on metallic substrates have been produced. The effect of adsorbed water vapor and alcohol on the electrical properties of films has been revealed by means of the method of charge-relaxation spectroscopy, that indicates the possibility of using the film as an active adsorbent material for chemical sensors. Studies of the influence of ion energy and other reactor parameters on the electrical properties of the deposited films continue.

 MAIN REFERENCES

1. N. V. Isaev, A. I. Chmil�, and E. G. Shustin. Ion Flows from a Beam�Plasma Discharge //Plasma Phys. Rep. 2004; 30: p.263
2. N. V. Isaev, A. A. Rukhadze, and E. G. Shustin. Mechanism for Ion Acceleration along the Normal to the Axis of a Beam�Plasma Discharge in a Weak Magnetic Field Plasma //Phys. Rep. 2005,31, p. 953  
3. N.V. Isaev, V.P. Tarakanov, E.G. Shustin Ion flows from area of beam plasma discharge at low magnetic field � physics and application //Problems of atomic science and technology, Serial 5. �Plasma electronics a. New methods of acceleration�, NAN of Ukraina, �, 2006 No5, p. 100
4. N. V. Isaev and E. G. Shustin. Acceleration of Ions in a Beam�Plasma Discharge in a Low Magnetic Field:Interrelation between the Electron and Ion Energy Distributions //Plasma Phys. Rep. 2007; 33 p. 38  
5. V. P. Tarakanov and E. G. Shustin. Dynamics of Beam Instability in a Finite Plasma Volume: Numerical Experiment. //Ibid.,p.130
6. E.G. Shustin, N.V. Isaev, M.P. Temiryazeva, Yu.V. Fedorov. Beam plasma discharge at low magnetic field as plasma source for plasma processing reactor. //Vacuum 2009, v.83 No11, pp.1350
7. Yu.I. Latyshev, �.G. Shustin, �.Yu. Latyshev, N.V. Isaev, �.�. Schekin, V.�. Bykov. Novel method of production of graphene films of large area inplasma discharge. //Report on 2 Intern. Nanoforum, Moscow 2009, http://rusnanotech09.rusnanoforum.ru/Public/LargeDocs/theses/eng/poster/09/10_Latyshev.pdf
8. N.V. Isaev, I.L. Klykov, V.V. Peskov, E.G. Shustin. Beam plasma processing reactor at diamond-like film deposition mode// XXXVII international conference on plasma physics and CF, February� 8 � 12, 2010, Zvenigorod. http://www.fpl.gpi.ru/Zvenigorod/XXXVII/Pt/en/OZ-Shustin_e.doc

 

Beam plasma reactor schematic