AVS2010 Session PS-TuP: Plasma Science and Technology Poster Session

The ultra thinned body fully depleted silicon on insulator (UTB-FD SOI) is the solution to problems of short channel effect by shrinking the gate length. This device has many advantages of high drive current, high conductance and ideal sub threshold slope¹². This device uses the TiN/HfO₂ gate stack . The metal/high-k stack structure is being the core technology because reduction of device is faced with physical limitations.

So, we have to study about this metal/high-k stack³.

In this study is investigated the dry etching characteristics of the TiN in the TiN/HfO₂ gate stack using inductively coupled plasma system. TiN thin film is etched by CF₄/Cl₂/He plasma. We investigate the etching chemistry of the CF₄/Cl₂ gas mixture. Etching parameters are gas mixing ratio, the RF power, the process pressure in this study. The chemical reactions on surface of the etched TiN and etched HfO₂ are investigated by X-ray photoelectron spectroscopy. The profile of the etched TiN is investigated by Scanning electron microscope.

Reference

¹S. Mukhopadhyay, K. W. Kim, X. Wang, D. J. Frank, P. Oldiges, C. T. Chuang and K. Roy, IEEE 27(4), 284 (2006)

²S. Eminent, S. Cristoloveanu, R. Clerc, A. Ohata and G. Ghibaudo, Solid-state Electronics 51(2), 239 (2007)

³D. P. Kim, X. Yang, J. C. Woo, D. S. Um and C. I. Kim J. Vac. Sci. Technol. A 27(6) 1320 (2009)

With the permanent scaling down of complementary metal oxide semiconductor (CMOS) devices, the thickness of the gate oxide is expected to be reduced to less than 1 nm for the 45 nm and 32 nm technology nodes. Continuing to reduce the gate insulator thickness using SiO₂ is problematic as gate leakage current due to direct-tunneling increase.^1-2 One solution to the problem is the replacement of SiO₂ by high-k material. HfAlO₃ have arisen as a promising material for gate oxide replacement due to their high dielectric constant, bandgap, and recrystallization temperature. Therefore, a further on the study of HfAlO₃ thin films is needed.³

In this work, HfAlO₃ thin films were etched in BCl₃/He plasma. The etching characteristics of HfAlO₃ thin films were investigated in terms of etch rates and selectivity as a function of gas mixing ratio, RF power, DC bias voltage, and chamber pressure. The total flow rate of BCl₃/He was fixed at 20 sccm. The other parameters were varied as follows; RF power = 400 ~ 600 W, DC-bias voltage = - 50 ~ - 200 V, process pressure = 1 Pa ~ 3 Pa. The plasma diagnosis was characterized by optical emission spectroscopy (OES) analysis. The chemical reaction on the surface of the etched the HfAlO₃ thin films was investigated with X-ray photoelectron spectroscopy (XPS). Field emission scanning electron microscopy (FE-SEM) was used to investigate the etching profile.

Refernce

¹G. D. Wilk, E. M. Wallace, J. M. Anthony. J. Appl. Phys. 89, 5243 (2001).

²A. I. Kingon, J. I. Maria, S. K. Streiffer, Nature 406, 1032 (2000).

³W. J. Zhu, T. Tamagawa, M. Gibson, T. Kurukawa, and T. P. Ma. IEEE Electron Device Letters 23, 649 (2002).

Quartz or pure fused silica is selected material for the fabrication of biochip devices and more specifically electrophoresis chips. Indeed, these materials benefit from transparency in the UV-visible range, and low dielectric breakdown. However material cost is higher in comparison to silica glass which offers similar properties with a low purity degree. Plasma deep etching techniques are well established for fused silica and quartz, but much more challenging for glass. In the present study, the etching simulator has been developed to study the etching of silica glass (Pyrex, D263, AF45, and Vycor) in SF₆/Ar plasma. The etching model is based on the development of the plasma kinetic model coupled to 2D Monte Carlo surface model to predict the etched surface morphology of glasses as a function of the operating conditions.

The gas phase kinetic model is based on the mass balance equations of reactive species. The kinetic constants of electron impact reactions are established as a function of electron temperature assuming maxwellian distribution of electron energy. The additional equation of power balance in the ICP reactor allows to determine the electron temperature evolution with the plasma discharge parameters (Rf power, reactor pressure and SF6/Ar flow rates).

Langmuir probe is used to measure the electrical parameters of SF6/Ar plasma mixture such as, electron temperature and density as a function of the plasma discharge parameters. A good agreement between the simulations and the experiments have been observed

One of the advantages of our model is the coupling between the plasma chemistry model and the surface etching model. The later is based on the Monte-Carlo approach which allows to describe, in a probabilistic manner, the surface mechanisms for silica glass etching.

The direct fluxes of the reactive species such as fluorine and ions are determined from the gas phase kinetic model and introduced as the input parameters in the glass etching model.

On the other hand, surface analyses such as the etch rate, surface roughness (profilometry), and surface topography (AFM) of silica glass as a function of operating conditions have been carried out.

The preferential redeposition mechanism of the etched products on the metallic sites seems to play an important role on the propagation of the etched surface roughness. A satisfactory agreement between experimental results and the model concerning the etching rate and the etched surface morphology have been obtained for different glasses.

As the critical dimension of metal-oxide-semiconductor field-effect transistor (MOSFET) shrinks to 45 nm and below, conventional poly silicon gates on ultrathin SiO₂ dielectric layers need be replaced by metal gates on high-k dielectric materials. However, the successful adoption of these new materials imposes new integration problems. Among many integration issues, selective etching of metal gate electrodes and the high-k gate dielectrics over the Si substrate is expected to be one of the critical steps in the process integration of the front end of the line. In the case of TiN etching on HfO₂ layer using conventional RIE etching, HfO₂ layer can be electrically damaged by charged particle leading to higher leakage current, the change of threshold voltage, etc. In order to solve these problems, in this study, we investigated etch characteristics of TiN on HfO₂ layer using low angle forward reflected neutral beam and compared with those by conventional RIE etch process.

As a result, we observed nearly unlimited etch selectivity of TiN/HfO₂ uisng HBr/Cl₂ gas mixing neutral beam by controlling energy (<100 eV). Also, using TEM and AFM, we observed an anistropic etch profile and smooth surface roughness (0.109 nm). Neutral beam for metal gate etching process turns out to be very promising for gate/high-k dielectric complementary MOSFETs due to lower interface trap generation during etching process.

III-V compound materials have been used for the devices such as high electron mobility transistors (HEMTs), light emitting diodes (LEDs), and quantum dot (QD) devices due to its excellent material properties including high carrier mobility, wide operating temperate range, direct energy band structure, etc. For the fabrication of these III-V compound materials devices, reactive ion etching techniques such as capacitively coupled plasma etching, inductively coupled plasma (ICP) etching, etc. are generally applied to obtain anisotropic etching properties. However, due to the energetic reactive ions involved in the reactive ion etch process, the surface of the etched III-V compound materials tends to be damaged physically and chemically by structural disruption, intermixing, stoichiometric modification, surface roughening, etc. In addition, it is difficult to control the etch depth precisely through the reactive ion etching due to the fluctuation of the etch process. To overcome these problems, various atomic layer etching techniques (ALETs) have been investigated especially as the application to the nano-device processing which requires atomic-scale precision in the etching in addition to the nearly no-damage to the surface during the etching .

The etch characteristics of III-V compound materials by ALET were investigated using a Cl₂-based ALET. The effect of ALET on surface modification and etch-depth control was also examined. Self-limited etching of III-V compound materials could be obtained using Cl₂ ALET. In addition, the significant improvement in the electrical properties of the III-V device could be obtained by etching the damage sensitive layer using ALET.

As the size of ULSI elements shirinks further, functional design for a top-down plasma processing will be strongly needed in order to solve many types of technological difficulties induced by plasma etching. Actually, under the present design rule for DRAM devices, a contact hole with high aspect ratio (> 20) is required.

The reactive ion etching (RIE) of high aspect contact hole (HARC) or inter layer dielectric has been traditionally performed by fluorocarbon gas chemistry in a two-frequency capacitively coupled plasma (2f-CCP) reactor. As is well known, SiO₂ etching in fluorocarbon chemistry proceeds under the competition of surface protection by the deposition of C_xF_y radicals and chemical sputtering by directional CF_x⁺ ions. Under a practical condition for SiO₂ etching where the radical flux is larger than that of ions, a reactive mixing layer (SiO_xF_y), formed under excessive F radicals assisted by high-energy ion bombardment, is always covered with thick polymer layer (C_xF_y). Consequently, the etching is essentially carried out through the removal of polymer layer and the chemical reaction in a mixing layer. The side wall is simultaneously protected against the energetic ions by C_xF_y polymer deposition.

Under the circumstance, we have developed the two-layer sufrace model for the simulation of SiO₂ etching profile in fluorocarbon gas chemistry. This model clarified the effects of reactive species (ions and radicals) on the SiO₂ etching profile [1] and the dependence of etch rate on the pattern size (RIE-lag) [2].

In this paper, we will propose a new surface model for SiO₂ etching which accounts for the selectivity between SiO₂ and underlying Si substrate in HARC processing. Feature profile evolution and the selectivity during SiO₂ etching can be coincidentally discussed as functions of flux of reactive species and impact ion energy. In addition, the effect of resist mask erosion will also be discussed.

[1] T. Shimada et al, Jpn. J. Appl. Phys. 45, p. 132, (2006).

Inductively-coupled chlorine-based plasmas (often also containing HBr, O₂ and Ar) are widely used in the microelectronics industry for selective, anisotropic etching of silicon, and are currently being investigated for etching of III-V materials such as InP for the fabrication of photonic devices.

We have constructed a reactor, identical in geometry to an industrial 200mm etch tool, but adapted for advanced diagnostics and supplied with Cl₂/HBr/O₂ and Ar gases. It is excited by a flat spiral antenna though a dielectric window. In parallel we are developing a hybrid simulation code based on the HPEM (Hybrid Equipment Plasma Model) of Mark Kushner. Experimental measurements of internal plasma parameters (electron densities, temperatures, and also gas dissociation fraction and temperature) will be used to test and improve the simulation code. The aim is to reliably predict the plasma behaviour as a function of these parameters, and for arbitrary reactor geometries.

The electron density was measured using a microwave hairpin resonator, and with a Langmuir probe. Measurements were made as a function of pressure and power in pure Cl₂ and as a function of composition in Cl₂/Ar mixtures. The electron density decreased from 5x10¹⁰cm^-3 to 2x10¹⁰cm^-3 as the Cl₂ pressure was increased from 0.5 to 10 mTorr. The electron density also decreases monotonically as the Cl₂ fraction is increased in Ar/Cl₂ mixtures. The Langmuir probe measurements of n_e showed the same trend as the hairpin probe, but gave lower values, particularly at the highest pressures and lowest powers and for high Cl₂ fractions.

In general, real time, closed loop control of plasma assisted processes has not been applied in IC manufacturing. In the case of etching, 'process control' is generally understood to mean ex situ statistical analysis of metrics such as etch depth, uniformity, anisotropy and selectivity and consequent adjustment of the process recipe, which is specified in terms of inputs such as gas flow rates, forward power and pressure. An alternative approach would be to specify a recipe in terms of plasma parameters such as ion fluxes and radical densities at the wafer surface and to regulate these in real time by adjusting the inputs with a suitable control algorithm. Such an approach would mitigate potential plasma process disturbances such as wall seasoning and substrate loading, leading to an improvement in process reproducibility. This presentation describes how suitable control algorithms for low pressure plasma processes may be derived from control-oriented process models. The stability and efficacy of the control algorithms are demonstrated using an plasma simulation. Some parameters of the control algorithm depend on unknown, possibly time-varying process parameters such as wall sticking coefficients. The presentation indicates how, given a process model and process measurements, the control algorithm may be adapted/gain-scheduled in order to maintain stability. Experimental implementation of control algorithms on a capacitively coupled plasma is presented and the results are compared to those of the simulation.

Recently, diamond-like carbon (DLC) films have been performed on polymer substrates for improving scratch resistance and gas barrier properties. However, the DLC films deposited directly on polymers often encountered the problem of poor adhesion, which can reduce the performance of the DLC films. Low adhesion of the DLC films is recognized as a consequence of a residual stress due to high atomic density in comparison to polymers. Plasma pre-treatment is one of the most effective methods to modify the top surface of polymers involving surface cleaning, ablation and surface chemical functionalization. Since the bonding states of the interface are formed at the initial stage of the film growth, the adhesion strength of the films is controlled by the condition of plasma pre-treated surface. However, there have been few reports that directly dealt with the relation between the interface properties and adhesion of the DLC films.

In this study, the DLC films have been prepared on polyethylene terephthalate (PET), polycarbonate (PC) and (PMMA) substrates using a pulse biased ICP-CVD method. Plasma pre-treatments using Ar, O₂, CO₂, N₂ and CH₄ gases were performed on polymer substrates prior to DLC (non-doped, Si-doped, and oxygen-doped) coatings. The plasma pre-treated surfaces have been investigated by XPS and FT-IR ATR. The adhesion of the DLC films on polymer substrates has been characterized with a scratch test method. The scratched areas have been observed with optical microscope and SEM. Regarding the adhesion on the PET, at this moment, the doping oxygen in the films and the plasma pre-treatment have shown no effects on the adhesion of the DLC films. On the PC, on the other hand, the oxygen incorporation in the Si-doped DLC films has resulted in the enhanced adhesion of films. Furthermore, formation of the interfacial layer with N₂-plasma pre-treatment has markedly increased the adhesive strength of the DLC films on the PMMA.

Since the nitrogen atoms or NH bonds are considered to be a key factor to improve the interfacial adhesion properties, we have examined formation of the Self-Assembled Monolayer (SAM) which has NH groups at the top of the SAM. The SAM is composed of a bundle of relatively long molecular chains. Thus, we can expect the SAM layer to have mechanical flexibility. This will brings about further improvement of the adhesion properties of the DLC films, such as prevention of film-peeling due to thermal history, which is now under investigation.

Long line-shaped plasmas are inevitable in material processing in manufacturing industries, such as solar cell film CVD, flat panel displays (FPDs), and various surface modification of large-area thin films. In this work, a newly proposed method of large-scaled line plasma production is studied. In this method, microwave power of frequency of 2.45 GHz in a narrowed and flattened rectangular waveguide is employed to produce a long uniform line plasma. Since the width of waveguide is very close to the cutoff condition, the wavelength of microwave inside the guide is very much lengthened, providing a condition of long line high density plasma with a great uniformity.

The narrowed rectangular wave-guide of 1.0 and 2.0 m in length and 5mm in height were prepared and the width of the waveguide is 62.0 to 61.5 mm which is very closed to the cut-off condition. The waveguide has a long slot on the top surface to launch the micro-wave into the discharge plasma chamber of 1.0 and 2.0 m in length. At the end of wave guide, a short plunger was quipped to adjust the phase of the standing microwave, hence the uniformity of the plasma thus produced. The plasmas of Ar at the pressures of 100 to 500 mTorr were produced by employing an extremely long microwave wavelength. The plasma thus produced was three-dimensionally measured by a Langmuir probe.