CONTACT EDGE ROUGHNESS IN THE ETCHING OF HIGH ASPECT RATIO CONTACTS IN SiO2* Shuo Huang, Chad Huard and Mark J. Kushner University of Michigan, Ann Arbor, MI 48109, USA shuoh@umich.edu, chuard@umich.edu, mjkush@umich.edu Seungbo Shim, Sangheon Lee, In-Cheol Song and Siqing Lu Samsung Electronics Co., Republic of Korea seungb.shim@samsung.com, s2009.lee@samsung.com, ic13.song@samsung.com, siqing.lu@samsung.com The 44th International Conference on Plasma Science, Atlantic City, New Jersey, USA 21-25 May 2017 Work supported by Samsung Electronics Co., DOE Fusion Energy Science and National Science Foundation. 1 1

University of Michigan Institute for Plasma Science & Engr. AGENDA High aspect ratio contacts (HARCs) in SiO2 Contact edge roughness (CER) Monte Carlo feature profile model (MCFPM) Reaction mechanism of SiO2 etching by Ar/C4F8/O2 3D profile simulation of HARC etching in SiO2 Single HARC: scalloped, convex and elliptic Multiple HARCs: rectilinear and honeycomb Concluding remarks University of Michigan Institute for Plasma Science & Engr. ICOPS_2017_S.Huang 2 2

HIGH ASPECT RATIO CONTACTS IN SiO2 Contact edge roughness (CER) becomes a major lithographic challenge as critical dimensions (CDs) decrease and aspect ratios (ARs) increase. Origin of CER is in part the randomness of lithography that produces photoresist (PR) mask and in part the mechanical stress on PR. CER in the mask is transferred to underlying materials being etched, resulting in pattern distortion (e.g., scalloped, convex and elliptic). In this talk, results from computational investigation of etching of single and multiple high aspect ratio contacts (HARCs) in SiO2. Side view Top view Contact Plug Tandou et al., Precis. Eng. 44, 87 (2016). Constantoudis et al., J. Micro/Nanolith. MEMS MOEMS 12, 013005 (2013). Takeda et al, IEEE Trans. Semicond. Manuf. 21, 567 (2008). University of Michigan Institute for Plasma Science & Engr. ICOPS_2017_S.Huang 3 3

MONTE CARLO FEATURE PROFILE MODEL (MCFPM) Multiscale model: Reactor scale (HPEM): cm, ps-ns; Sheath scale (PCMCM): μm, μs-ms; Feature scale (MCFPM): nm, s. MCFPM resolves surface topology on 3D Cartesian mesh. Each cell has a material identity. Gas phase species are represented by Monte Carlo pseudoparticles, which are launched with energy and angular distributions from PCMCM. Cell identities changed, removed or added for reactions such as etching and deposition. Resist SiO2 Si HPEM PCMCM MCFPM ne, Te, EEDF, ion and neutral densities Ion energy and angular distributions Etch rates and etch profile Reactor scale Sheath scale Feature scale University of Michigan Institute for Plasma Science & Engr. ICOPS_2017_S.Huang

FEATURE PROFILE REACTION MECHANISM SiO2 surface activated by ion bombardment. SiO2(s) + I+(g)  SiO2*(s) + I*(g) CxFy neutrals react with activated SiO2* surface to form complex layer (passivation). SiO2*(s) + CxFy(g)  SiO2CxFy(s) Further deposition of CxFy neutrals produces thick polymer layer (CxFy)n. Energetic ions and hot neutrals penetrate polymer layer to sputter O. SiO2CxFy(s) + I+(g)  SiFy(g) + CO2(g) + I*(g) Remaining Si is dominantly etched by F atoms through volatile SiF4. Thickness of polymer layer can be controlled through flux of O radicals. (CxFy)n(s) + O(g)  (CxFy)n-1(s) + COFx(g) Schematic of surface reaction mechanism for SiO2 etching by fluorocarbon plasma. Ref: Sankaran et al., JVSTA 22, 1242 (2004). University of Michigan Institute for Plasma Science & Engr. ICOPS_2017_S.Huang 5

MULTIFREQUENCY CCP – NEUTRAL, ION FLUXES Ar/C4F8/O2 = 0.75/0.15/0.1, 25 mTorr, 500 sccm. 3-Frequency CCP: 80/10/5 MHz = 0.4/2.5/5 kW = 170/385/1670 V, Vdc=-390 V. With significant dissociation, radical fluxes to wafer dominated by CFx, O, F. Large fluxes of non-reactive dissociation products (e.g., C2F4). Reactive neutral fluxes exceed ion fluxes by 1-2 orders of magnitudes. Ion fluxes dominated by Ar+ due to larger mole fraction of Ar. Large CnFx+ flux due to lower ionization potentials compared to CFx+. University of Michigan Institute for Plasma Science & Engr. ICOPS_2017_S.Huang 6

MULTIFREQUENCY CCP – IEADs TO WAFER Ar/C4F8/O2 = 0.75/0.15/0.1, 25 mTorr, 500 sccm. 3-Frequency CCP: 80/10/5 MHz = 0.4/2.5/5 kW = 170/385/1670 V, Vdc=-390 V. Sheath at wafer has components of both low and high frequencies. Combination of multi-frequencies and large range of ion masses (12 – 180 AMU) results in broad ion energy distributions. Fairly thick and collisional sheath for ions produces significant low energy component contributing to polymerization. University of Michigan Institute for Plasma Science & Engr. ICOPS_2017_S.Huang 7

CONTACT EDGE ROUGHNESS (CER) – SCALLOPING Transfer of PR pattern with periodic scalloped edge into underlying oxide. Slower etch rates as aspect ratio increases. Significant passivation (SiO2CxFy) at sidewall. Animation Slide Unit: nm University of Michigan Institute for Plasma Science & Engr. ICOPS_2017_S.Huang

University of Michigan Institute for Plasma Science & Engr. SCALLOPING – RESIST Time 1 1 2 3 2 3 Erosion of mask blurs scalloping. Scalloping in resist transferred to vicinity of oxide. University of Michigan Institute for Plasma Science & Engr. ICOPS_2017_S.Huang

SCALLOPING – SHALLOW OXIDE Time 4 4 5 5 6 6 The scalloping in mask can be transferred to ~300 nm deep in the oxide (AR=5). Reflection from sidewall with large angles blurs scalloping. University of Michigan Institute for Plasma Science & Engr. ICOPS_2017_S.Huang

SCALLOPING – DEEP OXIDE Time 7 8 9 7 8 Random onset profile deep in the oxide. Scalloping dissipates as etch depth increases due to finite angular distribution of ions and reflection from sidewalls. 9 University of Michigan Institute for Plasma Science & Engr. ICOPS_2017_S.Huang

SCALLOPING – ION ANGULAR DISTRIBUTION (IAD) Dielectric etch is more "ion – driven" process and so more sensitive to IEAD. Depth (AR) that scalloping can be transferred into oxide decreases as IAD varies from anisotropic to broad. Bowing due to reflected ions with broad IAD blurs scalloping. IAD with 100% anisotropy preserves scalloping through the entire depth, which can be used to reduce the corner rounding in, e.g., L-shaped and U-shaped contacts. =0 (anisotropic) =0.5 (narrow) =1.0 =2.0 (broad) Horizontal slices from top to bottom of final etch profile University of Michigan Institute for Plasma Science & Engr. Animation Slide ICOPS_2017_S.Huang

SCALLOPING – MAGNITUDES: 1.5 – 4.5 nm Periodic scalloped PR pattern with different magnitudes. Scalloping in PR is blurred with PR erosion, which contributes to smoother profile in oxide. Small increase in etch rate for circular via due to more vertically directed specular scattering. Small magnitudes almost fully smoothed as AR increases to 3. Large magnitudes transferred to deeper in the oxide (AR>5). Top view of pattern in PR Horizontal slices from top to bottom of final etch profile University of Michigan Institute for Plasma Science & Engr. Animation Slide ICOPS_2017_S.Huang

CONVEX – MAGNITUDES: 1.5 – 4.5 nm Short/breakdown between contact hole and poly gate due to asymmetric CER – convex. Compare convex with different magnitudes. Bowing in oxide removes some of the asymmetry and small convex is smoothed. Large convex cannot be smoothed. Rather, ellipse will be induced during the etching as the sharp corner is rounded. Contact plug Top view of pattern in PR Horizontal slices from top to bottom of final etch profile University of Michigan Institute for Plasma Science & Engr. Animation Slide ICOPS_2017_S.Huang

ELLIPSE – ELLIPTICITY: 0.4 – 0.8 Contact shape and area affect source/drain (S/D) contact resistance and saturation current. Decreased ellipticity (more circular profile) from top to bottom of oxide Broad view angle for small curvature and narrow view angle for large curvature. Larger etch rate in the direction of minor axis than major axis. S/D contact. Ban et al., J. Micro/Nanolith. MEMS MOEMS 9, 041211 (2010). Top view of pattern in PR Horizontal slices from top to bottom of final etch profile University of Michigan Institute for Plasma Science & Engr. Animation Slide ICOPS_2017_S.Huang

MULTIPLE HARCs – ALIGNED Section view showing non-uniform PR erosion Pattern PR CER in oxide at vicinity of PR for final profile SiO2 Thin and erodable PR with chamfered openings used to investigate interference between adjacent vias. As PR is eroded, position of ion reflection from PR changes (interrupts trajectories). More severe interference when PR sidewall evolves from vertical to purely chamfered. Si Thickness of PR: 50 nm Thickness of oxide: 680 nm Animation Slide University of Michigan Institute for Plasma Science & Engr. ICOPS_2017_S.Huang

ALIGNED MULTIPLE HARCs – PROXIMITY Top view of pattern in PR Contact proximity effect. Kuppuswamy et al., J. Micro/Nanolith. MEMS MOEMS 12, 023003 (2013). PR/oxide interface of final etch profile Same etch depth with varied proximity. Large proximity: each individual via can maintain circular profile with CER due to randomness. Small proximity: asymmetric profile (non-circular, convex) induced due to cross talk between adjacent vias. Interference between vias enhances distortion of any individual via and results in more CER in oxide. University of Michigan Institute for Plasma Science & Engr. ICOPS_2017_S.Huang

OFF-AXIS MULTIPLE HARCs – HONEYCOMB Top view of pattern in PR PR/oxide interface of final etch profile Half pitch HARCs. Kim et al., JVSTA 33, 021303 (2015). Same proximity and etch depth with different off-axis distance. Periodic boundary condition is applied. The vias are stretching along the proximity direction due to interference between adjacent vias. Incomplete isolation between vias in closely packed patterns results in distortion in oxide. University of Michigan Institute for Plasma Science & Engr. ICOPS_2017_S.Huang

University of Michigan Institute for Plasma Science & Engr. CONCLUDING REMARKS Contact edge roughness (CER) in the etching of single and multiple HARCs in SiO2 was investigated using 3D-MCFPM. Transfer of different types of roughness (e.g., scalloped, convex and elliptic) from PR pattern into oxide: CER dissipates as etch depth (AR) increases due to finite IAD and reflection from sidewalls with large angles. Depth that scalloping can be transferred into oxide decreases as IAD varies from anisotropic to broad. Interference between adjacent vias in closely packed pattern enhances distortion of any individual via and more CER in oxide. As proximity decreases, asymmetric profile (e.g., non-circular, convex, elliptic) induced due to interference between vias. Off-axis vias: edges of vias stretch along the proximity direction due to thin and incomplete isolation by PR. University of Michigan Institute for Plasma Science & Engr. ICOPS_2017_S.Huang 19 19