INVESTIGATING THE ROLE PROCESS NON-IDEALITY IN THE ATOMIC LAYER ETCHING OF HIGH ASPECT RATIO FEATURES* Chad Huard and Mark J. Kushner University of Michigan Ann Arbor, MI 48109-2122 USA chuard@umich.edu, mjkush@umich.edu Yiting Zhang, Saravanapriyan Sriraman and Alex Paterson Lam Research Corp., Fremont, CA 94538 USA * Work supported by Lam Research Corp., DOE Office of Fusion Energy Science and National Science Foundation. 1 1

NEED FOR ATOMIC LAYER ETCHING (ALE) IN 3D (finFET) PROCESSING In 3D structures many aspect ratios (AR) must be simultaneously etched, requiring aspect ratio dependent effects to be minimal. Thin stopping layers may be exposed for significant portions of the gate etch, requiring high selectivity for low damage. 14nm finFET – from EE Times 16nm gate I/O transistor – from EE Times pictures http://www.eetimes.com/author.asp?section_id=36&doc_id=1326319 http://www.eetimes.com/author.asp?section_id=36&doc_id=1328639 University of Michigan Institute for Plasma Science & Engr. MIPSE_2016

BENEFITS OF ATOMIC LAYER ETCHING Using self limited surface reactions during plasma etching: Decouple neutral and ion transport issues. Preserve profiles over wider ranges of aspect ratio (AR). Reduce damage with low ion energies. Mitigate non-idealities associated with low ion energy. Increase selectivity. pictures Experimental comparison of cw and ALE etch profiles (Ref: Lam Research.) http://http://electroiq.com/blog/2014/01/moving-atomic-layer-etch-from-lab-to-fab University of Michigan Institute for Plasma Science & Engr. MIPSE_2016

CHALLENGES OF ALE OF SILICON In order to fully benefit from ALE, both passivation and ion bombardment phases must be self limiting. For passivation phase this requires: No ion stimulated processes. No thermal etching. For ion bombard phase this requires: Ion energy > sputtering threshold of passivated SiClx Ion energy < sputtering threshold of bare Si No chlorinating species present (Cl, Cl+, Cl2+). pictures University of Michigan Institute for Plasma Science & Engr. MIPSE_2016

INTEGRATED MODEL HIERARCHY Multiscale model Reactor scale addresses plasma properties Sheath scale generates ion energy and angular distributions (IEAD) Feature scale model uses IEADs (and neutral fluxes) to simulate the evolution of device level structures Length and time scales are different in each model (cm, ps  nm, s) pictures University of Michigan Institute for Plasma Science & Engr. MIPSE_2016

MONTE CARLO FEATURE PROFILE MODEL Profile is defined on a 3D mesh, with cells representing a material. Gas phase pseudo-particles are launched with fluxes, energy and angular distributions derived from HPEM. Trajectories of particles are tracked until striking solid. Surface reaction mechanism defines outcome of collisions. Chemical reaction (material change) Etching Deposition All particles, including reaction products, are tracked until reacting or leaving the simulation domain pictures University of Michigan Institute for Plasma Science & Engr. MIPSE_2016

Ar/Cl2 ETCHING MECHANISM Etching of Si in Ar/Cl2 plasma is used to study the origins of ARDE Etch Mechanism Successive chlorination by radical Cl (Cl+, Cl2+) Si(s) + Cl(g) → SiCl(s) SiClx(s) + Cl(g) → SiClx+1(s) for x < 3 SiClx removal by energetic particles (Ar+, Cl+, Cl2+ and hot neutral counterparts) SiClx(s) + M+ → SiClx(g) + M Sputter probability increases with chlorination and ion energy Sputtering threshold: Si: 50 eV SiClx: 10 eV pictures University of Michigan Institute for Plasma Science & Engr. MIPSE_2016

IDEAL Ar/Cl2 ALE REACTION Passivation Phase (Cl): Cl flux: 7.51017 cm-2 s-1 No ions. Ion Bombardment phase (Ar+): Ar+ flux: 1.01017 cm-2 s-1 Ion energy: 24 eV Perfectly anisotropic angular distribution. No passivating species (Cl, Cl+ or Cl2+). Feature: 30 nm trench Mesh size = 0.3 nm Note: Images in (b) are not evenly spaced in time. pictures University of Michigan Institute for Plasma Science & Engr. MIPSE_2016

SURFACE PROPERTIES IN IDEAL ALE Ar+ phase: 0.64 s Cl phase: 0.64 s Etch depth shows exactly one monolayer (ML) removed in each cycle Roughness is measured using the surface area of the etch front. Roughness returns to 0 (perfectly flat etch front) after each pulse. During passivation phase, average surface chlorination rapidly rises to a steady state value. Ion bombardment phase returns surface to entirely bare silicon. All necessary purge times are excluded from the time scale. pictures University of Michigan Institute for Plasma Science & Engr. MIPSE_2016

ASPECT RATIO DEPENDENCE OF IDEAL ALE Previous example had an aspect ratio (AR) of 2 due to depth of resist. Same conditions in a feature with an AR of 10 results in: Retention of ideal ALE characteristics. Aspect ratio independent etch rate. Significantly longer time to steady state in passivation phase. Identical surface character in ion bombardment phase. pictures University of Michigan Institute for Plasma Science & Engr. MIPSE_2016

REACTOR SCALE PROPERTIES Reactor scale simulations of an inductively coupled plasma (ICP) were performed to provide non-ideal ion and radical fluxes, and IEADs. 20 mTorr w/ 200 sccm flow 150 W ICP, 10MHz Passivation phase: Ar/Cl2 = 80/20 Vbias = 0 V Ion bombardment Phase: Ar (100 ppm Cl) Vbias = 30 V, 10MHz Passivation Phase: Vbias = 0 V Bombard Phase: Vbias = 30 V pictures University of Michigan Institute for Plasma Science & Engr. MIPSE_2016

FLUXES AND ENERGY DISTRIBUTIONS IEDs with bias are broad and depend on masses. Even in the passivation phase, ions are not mono-energetic due to finite thickness of presheath. Passivation phase: Cl 7.5  1017 (cm-2 s-1) Ar+ 5.0  1014 Cl2+ 1.1  1016 Cl+ 1.7  1015 Ion bombardment phase: Cl 3.3  1015 (cm-2 s-1) Ar+ 1.0  1017 Cl2+ 3.7  1013 Cl+ 5.5  1014 pictures University of Michigan Institute for Plasma Science & Engr. MIPSE_2016

NON-IDEAL ALE SURFACE PROPERTIES ALE cycle persists, but is largely dominated by continuous etching due to non-self-limiting reactions. Etch rate is no longer aspect ratio independent. Roughness is significantly higher than ideal case and does not return to 0. Roughness achieves a steady state (i.e. not continually roughening). Steady state surface coverages of chlorine are now dependent on aspect ratio. Ion flux is nearly constant at different AR. Radical flux strongly depends on AR. pictures University of Michigan Institute for Plasma Science & Engr. MIPSE_2016

NON-IDEAL ALE WITH SHORT PULSES To combat continuous etching we can shorten pulse times. Cl = 30 ms, Ar+ = 160 ms Etch depth per cycle is only slightly more than 1 ML in AR = 2 case. Etch depth per cycle is less than 1 ML for AR = 10 case. Roughness is lower than longer pulses, and the pattern is much more similar to ideal ALE. Chlorination does not reach steady state for either depth. This solution restores much of the ALE behavior for AR = 2, but not at AR = 10. Higher aspect ratio feature would require longer passivation phase to achieve similar results. pictures University of Michigan Institute for Plasma Science & Engr. MIPSE_2016

PULSE TIME OPTIMIZATION With ideal conditions, ALE can be produced for nearly arbitrary combinations of passivation and ion bombard times. For non-ideal, every AR has sets of passivation and ion bombarding times that produce ALE behavior. (Note: If reactions during each phase are not 100% self-limited ARDE results.) Even with non-ideal reactions, there will always be some combination of Cl and Ar+ pulse time which will give ML etching. Optimal pulse times for ALE-ness = 1 change with aspect ratio. pictures ALE-ness vs pulse times Solid line: ALE-ness = 1 Shaded region: ).9 < ALE-ness < 1.1 ALE-ness > 1: sub-ML per cycle ALE-ness < 1: More than ML per cycle University of Michigan Institute for Plasma Science & Engr. MIPSE_2016

NON-IDEAL ETCHING CONTRIBUTIONS AR=4 Even with non-ideal reactions an ideal ALE-ness can be achieved by adjusting the pulse times. ALE-ness = 1 does not guarantee ideal etching. Non-ideal reactions introduce roughening, ARDE. Contribution of etching during the passivation phase, and sputtering during the ion bombardment phase as a function of pulse times. Minimizing these non-idealities requires both pulse times be as short as possible to achieve desired ALE-ness. pictures University of Michigan Institute for Plasma Science & Engr. MIPSE_2016

CLEARING 3D CORNERS IN GATE ETCH Etching the gate around 3D fins requires: Significant over-etch to clear 3D corners. High selectivity. Low damage. Selectivity and damage requirements often are achieved with low ion energy, which exacerbates need for over-etch. Approximately 14 nm process dimensions. Animations exclude purge time. pictures University of Michigan Institute for Plasma Science & Engr. MIPSE_2016

CLEARING 3D CORNERS WITH LOW BIAS RIE Material in corners is difficult to remove. 100% over-etch used, still not completely cleared in 3D corner. Vbias = 30 V, Ar/Cl2 = 70/30 Max ion energy: 50 eV Etch time: 132 s Highly tapered etch profile results in material buildup in corners. Cl+ = 3.6  1015 Cl2+ = 9.0  1015 Fluxes (cm-2s-1) Cl = 8.7  1017 Ar+ = 4.5  1014 pictures University of Michigan Institute for Plasma Science & Engr. MIPSE_2016

CLEARING 3D CORNERS WITH ALE Passivation phase: 30 ms Ar/Cl2 = 70/30, Vbias = 0 V Bombard phase: 160 ms Ar (100 ppm Cl2), Vbias = 30 V Etch time: 41 s active processing time (excluding any purge time). 216 pulse periods. Flat etch front maintained throughout etch. Requires 25% over-etch to clear corners. Some etching of fin cap, but no significant damage to oxide. pictures University of Michigan Institute for Plasma Science & Engr. MIPSE_2016

University of Michigan Institute for Plasma Science & Engr. CONCLUDING REMARKS First principles modeling of ALE for idealized and realistic process conditions. Model results indicate that: ALE under idealized conditions enable aspect ratio independent etching. Small systematic deviations from the ideal case can produce ARDE. Deviating from self limited reactions that occurs during realistic processing introduces significant aspect ratio dependence. Low ion energies in continuous processes can result in long over- etch requirements. Perfect self limiting reactions are not strictly necessary to benefit from ALE-like pulsed etching within limited etch regimes. pictures University of Michigan Institute for Plasma Science & Engr. MIPSE_2016