ALD2018 Session ALE1-TuM: ALE: Gas-phase and/or Thermal ALE

Controlling Ångstrom-thick film etching is essential for further development of sub-10 nanometer semiconductor manufacturing. The atomic scale era requires the use of decreasing film thickness together with stringent surface property control: preventing material damage and controlling over etching directionality and material selectivity.

Single digit nanofabrication requires the ability to achieve atomic scale etching control and material selectivity during pattern transfer. Atomic Layer Etching (ALE) satisfies these needs as critical dimensions continue to shrike. An ALE process consists of two sequential steps: A) surface modification: a thin reactive surface layer with a well-defined thickness is created B) layer removal: the modified layer is more easily removed than the unmodified material.

Here we study a Fluorocarbon(FC)-based ALE process using Ar plasma and CHF₃ gas for controlling the etching of silicon dioxide at the atomic level. During the saturative surface reaction, CHF₃ is injected in the steady state Ar plasma. CHF₃ breaks and forms some fluorocarbon polymer on the SiO₂ surface. CHF₃ is then purged from the chamber and FC polymer is a source of fluorine, which reacts with SiO₂ modifying its surface. For low energy Ar⁺ ion bombardment conditions, the physical sputter rate of the substrate vanishes, whereas the modified surface can be etched when FC reactants are present at the surface .

With the goal of achieving high selectivity FC-based ALE, we first investigated the etching per cycle (EPC) using spectroscopic ellipsometer on unpatterned surfaces. Using CHF₃-based ALE for SiO₂ etching, we proved ALE self-limiting behavior with etching rate of 6 Å/cycle. Figure 1 in Supplement shows SiO₂ features varying the ion power during the removing step (which decrease moving left to right) etched using different masks: ZEP and Chromium. Using a Cr mask (Figure 1: Row 2) the EPC is similar to the one of flat surfaces. Instead, SiO₂ features using ZEP mask (Figure 1: Row 1) have an EPC 50% higher than expected. Polymer mask (ZEP) is a source of carbon, hydrogen and oxygen, which interfere with the etch process bringing it out of the self-limiting window. SiO₂ features etched using lower DC bias (17 V) are aspect-ratio independent and results in a low degree of physical/ionic etching.

A successful application of the FC-ALE approach has been demonstrated. Overall, the cyclic CHF₃/Ar etch has proven to pattern features well with an hard mask, with great potential for significant improvement in overall etch performance.

Atomic layer etching (ALE) is one of the most attractive methods for overcoming the limitations of conventional processes. One of the advantages of ALE is low-damage etching. However, it is impossible to stop plasma-induced damage from occurring because ions, radiation, and radicals are irradiated on a surface simultaneously. In the case of low-energy incidences of ions (~ 50eV), the effect of radiation and radicals also has an impact on damage generation. In this presentation, I review the results of ion/radiation damage analysis.

Suppression of Si substrate damage caused by energetic ion bombardment is one of the most critical issues in advanced devices. The threshold energy for atomic displacement (Ed) of bulk Si crystal is reported to be 10-30 eV. When we use ion energy higher than the Ed, it is possible to generate damages in Si substrate. Si substrate damage during gate electrode etching causes a “Si recess” structure to form, which is reported to degrade device performance. Although the damage basically consists of a surface oxidized layer and underlying dislocated Si, the damage structure strongly depends on the incident ion species, ion energy, and oxidation during air and plasma exposure. As both the Si recess and remaining dislocated Si degrade electrical performance, precise control of incident ion energy is indispensable to minimize damage generation.

We also found that photons with longer wavelengths than optical bandgap energy could reach the underlying dielectrics/Si interface and cause interface damage. UV radiation transmitting through the upper dielectrics could cause the electrical characteristics of underlying MOS devices to fluctuate. Thus, we must control VUV/UV radiation from plasma, even in the case of ALE processes.

ALE is one of the pulsed plasma, so we investigated an effect of transient behavior of a pulse modulated inductively coupled plasma on the photon-induced interface defects. The time-resolved OES revealed that the optical emission overshoot appeared after ignition because of the variation of the electron temperature and number density in the early ON phase. As a result, the amount of UV radiation damage increased compared with CW plasma. Thus, control of transient behavior is essential for reducing photon-induced damage in pulsed plasma.

The key challenges of atomic level processes for plasma-induced damage reduction are almost same as CW plasma. The effects of VUV/UV radiation and radicals become particularly apparent in the case of low-energy incidences of ions. In addition, the control and the monitoring of the transient behavior of pulsed plasma become more important for atomic precision etching.

The demand for atomic layer/level etching (ALE) is increasing as the geometries of semiconductor devices continue to shrink. Remarkable progress has been made in the recent years to meet the enhanced requirements for a variety of materials to be atomically etched. One approach for selective isotropic ALE is rapid thermal cyclic ALE, which is cyclic repetitions of a self-limiting surface modification and a thermal desorption. Rapid thermal cyclic ALE of SiN, TiN, and W was developed by the authors [1, 2]. As for high-k lanthanum oxide (La₂O₃) whose boiling point is more than 4000°C, thermal etching via a low temperature (< 250°C) volatile organo-lanthanum complex has also been developed [3], although the cyclic process has yet to be confirmed.

In this work, we present thermal cyclic ALE of La₂O₃ exposed to a mixture of a ketone-based ligand and a stabilizer followed by a thermal annealing. Our preliminary cyclic experiments on powdered La₂O₃ demonstrated that the mass of the powder sample decreased as the number of repetition cycles increased. In one cycle of chemical exposure and annealing, the average loss in mass was 0.074 mg, which corresponds to about 30 nm/cycle for a La₂O₃ film.

Formation of an organo-lanthanum complex was quasi-self-limiting because the change in the mass of the powder sample had shown saturation behavior. The volatile organic species in preliminary experiments were collected and spectroscopically analyzed. The resultant data was identified as an organo-lanthanum complex bearing a ligand moiety and a stabilizer moiety.

Next, we examined the effect of temperature during the chemical exposure. In this experiment, both La₂O₃ and SiO₂ films deposited on silicon substrates were used as sample materials. Substrate temperatures during the chemical exposure were 140°C and 260°C. The films’ thicknesses were evaluated by scanning electron microscopy after the exposure. Because the etching of the La₂O₃ film was not detected when the temperature was 140°C, we concluded that the formation of the organo-lanthanum complex is self-limiting. On the other hand, continuous etching of the La₂O₃ film was observed when the temperature was 260°C, at which point the organo-lanthanum complex was volatile. The etching of SiO₂ was not detected at both temperatures. From these findings, we concluded that selective, thermal cyclic ALE of lanthanum oxide was successfully demonstrated.

[1] K. Shinoda et al., J. Phys. D: Appl. Phys. 50, 194001(2017).

[2] K. Shinoda et al., SPIE Advanced Lithography 10589-17 (2018).

[3] Y. Yamaguchi et al., AVS Symposium and Exhibition, PS+NS+SS+TF-ThM13 (2017).

The thermal atomic layer etching (ALE) of silicon was performed using an oxidation and “conversion-etch” mechanism (see Supplemental Figure S1). In this process, the Si surface is oxidized to a silicon oxide layer using O₂ or ozone. The silicon oxide layer is converted to an Al₂O₃ layer using trimethylaluminum (TMA). The Al₂O₃ layer is fluorinated by HF to an AlF₃ layer prior to the removal of the AlF₃ layer by ligand-exchange using TMA. Si ALE was studied using silicon-on-insulator (SOI) wafers in a warm wall reactor with a hot sample stage. In situ spectroscopic ellipsometry was employed to monitor the thickness of both the silicon and the silicon oxide layer during Si ALE. These studies observed that the silicon film thickness decreased linearly with number of reaction cycles while the silicon oxide thickness remained constant.

Using an O₂-HF-TMA reaction sequence, the Si ALE etch rate was 0.4 Å/cycle at 290°C (see Supplemental Figure S2). This etch rate was obtained using static reactant pressures of 250, 1.0 and 1.0 Torr, and exposure times of 10, 5 and 5 s, for O₂, HF and TMA, respectively. Reducing the O₂ pressure below 250 Torr reduced the etch rate. A lower etch rate of 0.2 Å/cycle at 290°C was observed at an O₂ pressure of 30 Torr. The order of the reactant sequence affected the Si etch rate. Changing the reactant sequence from O₂-HF-TMA to O₂-TMA-HF reduced the etch rate from 0.4 to 0.2 Å/cycle at 290°C. Comparable etching rates were observed using ozone instead of O₂ as the oxidant. Using an O₃-HF-TMA reaction sequence, the Si ALE etch rate was 0.5 Å/cycle at 290°C. The Si ALE etch rate decreased with process temperature for both the O₂-HF-TMA and O₃-HF-TMA reaction sequences. An oxide thickness of ∼10-20 Å remained after Si ALE at 290°C. However, this oxide thickness could be removed by sequential TMA and HF exposures without influencing the underlying silicon film.

This new thermal Si ALE process is expected to yield isotropic etching. Thermal Si ALE will complement the plasma Si ALE process based on directional Ar⁺ ions that yields anisotropic etching. Thermal Si ALE should be useful in advanced semiconductor fabrication. Thermal Si ALE could also be utilized for atomic-scale polishing and cleaning of silicon surfaces. In addition, there may be applications in other areas such as silicon-based optoelectronics, photonics and MEMS fabrication.

An increase in Al₂O₃ atomic layer etching (ALE) rates versus time was demonstrated using sequential, self-limiting exposures of hydrogen fluoride (HF) and trimethylaluminum (TMA) as the reactants with no purging. The normal purging expected to be required to prevent chemical vapor etching (CVE) or chemical vapor deposition (CVD) is not necessary during Al₂O₃ ALE. The purgeless, rapid atomic layer etching (R-ALE) was studied from 250°C to 325°C using various techniques.

In situ quartz crystal microbalance (QCM) measurements monitored Al₂O₃ R-ALE at 300°C. The Al₂O₃ R-ALE process produced linear etching versus number of R-ALE cycles. The net result of fluorination by HF and ligand-exchange with TMA was a mass change per cycle (MCPC) of -32.1 ng/cm² using sequential, 1 s exposures for both HF and TMA with no purging. This mass change equates to a thickness loss of 0.99 Å/cycle or 0.49 Å/s. The Al₂O₃ R-ALE etch amount per cycle at 300°C was nearly equivalent to the previously measured Al₂O₃ ALE etch amount per cycle at 300°C using 30 s purge times. However, the Al₂O₃ R-ALE etch rate versus time at 300°C was 78 times faster than the previously measured Al₂O₃ ALE etch rate versus time at 300°C using 30 s purge times. Ex situ spectroscopic ellipsometry (SE) and XRR measurements also revealed temperature-dependent etch rates for Al₂O₃ R-ALE from 0.02 Å/cycle at 270°C to 1.12 Å/cycle at 325°C.

Additionally, patterned SiO₂ wafers were used to compare Al₂O₃ R-ALE and normal Al₂O₃ ALE with purging in high aspect ratio structures. Scanning electron microscope (SEM) images revealed that the etching was uniform for both processes and yielded comparable etch rates per cycle in the high aspect ratio structures. The HF and TMA precursors were also intentionally overlapped by various amounts to explore the behavior when both precursors are present at the same time. Similar to atomic layer deposition (ALD), where precursor overlap produces chemical vapor deposition (CVD), precursor overlap during Al₂O₃ ALE leads to AlF₃ CVD. However, any AlF₃ CVD growth that occurs during precursor overlap is removed by spontaneous AlF₃ etching during the subsequent TMA exposure. This spontaneous AlF₃ etching by TMA explains why no purging is necessary during R-ALE.

Self-limiting atomic layer etching (ALE) is essential for enabling the manufacturing of complex transistor structures. Thermal ALE, based on thermal desorption of volatile etch products, is appealing for avoiding plasma-induced damages of sensitive device structures. However, thermal ALE process for metals is currently lacking. In our study, we demonstrated two thermal atomic layer etching processes based on the sequential oxidation-etch reaction schemes which allow controlled W to proceed for temperatures ≥ 275 and ≥ 200°C, respectively.

In the first part of our study, we investigated a thermal tungsten (W) ALE process using oxygen (O₂) and tungsten hexafluoride (WF₆). Based on insights gained from thermodynamic modeling, we proposed that W etching could proceed in two sequential reaction steps: 1) oxidation of tungsten, and 2) formation and removal of volatile WO₂F₂ during reaction with WF₆ (Supplemental, Figure 1a). Using in-situ Quartz Crystal Microbalance (QCM) analysis, we found the O₂ and WF₆ half reactions exhibit self-limiting characteristics therefore making the O₂/WF₆ process promising for W ALE. In addition, the etch rate is ~ 6.3 Å/cycle at 300°C but decreases to 0.34 Å/cycle when temperature decreases from 300°C to 220°C (Supplemental, Figure 1b). The significant decrease in etch rate is likely due to the limited volatility of WO₂F₂ at temperatures < 275°C as suggested by thermodynamic modeling results.

W etching with the O₂/WF₆ process is limited to temperatures ≥ 275°C, however lower temperature ALE processes are more desired for enabling selective deposition. Through further thermodynamic modeling studies, we find that a lower temperature W ALE process is viable when tungsten hexachloride (WCl₆) is used as the etchant instead of WF₆. Thermodynamic modeling results revealed that WCl₆ reaction with WO₃ can give rise to formation of volatile WO₂Cl₂ at temperatures ≥ 200°C (Supplemental, Figure 2a). Using in-situ QCM analysis, we found that W etching with O₂/WCl₆ process is self-limiting and proceeds readily for temperatures ≥ 220°C (Supplemental, Figure 2b).

In summary, we demonstrated thermally driven W ALE using O₂/WF₆ and O₂/WCl₆ processes, and our work on thermal W ALE serves to increase the fundamental understanding on ALE reactions and expand the base of available ALE processes for advanced material processing.

Alternating exposure to trimethyaluminum (TMA) and hydrogen fluoride (HF) has been used previously in the ALD of AlF₃ thin films. At substrate temperatures higher than approximately 225°C, the same exposure can result in the thermal atomic layer etching (ALE) of thin films of Al₂O₃. In previous work, we have shown that TMA/HF exposure performed in the presence of lithium fluoride (LiF) can significantly increase the Al₂O₃ etch rate and reduce the transition temperature where etching dominates over deposition.[1] In this work, we expand this approach to study the effect of other alkali halide compounds such as NaCl, KBr, and CsI. These materials are introduced externally into our vacuum chamber and influence the process behavior over surprisingly large spatial scales. All materials are observed to have an enhancing effect on the ALE process with respect to the etch rate of Al₂O₃ at a given substrate temperature. In some cases these materials can enable the reduction of the substrate temperature where continues etching of Al₂O₃ is observed to as low as 125°C, a regime where the deposition of AlF₃ would otherwise dominate the TMA/HF process. The choice of alkali material, choice of aluminum precursor, and process parameters like substrate temperature and precursor dose, can be exploited to tune the rate and selectivity of the thermal HF-based ALE process. We present characterization of these processes to etch Al₂O₃ in a self-limiting manner, and discuss applications of the method related to the performance improvement of aluminum optical components in the deep ultraviolet for space applications.

[1] J. Hennessy et al., J. Vac. Sci. Technol. A 35, 041512 (2017).