Mass flow limitations in reactive sputtering

Reactive sputtering may be accomplished by mixing the inert gas argon with some reactive gas during sputtering. To form ZrN, sputtered zirconium atoms must react with nitrogen. At a specific deposition rate, a certain number F_N0 of nitrogen atoms must arrive at the substrate. This requirement is fulfilled at the partial pressure P_N = P_N0. The minimum partial pressure P_N0 to obtain a nitride is often detected by a sharp decrease in sputtering rate at a specific N₂ partial pressure. However, during reactive sputtering of for example ZrN we found that the value of P_N0 was strongly dependent on the total gas throughout in the vacuum system. An increase in total gas throughput causes a decrease in the value of P_N0.

The decrease in sputtering rate has indirectly been measured by optical emission spectroscopy from the sputtering plasma.     

反应溅射的PID神经网络控制仿真

在反应溅射工艺中,反应气体流量和沉积速率、反应气体气压、靶电压之间存在迟滞关系.为了保证反应溅射稳定以及保持溅射率高和反应气体利用率高,需要对反应溅射系统进行精确控制.PID神经网络(PIDNN)控制器具有PID控制的快速收敛特点和神经网络的非线性逼近能力,适合对非线性反应溅射系统的控制.本文用PID神经网络(PIDNN)控制器对反应溅射系统进行精确控制.在S.Berg给出的反应溅射系统模型基础上,对反应溅射系统的PIDNN控制进行仿真.仿真结果表明,PIDNN控制器能够稳定反应溅射过程,并且收敛速度较快,输出响应迅速,抗干扰能力强.     

Effect of reactive gas mass flow on the composition and structure of AIN films deposited by reactive sputtering

The mass flow of the reactive gas during sputtering of AIN has been studied in order to control film stoichiometry as well as film structure and morphology. It has been observed that the film composition and sputtering rate depend strongly on the deposition conditions and, in particular, on the mass flow and the sputtering pressure. It has been possible to deposit transparent, almost stoichiometric as well as non-transparent, metal-rich AIN films. The metal-rich films had a sputtering rate which was a factor of six higher than that for the stoichiometric films. The crystalline texture and the morphology have been studied as a function of the total sputtering pressure for both film types. For the transparent films, this novel deposition procedure may be applied in surface acoustic wave devices. The non-transparent AIN films exhibit some similarities to films obtained by ion vapour deposition or ion-beam-enhanced deposition methods, and the films exhibit interesting tribological properties.     

Electrical resistivity, structure and composition of d.c. sputtered WNx films

WN_x films have been deposited by reactive d.c. diode sputtering in an Ar-N₂ gas mixture on Si substrates. The electrical resistivity, structure, microstructure and composition of the films were investigated. The parameters were the total and partial pressures, the discharge current and the negative bias voltage applied to the substrate. We have found that good W₂N films are obtainable with a dense structure and a resistivity of near 200 μΩ cm, but either the resistivity or the composition and structure suffer great variations with the discharge parameters. Depending on the nitrogen partial pressure and the bias voltage, the deposited layers are W₂N films, W₂N-W mixtures or W films.     

On the investigation of d.c. plasmatron discharges by optical emission spectrometry

We investigated the emission spectra resulting from high rate plasmatron sputtering in order to gain further information on the coating process. The measured relations between the peak intensities of the target material and such plasmatron discharge parameters as discharge current, discharge power and deposition rate were almost linear. However, the intensity showed a strong pressure dependence. In the case of reactive high rate sputtering there is a drastic drop in the spectral line intensity of the target material as the pressure of the reaction gas increases. The behaviour of both the line intensity and the discharge voltage clearly reflects the coverage of the target with reaction products as a function of the pressure ratio p_O2/p_tot. The effects of the residual gas on the metal sputter rate and the desorption of H₂O during r.f. substrate cleaning were verified. From these results it is seen that emission spectroscopy may be used to advantage for monitoring the process of high rate plasmatron sputtering.     

Time and energy resolved ion mass spectroscopy studies of the ion flux during high power pulsed magnetron sputtering of Cr in Ar and Ar/N2 atmospheres

Mass spectroscopy was used to analyze the energy and composition of the ion flux during high power pulsed magnetron sputtering (HIPIMS/HPPMS) of a Cr target in an industrial deposition system. The ion energy distribution functions were recorded in the time-averaged and time-resolved mode for Ar⁺, Ar²⁺, Cr⁺, Cr²⁺, N₂⁺ and N⁺ ions. In the metallic mode the dependence on pulse energy (equivalent of peak target current) was studied. In the case of reactive sputtering in an Ar/N₂ atmosphere, variations in ion flux composition were investigated for varying N₂-to-Ar flow ratio at constant pressure and HIPIMS power settings. The number of doubly charged Cr ions is found to increase linearly with increasing pulse energy. An intense flux of energetic N⁺ ions was observed during deposition in the reactive mode. The time evolution of ion flux composition is analyzed in detail and related to the film growth process. The ionization of working gas mixture is hampered during the most energetic phase of discharge by a high flux of sputter-ejected species entering the plasma, causing both gas rarefaction and quenching of the electron energy distribution function. It is suggested that the properties (composition and energy) of the ion flux incident on the substrate can be intentionally adjusted not only by varying the pulse energy (discharge peak current), but also by taking advantage of the observed time variations in the composition of ion flux.     

Elaboration and characterization of Fe1-xO thin films sputter deposited from magnetite target

Majority of the authors report elaboration of iron oxide thin films by reactive magnetron sputtering from an iron target with Ar-O₂ gas mixture. Instead of using the reactive sputtering of a metallic target we report here the preparation of Fe_1-xO thin films, directly sputtered from a magnetite target in a pure argon gas flow with a bias power applied. This oxide is generally obtained at very low partial oxygen pressure and high temperature. We showed that bias sputtering which can be controlled very easily can lead to reducing conditions during deposition of oxide thin film on simple glass substrates. The proportion of wustite was directly adjusted by modifying the power of the substrate polarization. Atomic force microscopy was used to observe these nanostructured layers. Mössbauer measurements and electrical properties versus bias polarization and annealing temperature are also reported.     

Investigation on the structure of TiO2 films sputtered on alloy substrates

Titanium dioxide (TiO₂) films have been successfully deposited on metal alloy substrates by radio-frequency magnetron reactive sputtering in an Ar+O₂ gas mixture. The effects of gas total pressure on the structure and phase transition of TiO₂ films were studied by X-ray diffraction and Raman spectra. It is suggested that the film structure changes from rutile to anatase while work gas total pressure changes from 0.2 to 2 Pa. The structure of TiO₂ films is not affected by the film thickness.     

Spectroscopic investigation of tantalum nitride thin film deposition by reactive sputtering in a triode system

In the present paper the change of the partial pressure of nitrogen during reactive dc triode sputtering of a Ta target in Ar+N₂ gas mixture is spectroscopically investigated by means of a quadrupole mass spectrometer. AES analysis of thin films, deposited at different N₂ partial pressures is carried out and the atomic concentration of the contaminants in the films is investigated. It is established that the change of nitrogen content in the sputtering gas and in the deposited films at different N₂ partial pressures is connected with a nitridation of the Ta target.     

射频磁控反应溅射制备Al2O3薄膜的工艺研究

采用射频磁控反应溅射法,以高纯Al为靶材,高纯O2为反应气体,在不锈钢和单晶Si基片上成功地制备了氧化铝(Al2O3)薄膜,并对氧化铝薄膜的沉积速率、结构和表面形貌进行了研究.结果表明,沉积速率随着射频功率的增大先几乎呈线性增大而后缓慢增大;随着溅射气压的增加,沉积速率先增大,在一定气压时达到峰值后继续随气压增大而减小,同时随着靶基距的增大而减小;随着氧气流量的不断增加,靶面溅射的物质从金属态过渡到氧化物态,沉积速率也随之不断降低.X射线衍射图谱表明薄膜结构为非晶态;用原子力显微镜对薄膜表面形貌观察,薄膜微结构为柱状.     

Quantification of the incorporation coefficient of a reactive gas on a metallic film during magnetron sputtering: The method and results

Reactive Magnetron Sputtering is a complex process and huge efforts are made addressing the understanding of its fundamental phenomena and the simulation of the deposition process by e.g. Particle in Cell/Monte Carlo (PIC/MC). One of the most uncertain parameters in this reactive sputtering process is the incorporation coefficient of the reactive gas in the growing layer, i.e. the real-time sticking coefficient during deposition. In this work, mass spectrometry is used to deliver more insights on this complex matter.Earlier, a method was developed to determine the incorporation coefficient of the reactive gas molecules in the growing metal film, using mass spectrometry combined with thin film analysis techniques (electron probe microanalysis and x-ray photoelectron spectroscopy). This method delivers a global, realistic incorporation coefficient which can be used in models for the reactive sputtering process. In this work, new insights have been added to the classical method. As previously only molecular reactive gas particles were considered, the here described method extends to determine the global incorporation coefficient of all reactive gas particles, i.e. both molecular as atomic. The incorporation coefficient of oxygen gas during the reactive magnetron sputtering of Cu, Al, Mg and Y, is determined. It can be concluded that for the first three metal systems, the obtained incorporation coefficient is mainly determined by the molecular oxygen, while for Y both atomic and molecular oxygen are important. Furthermore, a trend between the difference in electronegativity of the metal and the reactive gas, and the incorporation coefficient can be observed.     

Energetic negative ions in titanium oxide deposition by reactive sputtering in Ar/O2

The flux of energetic negative ions was measured during the preparation of TiO₂ in Ar/O₂ as a function of oxygen partial pressure. The Ti surface was so active that the small amount of O₂ contained in Ar was sufficient to oxidize the target surface. Ti target is also oxidized as similarly to the other metals such as Zr and Zn. Oxide islands form, which coalesce and cover the target surface with an oxide layer. The thickness of the Ti oxide layer on the target surface increased with increasing target current and Ar/O₂ gas pressure, which increased the oxygen flux incident on the target. This is due to the high reactivity of the Ti surface with oxygen. The data revealed that the flux of energetic negative ions in reactive sputtering of Ti in Ar/O₂ is fairly small compared with the sputtering of Zr and Zn. This indicates that the generation rate of negative ions on the Ti target during the reactive sputtering is very small.     

Optimization of Ti/TiN/Mo back contact properties for Cu(In,Ga)Se2 solar cells on polyimide foils

Molybdenum is conventionally used as electrical back contact for Cu(In,Ga)Se₂ (CIGS) solar cells. In this work, a multifunctional stack of Ti/TiN/Mo is introduced as back contact for flexible CIGS solar cells. The multilayer back contact was deposited on 25 μm thick polyimide foil by means of DC reactive sputtering.To optimize electrical conductivity and film stress of the alternative back contact sputter parameters such as total gas pressure, sputtering power, substrate temperature and RF substrate bias have been varied. XRD measurements and quantitative analysis of foil curvature revealed that the film stress is significantly influenced by the argon gas pressure and sputtering power. The electrical conductivity was improved by applying higher sputtering power or RF substrate bias. Analysis of the film microstructure with SEM shows that applied substrate bias influences the density of the sputtered film. The solar cells processed on Ti/TiN/Mo as well as on a conventional Mo bilayer back contact have been compared using standard current density to voltage (J-V) measurements and external quantum efficiency measurements. Conversion efficiencies of 13.4% for the alternative and 14.9% for the conventional design have been obtained.     

Effect of oxygen concentration and system geometry on the current-voltage relations during reactive sputter deposition of titanium dioxide thin films

Current-voltage relations at different magnetron sputtering systems and gas mixtures were studied during reactive sputter deposition of titanium dioxide thin films. The main goal of this work was to investigate the influence of reactive gas mixture (Ar + O₂) and system geometry on the electrical characteristics of the discharge. The geometries utilized were the conventional magnetron sputtering, hollow cathode magnetron sputtering and triode magnetron sputtering. A change in the system geometry leads to a change in the electric field distribution, which alters the working range of the discharge voltage and magnetron efficiency. It is noticed that the discharge voltage at constant current can be reduced when the geometry is altered from conventional magnetron to hollow cathode magnetron or triode magnetron, at the same time the magnetron efficiency is increased when hollow cathode magnetron or triode magnetron are used instead of conventional magnetron sputtering.     

SnO2-x薄膜非化学计量比对气敏性能的影响

采用反应磁控溅射法制备SnO2薄膜经常出现化学计量比的失衡问题。通过控制溅射过程中的氧分压制备不同化学计量比的SnO2-x薄膜,研究非化学计量比对薄膜结构、成分以及气敏性能的影响。XRD结果表明氧分压对材料结构和取向的影响非常显著。薄膜的O／Sn和表面化学成分通过XPS进行确定,分析发现氧分压的增加促使薄膜接近化学计量比,但表面化学吸附氧含量在0．33Pa氧分压下达到最大。气敏性能测试表明,非化学计量比主要影响薄膜表面的化学吸附氧数量,从而影响导电性和气体敏感性。氧分压对薄膜化学吸附氧的影响趋势与对气敏性能的影响趋势一致。0．33Pa氧分压下制备的薄膜拥有最多的表面吸附氧,同时对氢气的灵敏度高达45．6％。另外,在0．2～0．5Pa氧分压下制备的薄膜对氢气具有较好的选择性。     

总气压对直流反应磁控溅射制备TiO2薄膜的光学性质的影响

用DC(直流)反应磁控溅射设备在硅基底上制备TiO2薄膜，在固定电源功率、氩气流量42．6sccm、氧流量15sccm、溅射时间30min的条件下，通过控制总气压改变TiO22薄膜的光学性质。应用n＆k Analyzer 1200测量，当总气压增加时薄膜的平均反射率降低，同时反射低谷向短波方向移动，总气压对消光系数k影响不大；随着总气压的增加薄膜的折射率出现了下降的趋势，但当总气压达到一定量值时折射率的变化趋于稳定。通过XRD和SEM表征发现，随着总气压的增加TiO2的晶体结构由金红石相向锐钛矿相转变，薄膜表面的颗粒度大小由粗大变得微小细密。     

Magnetron sputter deposition: Linking discharge voltage with target properties

The discharge voltage is perhaps the most accessible parameter of the magnetron sputter deposition process. As its value can be easily monitored, research reports generally contain its value. Nevertheless, the interpretation of the discharge voltage and/or its behavior, especially during reactive magnetron sputtering, is less straightforward. To understand its behavior, it is necessary to look into the details of the magnetron discharge, the processes occurring at the cathode (or target) and the influence of the important discharge parameters such as discharge current, magnet configuration, and discharge gas pressure. The influence of these parameters can be partially understood from a general formula. This formula, based on the original work of Thornton, shows that the discharge voltage behavior during reactive magnetron sputtering finds its origin in the formation of a compound layer on the target. The discharge voltage behavior depends strongly on the material properties of the compound layer which is formed.     

Influence of the nitrogen partial pressure on the properties of d.c.-sputtered titanium and titanium nitride films

The properties of titanium and titanium nitride films deposited onto biased substrates in a d.c. sputtering system were studied as a function of the partial nitrogen pressure. The deposition rate was deduced from film thickness measurements. The film composition was determined by Rutherford backscattering analysis and the structure was studied using X-ray diffraction. The resistivity was measured by the four-probe method and the temperature coefficient of resistivity (TCR) was determined in the temperature range from ?196 to 25 °C.Around a critical nitrogen pressure p_c of 4 × 10^-2 Pa the deposition rate decreases rapidly, the film structure changes from h.c.p. titanium to nearly stoichiometric f.c.c. TiN. At the same pressure, the film resistivity and the TCR present minimum values.A general sputtering model which takes into account the gettering action of the deposited material is proposed. This model allows the calculation of the surface coverage of the target by the reactive gas or the metallic compound and the determination of the deposition rate as a function of the reactive partial pressure. A good agreement is found with the deposition rates measured experimentally.     

Resputtering-induced chemical inhomogeneity during the growth of highT c superconductor thin film

HighT _c films belonging to Y-Ba-Cu-O, Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O systems have been fabricated by reactive sputtering of single targets in a planar magnetron and Ar + O₂ sputtering atmosphere. Although it was possible to deposit films of correct composition, resputtering related composition variation was a problem. The key to obtaining correct chemistry was a proper control of the deposition parameters. The pressure and oxygen content of the sputtering gas were found to be the most critical parameters. Results of the variation of these parameters on the cation chemistry are presented in this paper. Results from Monte-Carlo simulation of the sputtering process are also presented showing that low pressure and oxygen content of the sputtering gas result in a higher yield of energetic reflected neutrals which can cause compositional variation in the film mainly due to preferential sputtering of the growing film. The effect was particularly noticeable directly underneath the target. The energetic particle bombardment can be controlled by using moderately high pressures and low oxygen concentration in the gas.     

白宝石上生长SiO2薄膜的工艺

用射频磁控反应溅射法,以高纯Si为靶材,高纯O2为反应气体,在白宝石上制备SiO2薄膜。对影响薄膜生长的工艺参数进行了分析,测试了薄膜的成分,并研究了薄膜的红外光学性能。结果表明,制备的薄膜中Si和O形成SiO2化学键,计算出的O与Si的原子比接近2:1,采用射频磁控反应溅射法在白宝石上能够沉积出SiO2薄膜。制备出的SiO2薄膜对白宝石衬底有较好的增透作用。