I. INTRODUCTION

In micro- and nanoelectronic technologies the quality of surface treatment of thin-film microcircuit substrate has great importance [1, 2], because it affects the film adhesion strength. During the production process these substrates contact with different environments, and full protection from the adsorption of various impurities is not possible. Today, many industries use the traditional liquid chemical cleaning. To provide efficiency of such cleaning it is necessary to know source and type of the contaminant, and nature of his behavior on the surface. Usually there are contaminants of different nature, organic and inorganic structure on the surface [1], therefore the cleaning is carried out consecutively by different reagents: surfactants, alcohols, alkalis and mineral acids. Expensive deionized water is used for substrate rinsing. Main operations are degreasing, etching, rinsing, drying in a centrifuge. Thus, the main problem of the liquid cleaning is high cost of the process, which includes the cost of buying new reagents, and the utilizing their waste. Moreover, these reagents harm the environment anyway. For the substrate cleaning usage of gas-discharge plasma and ion beams is known [3-6]. Replacement of cleaning liquid reagents with plasma gives high degree of purity, a reduction of reagent cost and the job time and full elimination of waste utilisation problem. Volatile compounds formed as a result of plasma-chemical reactions at the substrate surface are removed via vacuum chamber pumping. Plasma-forming gases are used as reagents, common examples are oxygen, hydrogen, argon. Today there are many plasma substrate cleaning system suggestions on the market [6-10]. However, after the purchase and application of these systems one problem still remains. Substrate contamination occurs during

unloading / loading and transportation stages between two consecutive technological operations − cleaning and film deposition, because in this case these operations are performed in different chambers and systems. Therefore, performing of substrate cleaning in vacuum chamber just prior to film deposition is preferable. Undoubtedly, the maximum adhesion strength of films can be achieved only by using deposition system with built-in plasma cleaning module. However, the cost of these universal systems is very high. Thus, in some cases the rational solution is to equip and upgrade existing deposition system at a manufacturing plant by compact module of plasma cleaning. In addition, the effectiveness of plasma treatment of the substrate surface can be increased using ion bombardment. It allows combining both chemical and physical treatments. Considering these circumstances, large area plasma sources which operate in ion emission mode [11-13], can be potentially used for substrate cleaning directly before film deposition. However, the choice of a suitable plasma emitter is often difficult and there is important question of comparative quality indicators of proposed ion-plasma cleaning. Thus, the purpose of this paper is to conduct research aimed at identifying the performance of ion-plasma and liquid acid-alkaline cleaning methods and validate the expediency of using these treatment types at manufacturing plant.

II. METHODOLOGY

The studies were conducted in a manufacturing

“Stavropol radioplant “Signal”. “Polycor” plates (glass-ceramic composite based on polycorundum Al2O3) of 60×48 mm are used by this company as substrates for integrated hybrid microcircuits of high frequency electronics [1].

One group of these substrates were cleaned by liquid reagents according to the current manufacturing technologic multi-step process, involving alcohol

Ion-plasma Cleaning Module for Vacuum Thin-film Deposition Systems as Alternative to Acid-alkaline Cleaners

E. F. Shevchenko and M. Yu. Shevchenko

Scientific Educational Center of Nanotechnologies, North Caucasus Federal University, Russia

Abstract—The ion-plasma flow generated by a novel DC gas discharge plasma source with cold hollow cathode and

3D control system of current density distribution was applied for the cleaning of thin-film microcircuit substrates on a microelectronics factory. The atmospheric air was used as the plasma-forming gas. A separate group of substrates was cleaned by liquid reagents according to the current manufacturing technological multi-step process. The comparison of performance indicators of the ion-plasma and liquid cleaning was carried out. The hydrophilicity and adhesion strength of the films for all treated substrates were tested. The improved method for determining the adhesive strength of the films was developed. Conclusions about the expediency of the using of the ion-plasma cleaning for substrates in the thin-film microcircuit production process as an alternative to liquid acid-alkaline cleaning were made.

Keywords—Gas discharge plasma, ion source, substrate cleaning, film deposition, thin-film microcircuits

Corresponding author: Evgeny Shevchenko e-mail address: e.shevchenko@mail.com

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degreasing, rinsing in an ultrasonic bath with distilled water at 75ºС, heating in three bathes: ammonia-peroxide solution (NH4OH : H2O2 : H2O = 1: 1: 4), 36% hydrochloric acid and 80% sulphuric acid. After each bath step substrates were rinsed with distilled water. Finally, substrates were rinsed in a three-stage bath with deionized “A” mark water and dried in a centrifuge. Total time chemical cleaning process took is 180 minutes.

Another group of the substrates was treated by ion-plasma flow (IPF) in vacuum coater with the developed

plasma source of large area (Figs. 1, 2) for 10 minutes. In comparison with well-known systems based on standard filament‐type cathodes such as Kaufman type sources [12, 14], the proposed discharge system with cold cathode has longer operating life in a reactive plasma.

However, the main feature of the applied ion-plasma source is a 3D control system of current density distribution on the emission surface with respect to two coordinates (x, y)  and on the axis of the source (third coordinate, z) [13]. The control of size and shape of the

Fig. 1. Scheme of the ion-plasma source (a) and various positions of control electrodes at treatment with circle (b) and rectangular (c) IPF profile: 1 – hollow cathode, 2 – anode, 3 – ring magnet (B ≈ 80 mT), 4 – cathode-reflector – walls of vacuum chamber, 5 – control electrodes, 6 – carousel with substrates, 7, 8, 9 – power supply sources, 10 – isolator, 11 – treated area, 12 – table with substrates, φ1 = 20º and φ2 = 70º – min

and max tilt angles of control electrodes.

Fig. 2. The ion-plasma cleaning module with control electrodes developed with «Solid Works» computer program (a) and assembled after production (b).

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IPF cross-section was effected by varying: the tilt angles φ of the control electrodes 5 and their potentials, the currents of auxiliary and main stages of the discharge I1 / I2. It should be noted that IPF cross-section shape may be varied: circle (Fig. 1b) or rectangle (Fig. 1c) without loss of ions. All technologic vacuum coaters have specific shape, size, constriction, but the developed ion-plasma module can be optimally tuned for each vacuum system without constructional changes. Using control system the ion-plasma source was easily adapted to the geometric characteristics and other features of industrial vacuum coater UVN-75 and the IPF with rectangular cross-section of 220×160 mm was formed for energy-efficient ion-plasma treatment on a rotating substrate holder carousel (conveyor style of process) with good evenness of the treatment (unevenness does not exceed ± 10%). During the process of ion-plasma cleaning, paired electrodes 5 along the axes x were tilted at φ = φ2 = 70º, auxiliary stage current I1 was 150 mA, currents on the paired electrodes 5 along the axes x and y were I2x = 150 and I2y = 350 mA respectively and IPF current was 50 mA at acceleration voltage of ions of U3 = 560 V along the axis z.

Non-pretreated atmospheric air is supplied into the hollow cathode 1. Gas pressure in the vacuum chamber was 50 mPa. Usage of atmospheric air as the plasma-forming gas is explained by the following factors. First, it is easily accessible and free substance that always surrounds us. Secondly, the air is not dangerous for the environment. Third, the air contains oxygen ions which effectively remove organic contaminants. Fourth, as it was shown our previous research [15], used discharge system could be stably lit for a long time with air.

As for the analysis of used methods of substrate surface purity evaluation, it is necessary to note the following. It is known the hydrophilicity of the substrate surface is an important criterion in assessing the degree of purity, which gives information about concentration of hydrophobic organic contaminants on the surface. For hydrophilicity test the deionized water was sprayed over substrates with various types of cleaning and then the water spreading was fixed on of the surface after 60 seconds from the spraying (Fig. 3).

The test of film adhesion strength was carried out according to our designed method, because known standard methods based on the scribing [16, 17] or the normal pattern set apart of a certain portion of the film area, were not applicable due to the high adhesion strength and hardness of the resistive and adhesive underlayer RS3710 (37% chromium, 10% nickel, 53% silicon). In this case, the obtaining of scribing effect was not possible, and the evulsion of the film with a predetermined pattern area in a number of experiments turned out to be incomplete, since the substrate had «islands» of covering intact. Proposed technique of determining the high-value adhesion strength was as follows. Thin-film RS3710-Cu structure was deposited on the cleaned substrates by magnetron sputtering. Butts of wire segments were soldered to four evulsion points on the edge of structure (Fig. 4a). Normal evulsions and synchronous measurements of evulsion force were carried out with dynamometer. Evulsion points were situated on the edge areas of structure, because their adhesion strength is usually less than in the center pieces. Thereafter, areas of removed thin-film complex figures were defined with computer microscope. Graphic program converting reduced color depth of microscope images (Fig. 4b) to monochrome format (Fig. 4c). Then the program calculated quantity of white pixels on the every image. White pixels form useful interface − figure of removed film, their quantity is associated with the area of this figure proportionally to the scale and ratio of pixels per square meter. The film adhesion strength was calculated using the following expression:

чп

бп

F NP

S M N

(1)

where F - evulsion force, S□ - area of rectangular image, which includes useful interface (white pixels), Nw and Nb − quantity of white and black pixels, respectively, M - scale. Obtained data of four experiments for each sample were averaged.

Fig. 3. Scheme of the hydrophilicity test: 1 – light source, 2 – camera 3 – computer and photoprinter, 4 – water sprayer, 5 –

spreading water, 6 – substrates.

Fig. 4. Adhesion strength definition: points of film evulsion 1 - 4 at

the edge parts of the thin-film structure (a), typical photo of the interface obtained with the microscope (b) and converted with a

graphical editor for calculating quantity white pixels and area (c).

Shevchenko et al. 159

This technique of area definition has some advantages in comparison to the procedure employed in [18] based on setting of contour of figure and area calculation using mathematical functions such as definite integrals. Advantages include increase in the speed and accuracy of the calculation due to automatic processing of digital photos with consideration of the smallest features of a complex figure: interior «islands» and external protrusions.

III. RESULTS AND DISCUSSION

Results of hydrophilicity test for microcircuit substrates that undergone different types of cleaning are shown at Fig. 5a. Superior hydrophilicity obtained in the case of ion-plasma cleaning, denominated in the maximum coverage area of spreading deionized water, indicating the lowest content of hydrophobic organic contaminants on the substrate surface.

Measurements have shown that adhesive strength of microcircuit thin-film layers increased significantly in the case of the ion-plasma cleaning in the technologic process (diagram on Fig. 5 b).

The scatter of the adhesion strength for various evulsion points on a single microcircuit after ion-plasma cleaning did not exceed 7%, and 10% after chemical relatively to the average value. The difference in the magnitudes of average adhesion strength for the side and central substrates mounted on a substrate-holder carousel 7 (Fig. 1) does not exceed 5%. These results indicate high evenness of substrate cleaning with proposed ion-plasma module.

It should be mentioned that in case of liquid chemical cleaning following tendency is observed. The values of adhesion strength at evulsion points 3 and 4 (Fig. 4) for all substrates that undergone chemical cleaning are 20–30% lower than at points 1 and 2. Apparently, it is due to the vertical position of substrates in the baths of chemical reagents and location of points 3 and 4 is near the bottom of the bath. The regulations of manufacturing plant allow re-use of some reagents in such baths, for example chromate salts and strong acids, and it is possible reason of our observations. In these cases the bottom of baths can contain substances that unexpected by technologic process (sludge). It degrades the quality of liquid cleaning, especially for bottom substrate area (points 3 and 4).

The process of low-energy ion-plasma treatment of the substrate surface includes two sequential stages:

I. etching of a contaminants / oxides layer, II. modification of the basic substrate material

surface layers. The time that required for the completion of the first

step is unknown. Furthermore, for each loaded substrate the time has different values and depends on the chemical composition, thickness and uniformity of contaminant layer spreading. Therefore, stage separation is very difficult. A technologist must envisage all consequences of each stage.

Both stages of the ion-plasma treatment are characterized by the same operational parameters. Primarily it is composition and pressure of plasma-forming gas, which in current case is air. In a well-ventilated room air is a mixture of gases with content in mass percents: N2 - 75.5%, O2 - 23.15%, Ar - 1.292%, CO2 - 0.046%, Ne - 0.0014%. There are also other components with less than a thousandth of a percent content, and water vapor (0.05-0.5%). During the 10-minute treatment 30 cm3 of air were supplied to the ion-plasma module where all air components were ionized by electron impact in the glow discharge. It is known that the plasma density in the hollow cathode of reflex discharge is 1012-1013 cm-3. At this plasma density the IPF contains a large amount of radicals that are involved in etching. They are simultaneously involved in different surface processes such as adsorption, chemisorption, physical destruction and sputtering. Nitrogen ions behave more like particles of inert gases, because these ions are not chemically active at given treatment conditions (low temperature of plasma and the substrates). Particles of inert gases destroy the contaminant layer via physical mechanism of impulse and energy transmission. It is especially effective for removal of contaminant nanoislands which did neither enter chemical reactions nor form the volatile compounds during process. Ion bombardment does not change the chemical nature of the etching, but it causes the activation of the surface due to rupture of atom bonds which have weak adsorption to the surface, and activation of plasma particles chemisorption.

Fig. 5. Results of the hydrophilicity (a) and the adhesive strength (b) tests of the microcircuit substrates after various types of cleaning: 1 – liquid, 2 – ion plasma, 3 – dedusting by gas stream of nitrogen, 4 –

liquid + ion-plasma (sequentially).

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Therefore, reaction rate between plasma particles and surface increases, some reactions that do not occur under usual conditions (without ion bombardment) initialize. It is must be noted that stimulation of chemical reactions is an important function of ions of nitrogen (main component of air) and inert gases. This function can come to the first plane at low ion energy (up to 1 keV) when the sputtering rate is small.

Thus, the combination of physical and chemical ion-plasma etching has the effect of multiplicative amplification, and air is a natural mixture of components which is quite suitable for ion-plasma etching of conventional contaminants. It can be assumed that such effect can be also achieved by usage of argon-oxygen plasma-forming mixture with approximate ratio of 1:1. Mass, impulse and energy of argon ions are higher than analogical parameters of nitrogen ions; therefore, argon ions make a greater contribution to the surface cleaning. Both air and argon-oxygen mixtures are non-toxic and environmentally clean. However, the usage of air as plasma-forming gas has some obvious advantages: this substance is always available, free of charge, does not require any means for transportation and storage. It should be noted that the developed ion-plasma module operates in stable way in both air and argon-oxygen flow due to usage of low-voltage glow discharge with cold cathode.

The composition of the plasma gas is not the singular critical parameter that determines results of ion-plasma treatment. Since substrate material is a dielectric, processing is hampereddue to uncompensated positive charge on the substrate surface. Furthermore, sometimes at low pressures there is volume charge in the ion accelerating gap. During substrate treatment process, these charges must be compensated by electrons or surmounted by high energy of accelerated ions (typically above 10 keV). However, usage of high-energy ions is undesirable due to probability of emergence of surface radiation-stimulated defects. It is known that at gas pressures p ≥ 20 mPa surface charge is compensated by the electrons from a specific formation, known as ion-beam plasma [19]. Formation of ion-beam plasma explains ion current I3 increase from 30 to 50 mA with synchronous increase of pressure from 10 mPa to 50 mPa, when approximately 40% of ions are created in the accelerating gap. Thus, I3 = 50 mA and p = 50 mPa (used experimental parameters) ion-beam plasma contains enough electrons for charge compensation. Ion energy depends on the accelerating voltage U3 = 560 V and partly on the discharge voltage and is less than 1 keV. With U3 increase current I3 and etching speed grow, but also ion energy increase and it connotes danger of surface radiation-stimulated defects. Therefore, the value of U3

that is close to optimal (near 0.5 kV) was used in our experiments.

After the contaminant layer removal, step II - modification of the surface of the main substrate material, polycorundum ceramic Al2O3, follows. However, due to high chemical inertness and low sputtering coefficient of this material, as well as relatively low ion energy the modification appears to be extremely low since almost no

ions are implanted and their chemical reactions with polycorundum at low temperature are minimal. At noted conditions even reactive oxygen particles cannot cause modification of the ceramic. There is possibility of defect formation in the crystal structure under the influence of plasma-emitted UV rays. This effect is observed for some crystalline and polycrystalline Al2O3 modifications, as their optical properties can be changed by UV radiation.

Thus, all kinds of plasma emissions, such as ions, radicals, neutrals, electrons, UV photons directly involved in surface treatment. Most of them manifest themselves on stage of contaminant layer removal via physical or chemical mechanisms, but some of them can modify the substrate surface. Anyway, results of ion-plasma processing can be monitored by critical process parameters: composition and pressure of plasma-forming gas, energy and density of ions, electric potentials and geometric factors of the discharge chamber.

IV. CONCLUSION

The usage of the ion-plasma substrate cleaning in a

vacuum coater with developed ion-plasma module was able to increase adhesion strength of functional microcircuit thin-film layers deposited after the cleaning in the same vacuum cycle. Ion-plasma cleaning was carried out with high energy efficiency through new 3D ion current density control system.

In addition to the quality improvement (hydrophilicity, adhesive strength, homogeneity of treatment), replacement of liquid cleaning by ion-plasma cleaning gives drastic reduction of material expenses, that mainly related to reagent purchasing and work hour payments, it also gives the possibility to renounce usage of harmful or dangerous for the environment substances.

Results of experiments of the substrate hydrophilicity and the adhesive strength of the films assessment showed that the ion-plasma cleaning has higher efficiency than liquid chemical cleaning. In addition, the application of ion-plasma cleaning as an alternative to liquid cleaning can achieve significant economic effect associated to the reduction of material expenses. Thus, ion-plasma cleaning allows to improve qualitative and economical indicators.

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