AVS2011 Session PS-ThP: Plasma Science and Technology Poster Session

We report on an investigative study into the anomalous copper contamination observed on silicon test wafers run in hydrogen plasma processing systems using substantially oxygen-free plasmas. The copper is believed to originate from trace copper constituents of aluminum and other alloys used in construction of the vacuum system components. The transport of the copper to the wafer surface is consistent with the formation of meta-stable CuH.

Several plasma-ash systems from multiple vendors were tested using VPD/ICP-MS on silicon test wafers processed using hydrogen plus nitrogen gas mixtures. Anomalously high copper contamination levels in excess of 5x10¹¹ atoms/cm² were measured. The copper contamination was found to increase linearly with plasma process time and the rate was highly system dependent. Clean systems tended to have higher contamination rates as compared to systems that have run large amounts of mixed processes, suggesting carbon or hydrocarbon deposits on chamber walls can somewhat suppress the CuH formation. Cycling these systems with oxygen plasmas increased the copper contamination rate. Additionally the contamination rate increased with increasing chamber wall temperature suggesting strongly that the chamber construction materials are the source of the contamination. Additionally several systems exhibited increases of cadmium, germanium and nickel. Each of these metals also has stable or meta-stable metal hydrides.

It was found that adding oxygen to the plasma in excess of 1% by volume, suppressed completely the anomalous copper contamination. VPD/ICP-MS measurement showed that 1% or higher oxygen concentration reduced copper contamination to below 1x10⁹ atoms/cm2.

Platinum nano-particles (NPs) and platinum thin films were deposited by using inductively coupled plasma atomic layer deposition (PEALD). In this PEALD process, the Ar/O₂ reaction plasma was applied to decompose the ligands of Pt precursor (MeCpPtMe₃) at varied substrate temperatures between 150 to 300 ^oC. The partial pressures of Ar and O₂ were held at 1.5 and 0.5 Torr during the PEALD Pt NPs process. Next the PEALD Pt thin films deposited by using the lower pressure of Ar and O₂ at 0.3 and 0.1 Torr. X-ray diffraction (XRD) patterns show that Pt NPs and thin films are polycrystalline and a preferred orientation along the (111) plane of the fcc structure. Based on the Transmission Electron Microscopy (TEM) measurement, the growth rate of Pt NPs size and Pt thin films thickness is about 0.04 nm/cycle. The Pt thin film 4f XPS spectra consists with the Pt 4f data that observed metallic Pt peaks deposited at 200 ^oC on Si wafer using Ar/O₂ plasma of 400 W. The step coverage was approximately 100% coated in 170 nm (open diameter) trench structures with 3.5： 1 aspect ratio. The low resistivity of Pt NPs and Pt thin films prepared by using 600 and 200 cycles was 1.83×10^-5 and 1.62×10^-5 Ω-cm, respectively.

Keywords: inductively coupled plasma, PEALD, platinum NPs

The Indium zinc oxide (IZO) is a wide band gap transparent conducting oxide that has provoked a great deal of interest recently, due to its many potential applications. It allows the fabrication of devices with a low operating voltage, and the design of such devices will lead the way to the next generation of display technologies based on transparent and flexible devices. IZO thin films have attracted a great deal of attention because of their excellent optical transmission, high conductivity, chemical stability, thermal stability and low compressive stress. Therefore, amongst the various patterning techniques used for IZO thin films, plasma etching is preferred, because it allows for high resolution pattern transfer for optoelectronic device structures. Recently, although the growth characteristics of IZO thin films have been well optimized, the development of an efficient pattern transfer process remains to be achieved. Among the various patterning techniques, the dry etching process has several advantages over the conventional wet etching process, including high resolution and easy process automation. Moreover, there have only been a few studies relating to IZO dry etching using the high density plasma sources favored by modern microelectronic technology. As a result, the influence of the process parameters on the IZO etch rate has not been explored in detail and the IZO etch mechanism is not quite clear.

In this work, we investigated the etching characteristics of IZO thin film using a high density plasma (HDP) system. The etching characteristics of the IZO thin film were investigated in the terms of the selectivity of IZO over SiO₂ as a function of the etch chemistry. The chemical states on the etched surface were investigated by X-ray photoelectron spectroscopy (XPS). Field emission Auger electron spectroscopy (FE-AES) was used for the elemental analysis of the etched surfaces.

The availability of particle counters which may be operated at ambient pressures of up to 2x10^-4 Torr is highly desirable in many current research applications. In one field in particular, it allows mass spectroscopy of gas analysis and processing plasmas to be performed using pressures of this order in the quadrupole mass spectrometer (QMS). These pressures are much closer to those of many processing plasmas so that the sampling of neutral species, in particular, from the plasma is improved. The sampling of ion and neutral species from magnetron plasmas is a good example. The particle counter used in the present investigation, could be mounted axially so as to have a direct line-of-sight view of the sampling orifice of the QMS. Consequently, energetic neutral species such as metastable atoms of helium, which are produced in helium plasmas and have long lifetimes against spontaneous de-excitation, may travel to the detector and have sufficient energy to be counted there. The detection of metastable helium may be of importance in the study of reaction processes during hydrogen (H/HD)-deuterium (D₂) plasma fusion, of which the residual fusion byproduct is helium ash. Furthermore, collision processes in the ionisation source of the QMS, (including Penning ionisation), which are insignificant at the more usual source pressures of below 1x10^-5 Torr, generate product ion species whose study helps the interpretation of the processes occurring in the plasma reactor. Typical data from neutral gases and plasmas in a range of gas mixtures which include helium or argon with molecular gases such as hydrogen and nitrous oxide are shown. The majority of the data presented consist of electron impact threshold ionisation efficiency curves obtained by scanning the energy of the electrons in the QMS source. The results presented are discussed in terms of processes which include collisions between metastable species such as He (2¹S and 2³S) at 20.61eV and 19.82eV respectively^[1] having radiative lifetime states of 20±2ms^[2], and also other plasma constituents. The measurements suggest new avenues of research into gas analysis and processing plasmas in which energetic, long-lived, metastable species may have important roles.

[1] Laborie. P, Rees, J.A. Electronic Cross Sections and coefficients, Hydrogen and rare gases, Dunod, 1968

[2] Van Dyck, Robert,S et al. Phys. Rev. A 4, 1327–1336 (1971)

Wave-cutoff method is an electron density measurement tool. Most of the diagnostic tools, including cutoff probe, need a few seconds to measure the plasma parameters. In this presentation, a fast measurement method using Fourier analysis will be newly introduced. A wave-cutoff probe system consists of two antennas and a network analyzer. A network analyzer provides the transmission spectrum and the reflection spectrum by frequency sweeping. The plasma parameters such as electron density can be obtained through these spectra. The frequency sweeping time, the time resolution of the wave-cutoff method, is about 1~10 seconds.

We proposed a new method to use an impulse generator and an oscilloscope instead of a network analyzer. This method was named 'Fourier cutoff probe method'. A short impulse signal with a few GHz band and its transmitted signal were detected by an oscilloscope, and the Fourier analysis of the signals made a transmission spectra of plasma. The data acquisition time can be reduced with this method.

In this work, 3 diagnostic methods, Langmuir probe, network analyzer cutoff probe, and Fourier cutoff probe, were also compared. The measurement results were well matched. This method provides electron density with time resolution of 10 nano seconds, and it took about 100 milli seconds to make a measurement. This technique is found to be very useful especially in pulsed plasma. Fourier cutoff probe was 40 times faster than Langmuir probe in a pulsed plasma measurement.

Measurement of plasma density, a key parameter that control the property of processing plasmas and hence the processing results, has been the focus of extensive studies in recent years, not only for characterization of the plasmas but also for development of tools for monitoring of the plasma based processes¹. In this study, a multi-mode plasma absorption probe² was investigated by employing a full wave electromagnetic simulation using a finite element code, High Frequncy Structure Simulation (HFSS, ANSOFT CORP). The spherically-shaped probe was formed by two isolated hemispherical conductors, which was enclosed by a layer of dielectrics (alumina). In the simulation analysis, a coaxial line was attached to the spherical probe head and the "driven mode" was adoped, i.e., the wave was fed into the coaxial line and the absorption specturm, reflection coefficient vs frequency, was then analyzed. The probe was in a plasma, characterized by the collisional electromagnetic plasma permittivity, determined by plasma density, wave frequency and momentum transfer collisional frequency. Simulation results show that the absorption spectrum exhibits a number of resonances, as a result of cavity-like structure resonant modes, as well as the plasma density, similar to the results reported in a previous work based on an electrostatic analysis². The electromagnetic analysis adopted in this work, however, reveals that the wave also propagates along the interface between the plasma and the coaxial cable, similar to the characteristics of a conventional plasma absorption probe³. This effect thus contributes to resonant peaks in the absorption spectrum that did not appear in the electrostatic analysis. A parametric study by varying the plasma density, probe size and dielectric thickness have been conducted and the results will be presented.

¹ M. A. Lieberman and A. J. Lichtenberg, Principle of Plasma Dishcarges and Material Processing, Wiley, New York, 2005.

² C. Scharwitz, et al, Appl. Phys. Lett., 94, 011502(2009).

³ H. Kokura, et al, Jpn. J. Appl. Phys., Part 1, 38, 5262, 1999.

* Work supported by the National Science Council of Taiwan, ROC.

As the size of the semiconductor device is drastically scaled down to nano-scale, the etching technology of high aspect ratio contact (HARC) is increasingly important. To etch the nano patterned HARC, the multi-layer resist (MLR) structure conventionally composed of a PR, SiON layer, and amorphous carbon layer has been developed because it is difficult to etch HARC directly using a photoresist mask only. However, even though the etching of HARC tends to show higher etch selectivity and proper etch profile with MLR structures, for the next-generation nano-scale dynamic random access memory devices, the etching of nano-scale SiO₂ HARC using fluorocarbon plasmas is becoming more and more important.

Nanoscale SiO₂ contact holes were etched by using C₄F₈/CHF₃/O₂/Ar gas mixtures in dual-frequency capacitive coupled plasmas (DF-CCPs) where 60 MHz source power is applied to the top electrode while 2 MHz bias power is applied to the bottom electrode. we added CHF₃ and O₂ gases to control the etch rate and improve the etch selectivity. Proper additive gas ratios are needed to get the high etch selectivity and maintain CD size. We measured radical intensity, etch rate and SiO₂ surface by using optical emission spectroscopy(OES), X-ray photoelectron spectroscopy(XPS), field emission scanning electron microscopy(FE-SEM) respectively.

In this study, we confirm that the etch rate and etch selectivity are varied as a function of CHF₃/O₂ gas ratios. The SiO₂ and amorphous carbon layer(ACL) etch rate are increased with increasing the HF power due to the improvement of gas dissociation. But as increasing the HF power, etch selectivity is saturated. In this study, Highest etch selectivity of about 6 is obtained at HF power 300 W, LF power 500 W, Ar/C₄F₈/CHF₃/O₂ = 150/20/25/5 sccm, process pressure of 20 mTorr.

Recently, there are many researches in order to increase the deposition rate (D/R) and improve film uniformity and quality in the deposition of microcrystalline silicon thin film. These two factors are the most important issues in the fabrication of the thin film solar cell, and for the purpose of that, several process conditions, including the large area electrode (more than 1.1 X 1.3 (m²)), higher pressure (1 ~ 10 (Torr)), and very high frequency regime (VHF, 40 ~ 100 (MHz)), have been needed.

But, in the case of large-area capacitively coupled discharges (CCP) driven at frequencies higher than the usual RF (13.56 (MHz)) frequency, the standing wave and skin effects should be the critical problems for obtaining the good plasma uniformity, and the ion damage on the thin film layer due to the high voltage between the substrate and the bulk plasma might cause the defects which degrade the film quality.

In this study, we will propose the new concept of the large-area multi-electrode (a new multi-electrode concept for the large-area plasma source), which consists of a series of electrodes and grounds arranged by turns. The experimental results with this new electrode showed the processing performances of high D/R (1 ~ 2 (nm/sec)), controllable crystallinity (~70% and controllable), and good uniformity (less than 10%) at the conditions of the relatively high frequency of 40 MHz in the large-area electrode of 280 X 540 mm². And, we also observed the SEM images of the deposited thin film at the conditions of peeling, normal microcrystalline, and powder formation, and discussed the mechanisms of the crystal formation and voids generation in the film in order to try the enhancement of the film quality compared to the cases of normal VHF capacitive discharges. Also, we will discuss the relation between the processing parameters (including gap length between electrode and substrate, operating pressure) and the processing results (D/R and crystallinity) with the process condition map for μc-Si:H formation at a fixed input power and gas flow rate.

Finally, we will discuss the potential of the multi-electrode of the 3.5G-class large-area plasma processing (650 X 550 (mm²) to the possibility of the expansion of the new electrode concept to 8G class large-area plasma processing and the additional issues in order to improve the process efficiency.

The Neutral Beam Etching (NBE) system has been expected to provide a damage-free top-down process for organic and inorganic materials and facilitate the practical development of future Nano scale devices

In order to examine the mechanical damage-free effect by NBE, we have conducted NBE surface treatment to microcantilever formed over an 8 –inch wafer from the upper surface several times, and measured the resonance frequency and the quality factor (Q-factor) of 52 cantilevers after each treatment.

The NBE treatment removes about 50nm, 100nm, and 500nm thick silicon surface layer, respectively.

The Q-factor of cantilevers is attributed by summation of various kinds of energy dissipation mechanisms such as air damping, thermo elastic damping, support, and surface dissipation.

Based on the theoretical consideration of mechanical vibration, we have designed the microcantilever having 30 um in width, 150 um in length and 2 um in thickness, by which the surface energy dissipation become dominant compared to other ones in high-vacuum condition.

In general, Q-factor is a function of the resonant frequency (fr) so that we can’t argue the effect of NBE treatment from the Q-factor change as it is. Therefore we adopt the values Q-factor divided by resonant frequency (Q/fr value) which becomes independent of the thickness in case that the Q-factor is dominated by the surface dissipation and is a good parameter to represent surface dissipation.

From the viewpoint of transition of Q/fr value taken after each NBE treatment, surface dissipation of the cantilever was increased with each treatment process and saturated after adequate NBE treatment.

This indicates that surface damage introduced during fabrication process can be removed completely by slight surface etching using NBE.

In the conference, we will report the precise results of the NBE treatment experiment.

In this work, to study the effect of surface charging on the removal of the barrier layer, the Cl₂/BCl₃ neutral beam etching has been used and the results were compared with the AAO etched by an ion beam etching. By the neutral beam etching with BCl₃ containing gas mixtures, the barrier layer was successfully removed due to the formation of volatile BO_xCl_y during the etching. However, when the barrier layer was etched using the ion beam with the BCl₃ containing gas mixtures, even though AAO itself is etched, the barrier layer located near the bottom of the AAO pore was not easily etched due to the charging of the AAO pore similar to the case of conventional reactive ion etching.

This letter reports on a different method of negative beam formation that has potential application to neutral beam etching. In particular, we obtain a flux of fast neutral atoms with narrow energy and angular distribution from an initially negative ion beam which is extracted from the discharge volume with energy of 10 – 500 eV and then neutralized. By increasing the pulse frequency of the antenna voltage to 20 kHz and applying a synchronized bipolar potential to the accelerating electrodes, it is possible to produce a high-density negative ion beam whose energy level is determined by the amplitude of the extraction potential. During the discharge, a positive potential is applied to the extraction electrode and a negative potential to the focusing electrode. In the afterglow period, these potentials are switched, allowing high-energy negative ions to escape the source. The lower and upper limits of the bipolar extraction potential on the two electrodes can be varied independently, so that the ratio of negative ions to positive ions in the resulting beam can be adjusted from 0% to 100%.

Amorphous silicon deposition and nano-crystallization experiments were performed in a novel reactor with rotating substrates. Separate plasma sources were used for deposition (a capacitively coupled silane/helium plasma) and crystallization (an inductively coupled hydrogen plasma). Substrates were exposed sequentially and repeatedly to the deposition and crystallization plasmas for different times by varying the rotation speed. Radical concentrations in the plasma were monitored by optical emission spectroscopy while stable products were measured using a mass spectrometer. Films were characterized by spectroscopic ellipsometry (thickness, optical constants), Fourier transform infrared absorption (silicon-hydrogen bonding, hydrogen concentration), Raman spectroscopy and X-Ray diffraction (crystalline structure, degree of crystallization). Results will be presented for different plasma operating conditions, substrate temperatures, and exposure times in the deposition and crystallization plasmas. The effect of “cross-talk” between the two plasmas and methods to prevent it will also be discussed.

Supported by the University of Houston GEAR Program and DoE

Microwave plasma sources are widely used in industrial plasma processing devices for their favorable characteristics. In particular, electron cyclotron resonance (ECR) reactors offer a number of desirable characteristics for plasma etching and deposition, including higher plasma density, lower ion energy, lower pressure operation, and higher uniformity compared with the conventional radio frequency (RF) type plasmas. ECR devices designed for commercial use usually provide the user with very limited access for controlling and characterizing the plasma conditions.

Retarding field energy analyzers (RFEAs) are generally used to measure the ion energy distribution function (IEDF) at electrically grounded surfaces. An RFEA can also be used to measure IEDFs at the surface of radio-frequency (RF) driven electrodes by electrically isolating it from the ground reference. The RFEA then “follows” the RF bias without disturbing it. Using the same principle, but applying grid bias signals of opposite polarity, one can also measure the electron energy distribution function (EEDF) at the surface of RFEA. From the measured IEDFs and EEDFs it is possible to calculate the respective ion and electron flux and energy. These quantities are key parameters in plasma surface processing. Therefore, the metrological capabilities of the RFEA together with its non-perturbing in situ operation makes it a convenient diagnostic tool for characterizing plasmas in devices like ECR etching reactors.

In this work we present IEDF measurements at the surface of an RF driven electrode using a RFEA installed in an industrial ECR etching reactor. The RFEA is mounted at the RF biased electrode (on which the SiO₂ wafer is mounted) and is connected through high input impedance low pass filters to the ground referenced grid bias signals. The RFEA installation does not require any modifications to the reactor since the signal cabling is taken out to atmosphere through an existing unused port located in the vacuum pump tunnel. The discharge control parameters were varied as follows; pressure 5 to 20 mTorr, flow rate 10 to 50 sccm of pure argon and oxygen, microwave input power 750 to 1500 W, RF input power 0 to 250W, and a variety of magnetic field profiles. The radial location of RFEA has also been varied. The measured fluxes and energy distribution of the ions and electrons impacting the wafer surface are presented and discussed.

RF plasma sources used in modern plasma etch applications are required to provide a high plasma uniformity and variety of plasma controls, including independent plasma profile, plasma density and ion energy controls. They must be able to sustain a stable plasma in a very different gases and under very different conditions (gas flow, gas pressure, etc.). Finally, going forward the system has to produce a minimum impact on environment (energy consumption, EM emission). Mattson Technology has developed an etch tool with a new and efficient ICP source that uses two coils operating at significantly different frequencies (13.56 and 2 MHz) by utilizing a ferromagnetic core in a low frequency coil. Tests on the tool revealed that

· It provides high-density plasma generation, with no capacitive coupling between plasma and the source;

· There is no interference between higher and lower frequency RF subsystems;

· It provides good plasma and process profile control;

· It has wide processing window (gas pressure, gas flow, gas composition, source and bias power);

· High efficiency and stability of operation with both electropositive and electronegative gases.

To produce semiconductors at low cost, the transition to 450 mm-sized wafers is being considered. For semiconductor fabrication on 450 mm substrates, the etching performance is expected to equal or exceed 300 mm substrate capability with improved productivity requirements. A 450 mm substrate capable apparatus was developed on a proven plasma source using microwave electron cyclotron resonance (ECR) currently utilized for 300 mm equipment. We have evaluated plasma uniformity by using: 1) Single Langmuir probe with -50 V bias to measure ion current flux (ICF) distribution in the plasma reactor. The plasma generated area and the spatial distribution of ICF were scanned in the radial and z-axis directions in the reactor. 2) A 41-point wafer stage to measure ICF distribution. 3) Etch rate uniformity of poly-Si and SiO2 using coupon samples attached on 450 mm bare Si wafers. We used HBr/Cl2/O2/Ar gas and 0.1-10 Pa pressure range as test conditions.

We found that the high density plasma generated at the ECR zone and the shape of the plasma was ring-shaped. This ring-shaped plasma diffused and became increasingly uniform, finally becoming almost flat at a height of 25 mm above the wafer stage. To evaluate ICF distribution of the wafer stage, 41 pads were used to measure plasma density distribution using about 500 test conditions. By controlling the magnetic fields, non-uniformity of ICF below ±5% was achieved for several test conditions of gas and pressure. Poly-Si and SiO2 etching rate were observed to be very similar to rates seen using 300 mm substrate equipment with non-uniformity below ±1% within a wafer.

New pulsed plasma generator for reactive magnetron sputter processes was developed. (old sentence)

A new pulsed plasma generator for reactive magnetron sputter processes has been developed. (new sentence)

Compare with existing pulsed DC plasma generators new generator has high current capabilities in the range of 100 – 300 A.(old sentence)

Compared to existing pulsed DC plasma generators, the new generator has high current capabilities in the range of 100 – 300 A. (new sentence)

In proposed method near arc free magnetron discharge in reactive atmosphere can be generated by adjusting the frequency and amplitude and shape of the voltage pulses. (old sentence)

In the proposed method, near arc free magnetron discharge in reactive atmosphere can be generated by adjusting the frequency and amplitude and shape of the voltage pulses.(new sentence)

The application of new pulse plasma generator for reactive sputtering of SiN, SiO₂, AlN and TiO₂ films at ENDURA 200 mm tool (AMAT) will be discussed. (old sentence)

The application of new pulse plasma generator for reactive sputtering of Si₃N₄, SiO₂, AlN and TiO₂ films at ENDURA 200 mm tool (AMAT) will be discussed.(new sentence)