Fundamentals of Plasma Etching Part 1 Focus on the Plasma and Ion Energy Control Jim McVittie <mcvittie@stanford.edu> Stanford Nanofabrication Facility Stanford University 2008 NNIN Etch Workshop 1
Outline Etcher Overview RF Plasma Why we use RF excited plasmas The Capacitive Coupled Plasma (CCP) How the rf current across sheath leads the DC bias Why controlling DC bias is important for etching Use of Inductive coupled plasmas (ICP) as low bias source Use of ICP with CCP to control DC bias (Ion Energy) Beyond simple DC biasing for ion energy control 2
Basic Etching Process RF Power Electrons gain energy from RF or µw fields Electrons impact with feed gas to generate ions, reactive neutrals and more electrons Ions and reactive neutrals diffuse and drift to wafer surface where they remove and deposit material 3
Ion Enhanced Etching Effect Spontaneous (Chemical) Etching Ion Enhanced Etching From Coburn and Winters Physical Etching (Sputtering) 25x Etch Rate increase Ions + Adsorbed Reactive Neutral High Etch Rates 4
Ion Directionality V Sheath 0 Position Wafer 5
Plasma Etch Reactors Capacitive Coupled CCP RIE Type High Density Inductive Coupled ICP Type Downstream Plasma Plasma stop Rf Bias Reactive Neutral Etch Product Wafer 6
Why We Use RF DC plasmas Wafer Damage Leads to charging and DC currents through wafer Microwave Plasmas No self (DC) bias (Needed for directional etching) RF plasmas RF current through wafer causes no damage No charging damage if plasma is uniform Exception is electron shading caused charging in high aspect ratio structures Easy to get induced self or DC bias 7
Capacitive Coupled Plasma (CCP) Vacuum Chamber RF 13.5 MHz Glow or Plasma Region Sheaths Wafer Matching Network Gas In To pump Driven electrode To start, initial voltage must exceed V breakdown Depends on gas, pressure and spacing ~ 300 to 600 V RF current through gas maintains steady state discharge by heating electrons Ions and electrons from ionization balance their losses 8
Glow or Plasma Region Generation Region -- Ion, electrons, excited species and molecular fragments generated here Relaxation of excited species produces glow (τ ~1 ns) Reactive fragments important for etching and CVD Quasi-neutral gas -- n i+ = n e- + n i - pos ions n i+, neg ions n i-, electrons n e - n i- can not make it to wafer -- often can be neglected Only weak E fields < 10 V/cm Neutral density >> ion density n o >> n i, n e n i /n o = 10-3 to 10-6 n i ~ 10 9 10 12 cm -3 Electrons carry the RF current in this region Plasma Potential V between plasma and gnd 9
Ionization, Radical Generation and Electron Temperature dn(ε)/dε Electron Energy Distribution Dissociation Ionization Expontential Boltzmann Tail exp(-ε / T e ) 0 T e /2 ε d ε iz Energy, ε T e ~ 4 5 ev 10
Sheath Region Electron depletion region forms at all surfaces to keep electrons in plasma region (n i+ >> n e- ) Dark -- Few electrons no excited species no light Pos Charge High E field (up to a few KV/cm ) Most electrons returned to plasma Few percent make it across Pos ions accelerated toward surface Ions gain energy and directionality Ion current determined by plasma density n e RF current carried by displacement (capacitor) current This is the source for the name capacitive coupling Plasma Ions electrons + - Sheath 11
CCP Currents I rf Plasma E Ions + - J i J e electrons Sheath J Disp Sheath osc Plasma Region Small E field Quasi neutral n i+ = n e - e - lighter & faster v e ~ 100x v i e - carries current J rf = J e >> J i Sheath Regions Large E field to keep mobile e - in plasma region e - depletion n i+ >> n - e e - cannot carry current Irf J rf >> J e ~ 98% of e - are returned to plasma by sheath Conduction currents over area balanced over rf cycle J i A= - J e A J rf carried by displacement (capacitor) current J rf = J disp Charge transfer by sheath width oscillation Sheath Charge Dc bias J is current density 12
Oscillating RF Sheath RF current crosses sheath by displacement i rf = dq/dt For i rf = i o sin ωt, a charge of i o /ω cos ωt builds up on each of the sheath On plasma side of sheath there is no electrode, displacement current develops by the sheath moving and generating a dq/dt by depleting and restoring the e s as the plasma edge oscillates in and out. n o n s n Have neglected pre-sheath region n o =n i =n e n i =n e n e ~ 0 - n i - Plasma + - n e (t) + + - + n + - + + e + 0 S(t) Sm X electrode 13
RF Sheath Analysis Assume J rf = J o sin ω t Sheath oscillation is near sinusoidal s ~ s o sin ω t Max Sheath width s m ~ 2s o Analysis gives s ο = J ο ε ω n s s After Lieberman ω t Sheath width, s, increases with J rf s decreases with frequency and plasma density Charge stored in sheath Q = e sm n n sh ( o i ) e Poisson s Eq d 2 V dx DC Sheath voltage s o o DC sheath voltage increases with RF current and decreases with RF frequency / V 2 = e dx ( n ) i ( x) ne ( x) / ε o 2 1.3 J eε ω n s n i + + + + n + + e 0 s 14 n
V dc Depends on I rf and Electrode Geometry Self bias voltage V dc is the externally measured voltage V dc is sum of two sheath sheath voltages Asymmetric J rf = I rf / A S 1 S 2 V(t) A > A J 1 rf 1 2 < J rf 2 (Used to avoid sputtering gnd electode) V V s1 < V s2 V p 0 X Typically V dc = V s1 V V s2 s1 << V V s2 s2 and V s1 10 to15v V dc Self Bias V rf Swing V dc 2 1.3 I rf eε oω ns A2 15
Summarizing CCP Characteristics The plasma is generated by RF current flow between electrodes Plasma density (n e ) tends to increase linearly with RF power RF current across a sheath generates a dc voltage Ions gain energy from the dc sheath voltage In CC plasmas, n e and E ion tend to be coupled and increase together Ways to gain independent control of n e and E ion Use non-ccp method, such as ICP or ECR, to generate plasma and use CCP for bias (energy) control Use high freq ( > 50 MHz) RF to generate n e and low freq (< 10 MHz) for bias plasma generation 16
Use of Inductive Coupled Plasmas (ICP) as Low Bias Source Simple ICP Lam Style ICP Current in coil induces current loop in plasma in glass tube Toroid of high density plasma B field lines have been compressed because opposing B field from induced current loop in plasma toroid In ICP power is transferred to plasma by the oscillating B field. There is minimum rf current going across a sheath, so the sheath voltage is usually small 17
ICP Configurations RF for plasma generat Substrate Chamber RF bias Inductive coupling can generate high density plasmas with low sheath voltages. ICP power controls plasma density, n e. Capacitive coupling of a 2 nd rf source drives rf current through wafer sheath and is used to control ion energy, E i. 18
Ion Directionality Plasma + T i Ions enters sheath with transverse energy of T i Sheath E + Vsẖ + Wafer Free-fall Collisional At 13.6 MHz most ions respond only to the average (DC) sheath field Ions gain directionality and energy crossing the sheath Ion directionality strongly affects Etch bow (side wall etching) Electron shading type charging - - + - - + + 19
Collisionless Sheath Ion Directionality E V s σ θ Direction of mean ion σ = tan 1 θ T i ev s IAD T i Ion directionality determined by V s and T i at sheath edge Mean ion arrives at wafer σ θ degrees off the normal T i is determined by collisions in pre-sheath and energy at ion creation. Typically, T i 0.5 ev Example: If T i = 0.5 ev and V s = 100V σ θ ~ 4.0 For anisotropic etching, typically we need σ θ 4.0 Sheath voltage control is essential for etch control 20
For Ar @ 1 mt Beyond Simple DC Biasing: RF Effects on Ions Crossing Sheath V sdc = 100V T e = 2eV T i = 0.05ev n e = 5 x 10 9 After Barnes et al, 1991 V sdc = 27V T e = 5eV T i = 0.5ev n e = 1 x 10 11 50 MHz 50 MHz 25 MHz 10 MHz 5 MHz 2.5 MHz 1.0 MHz 0.5 MHz For an oscillating rf sheath, the ion energy distribution (IED) at wafer surface depends strongly on sheath transit effect IED tends to be bimodal with ε ion decreasing with increasing RF frequency IED strongly affected by ion mass, sheath thickness, and V sheath waveform IED can strongly affect etch profile Higher energy ions will have smaller Ion Angular Distribution (IAD) 25 MHz 10 MHz 5 MHz 2.5 MHz 1.0 MHz 0.5 MHz 21
Using Bias Frequency to Control Etch Profile After Schaepkens 1999-85V, 1.3 MHz -120V, 1.3 MHz -120V, 10.5 MHz -85V, 10.5 MHz 22
AMAT Oxide Etcher With Dual Bias Frequency AMAT PEUG 2007 Talk 2 MHz and 13 MHz for bias VHF for plasma generation VHF low V p ICP not used for Ox etch 23
Use of Mixed Bias Freq to Improve Ox Etch AMAT PEUG 2007 Talk 13 MHz only 13 MHz only 13 MHz/2 MHz 13 MHz/2 MHz 24
Summary DC self bias is a result of rf current flowing across a plasma sheath Increases with rf current and decreases with rf frequency RF biasing applied to wafer to control E i in high density plasma systems Biasing is needed for controlled anisotropic etching Recent etch equipment designs go beyond simple DC biasing to shape energy distribution of ions bombarding wafer surface to better control etch characteristics 25