E SC 412 Nanotechnology: Materials, Infrastructure, and Safety Wook Jun Nam

E SC 412 Nanotechnology: Materials, Infrastructure, and Safety Wook Jun Nam

Lecture 10 Outline 1. Wet Etching/Vapor Phase Etching 2. Dry Etching DC/RF Plasma Plasma Reactors Materials/Gases Etching Parameters Bosch Process Cryogenic Process

Top Down Approach

Etching: Some Key Terminology Mask- the word mask is used in etching to mean a protective layer (covering). Ideally a mask material is not etched at all. Etch rate-how fast material is removed (usually in nm/sec) Selectivity-how good an etching process is at attacking one material and leaving another alone Isotropic-etching which attacks a material equally in all directions Anisotropic-etching which attacks a material mainly in one direction

Isotropic / Anisotropic Etching http://home.comcast.net/~dwdm2/mems_micromachining.html

Wet / Vapor Phase Etching

Wet Etching Advantages: Relatively simple, easy, fast, and economic (e.g., batch process) High etch selectivity No physical damages on a substrate Disadvantages: Etch rate is not reproducible Usually Isotropic etching Chemical wastes

J. D. Plummer, M. Deal, and P. D. Griffin, Silicon Copyright VLSI Technology 2014 by Fundamentals, Wook Jun Practices, Nam and Modeling, Prentice Hall, 2000 Wet Etching: Typical Materials / Etching Chemicals

Vapor Phase Etching (XeF 2 ) Selectivity: XeF 2 shows very high selectivity vs silicon to the majority of semiconductor materials (e.g., photoresist, silicon dioxide (>1000:1), silicon nitride (>100:1), and aluminum). Isotropic etching Safety issues when loading/unloading samples. 2XeF 2 + Si SiF 4 + Xe

Vapor Phase Etching (XeF 2 ) No release stiction XeF 2 etching is a dry process so no drying is needed which avoids the sticking issues that often plague wet release processes. Delicate structures are safely released Since XeF 2 etching is a dry, room temperature process delicate structures can be released. This is particularly useful for releasing delicate devices (e.g., micro-mirrors).

DC / RF Plasma

Reactions in Plasma very reactive radicals photon generation: plasma glow very reactive radicals

DC Glow Discharge (Paschen Curve) small pd area large pd area http://commons.wikimedia.org/wiki/file:paschencurve.jpg When a high DC bias is applied between two electrodes in a gas, a breakdown is occurred. Small pd: either too low pressure or too close space between the electrodes electrons move across the space with no or few collisions. Large pd: either too high pressure or too big electrodes space not enough energy transfer by collisions.

J. D. Plummer, M. Deal, and P. D. Griffin, Silicon VLSI Technology Fundamentals, Practices, and Modeling, Prentice Hall, 2000 RF Plasma Electrons oscillate between the electrodes wit the AC voltage. No need for electron emission from cathode Can sustain plasma at lower pressure than DC plasma. Can etch dielectrics as well as metals.

RF Plasma (continued) powered electrode (cathode) grounded electrode (anode) V T = V DC + V p The smaller electrode has greater voltage drop. The anode should be bigger than the cathode : the anode is usually connected to the chamber wall to increase the area. The big anode area reduces V p reduce the plasma induced damage on the chamber wall.

Plasma Reactors

J. D. Plummer, M. Deal, and P. D. Griffin, Silicon VLSI Technology Fundamentals, Practices, and Modeling, Prentice Hall, 2000 Capacitively Coupled Plasma (CCP) Powered electrode is directly coupled to the plasma. High electric field is formed near the powered electrode. Power transfer efficiency is relatively low, but very uniform plasma generation. Applied Power (e.g., DC, RF (13.56 MHz), VHF (>30MHz)).

High Density (HD) Plasma High etch rate requires high plasma densities (>10 11 /cm 3 ) Higher process pressures higher plasma densities short mean free path less directional Different plasma systems are needed to generate HDP at low pressure Inductively coupled plasma (ICP) Electron cyclotron resonance (ECR)

High Density (HD) Plasma (continued) HD plasma offers; Good etch selectivity High Etch rate Anisotropic etch profile Low plasma induced physical damages Good control in critical dimension (CD)

ICP: Operation Also called as transformer coupled plasma (TCP). Upper part of chamber: ceramic or quartz Source RF inductively couple with plasma (remote plasma) RF source does not directly contact with plasma (no contamination) Source RF generates plasma and controls ion density (~10 12 /cm 3 ) Bias RF controls ion bombardment energy. Ion energy and density independently controlled.

ICP: Typical Tool Configuration J. D. Plummer, M. Deal, and P. D. Griffin, Silicon VLSI Technology Fundamentals, Practices, and Modeling, Prentice Hall, 2000

ECR: Operation An electron in a static and uniform magnetic field will move in a circle. Applying an alternating electric field will results in a cycloid. The frequency of this cyclotron motion is given by This is called electron cyclotron resonance frequency When the frequency of the electric field set to electron resonance occur. For commonly used microwave frequency, 2.45 GHz, the resonance condition is met B=875.

ECR: Typical Tool Configuration J. D. Plummer, M. Deal, and P. D. Griffin, Silicon VLSI Technology Fundamentals, Practices, and Modeling, Prentice Hall, 2000

Magnets/ Magnetic Field Long MFP, insufficient ionization collisions In a magnetic field, electron is forced to spin with very small gyro-radius Electrons have to travel longer distance/more collisions Increasing plasma density at low pressure Magnetic field increasing electron density in sheath layer Less charge difference in sheath region Lower DC Bias Effects on ion bombardment increasing ion density reducing ion energy

Wafer Cooling Ion bombardments generate large amount heat. High temperature can cause PR reticulation/low etch selectivity. Need cool wafer to control temperature. Helium backside cooling is commonly used. Helium transfer heat from wafer to water cooled chuck.

Mechanical Chuck (Clamp Chuck) Clamp Ring Seal O- ring Wafer Water-cooled pedestal, cathode, or chuck Helium

Electrostatic Chuck Helium needs to be pressurized Wafer has high pressure at backside because low chamber pressure Need mechanisms to hold wafer Either mechanical clamp or E-chuck Clamp ring causes particles and shadowing effect E-chuck is rapidly replacing clamp ring

Materials / Etching Gases

Materials & Etching Gases J. D. Plummer, M. Deal, and P. D. Griffin, Silicon VLSI Technology Fundamentals, Practices, and Modeling, Prentice Hall, 2000

J. D. Plummer, M. Deal, and P. D. Griffin, Silicon VLSI Technology Fundamentals, Practices, and Modeling, Prentice Hall, 2000 Dry Etching: Processes at the Etched Material Surface

Chemical/ Physical Etching

Anisotropic Etching J. D. Plummer, M. Deal, and P. D. Griffin, Silicon VLSI Technology Fundamentals, Practices, and Modeling, Prentice Hall, 2000

Anisotropic Etching: Inhibitors H 2 consumes F, and forms HF which does not contributes for Si etching. The low concentration of F reduces the chemical reaction to form SiF 4, and slows down the etch rate. J. D. Plummer, M. Deal, and P. D. Griffin, Silicon VLSI Technology Fundamentals, Practices, and Modeling, Prentice Hall, 2000

J. D. Plummer, M. Deal, and P. D. Griffin, Silicon VLSI Technology Fundamentals, Practices, and Modeling, Prentice Hall, 2000 Anisotropic Etching (continued) Hydrogen consumes F. Too much addition of H 2 will cause too slow etch rate.

Anisotropic Etching (continued) J. D. Plummer, M. Deal, and P. D. Griffin, Silicon VLSI Technology Fundamentals, Practices, and Modeling, Prentice Hall, 2000

Anisotropic Etching (continued) : ICP Si Etching Cr Si CF 4 : 30sccm, SF 6 : 20 sccm 80 sec etch time CF 4 : 35sccm, SF 6 : 15 sccm 80 sec etch time All other etching conditions (e.g., rf power, etch time, process pressure) are the same

Anisotropic Etching (continued) : ICP Si Etching CF 4 : 35sccm, SF 6 : 15 sccm 80 sec etch time CF 4 : 40sccm, SF 6 : 10 sccm 80 sec etch time All other etching conditions (e.g., rf power, etch time, process pressure) are the same

Anisotropic Etching (continued) : ICP Si Etching CF 4 : 45sccm, SF 6 : 5 sccm 80sec etch time CF 4 : 45sccm, SF 6 : 5 sccm 120 sec etch time All other etching conditions (e.g., rf power, process pressure) are the same

Z. Cui, Nanofabrication: Principles, Capabilities, and Limits, Springer (2008) Macro-loading Effect Etch rate is decreased as the overall etch area is increased

Z. Cui, Nanofabrication: Principles, Capabilities, and Limits, Springer (2008) Micro-loading Effect Micro-loading effect is caused by localized pattern density. Micro-loading effect is related with localized depletion of reactive species or accumulation of etch by products.

Z. Cui, Nanofabrication: Principles, Capabilities, and Limits, Springer (2008) Aspect Ratio Effect (Aperture Effect) The aspect ratio effect is strongly dependent on dimensions of pattern. The etch rate for small features is slower than bigger ones. The mechanism for the effect is very complicated, and is related with available reactive species and reaction byproducts.

Aspect Ratio Effect (Aperture Effect) http://cmi.epfl.ch/etch/601e.php

Z. Cui, Nanofabrication: Principles, Capabilities, and Limits, Springer (2008) Micro Trenching Effect Micro-trenching effect is a phenomenon that the etch rate near the trench corner is faster than the center. The effect is caused by the impact of high energy ions at grazing angles (> 80 ) on the side walls then reflected to the bottom of the trench. Both side wall slope angle and the incident angle of the ions can significantly influence the resulting etch profile.

Micro Trenching Effect (continued)

Z. Cui, Nanofabrication: Principles, Capabilities, and Limits, Springer (2008) Notching Effect (DRIE) The addition of etch stop layer is very helpful for removing loading effects. The etch stop layer (e.g., SiO 2 ) can cause a notching effect as the layer is locally charged.

Bosch / Cryogenic Processing

Bosch Process: Deep Reactive Ion Etch (DRIE) The Bosch process is used for high aspect ratio etching by alternating passivation (C4F8 plasma) and etching (SF6 plasma) cycles. http://cmi.epfl.ch/etch/601e.php

http://www.iue.tuwien.ac.at/phd/ertl/node68.html Bosch Process: Deep Reactive Ion Etch (DRIE) The deposition of a passivation layer protects the side walls from chemical etching during the subsequent etching cycle. Directional etching caused by ion bombardment removes the passivation layer at the bottom, so that the radicals are able to attack the substrate.

http://en.wikipedia.org/wiki/deep_reactive-ion_etching Bosch Process: Scalloping Issue Lateral roughness due to the scalloping is about 150nm or more!

Bosch Process: Scalloping Issue (continued) (a) (b) Sidewall roughness can be tuned little bit! : (a) SF6/C4F8 = 7s/2s (b) SF6/C4F8 = 3s/1s. http://cmi.epfl.ch/etch/601e.php

Cryogenic Process In cryogenic-drie, the wafer is chilled to 110 C (163 K). The low temperature slows down the chemical reaction that produces isotropic etching. However, ions continue to bombard upward-facing surfaces and etch them away. This process produces trenches with highly vertical smooth sidewalls.

Cryogenic Process (continued) Very high selectivity over photoresist (to 100:1) and SiO2 masks (to 200:1) Simple and extremely clean plasma chemistry: SF6-O2 plasma (no fluorocarbons) instead of SF6-C4F8 plasma. - almost no chamber cleaning The primary issues with cryo-drie is that the standard masks on substrates crack under the extreme cold, plus etch by-products have a tendency of depositing on the nearest cold surface, i.e. the substrate or electrode.

Lecture 10 Outline 1. Wet Etching/Vapor Phase Etching 2. Dry Etching DC/RF Plasma Plasma Reactors Materials/Gases Etching Parameters Bosch Process Cryogenic Process