Dry Etching and Reactive Ion Etching (RIE)

Transcription

1 Dry Etching and Reactive Ion Etching (RIE) MEMS 5611 Feb 19 th 2013 Shengkui Gao Contents refer slides from UC Berkeley, Georgia Tech., KU, etc. (see reference) 1

2 Contents Etching and its terminologies Wet etching Dry etching Plasma systems Sputtering (Ion milling) Reactive ion etching Lab introduction 2

3 Etching or Lift-off? Two principal means of removing material. Etching: top to bottom methodology. Chemical reaction (Most metals, Si products, organics) Lift-off: bottom up methodology. Gold, Platinum, etc. 3

4 Etching Classic Procedures Lift-off Si wafer Si wafer Al Si wafer Deposition Spin-coating Photoresist Si wafer Spin-coating Photolithography Photoresist Al Si wafer Photolithography Si wafer Deposition Al Si wafer Al Al Si wafer Al Etching Al Si wafer Al Lift-off 4

5 Etching Techniques Chemical etching in liquid (wet etching) or in gaseous form (dry etching) is used to remove any barrier materials protected by hardened PR (or mask). Depending on the sources used for etching 5

6 Terminologies in Etching Process Etch Rate Selectivity Anisotropy Uniformity 6

7 Etch Rate & Selectivity Etch Rate: How fast the material is removed in the etch process Etch Rate = Thickness before etch - Thickness after etch Etch time Selectivity: Ratio of the etch rates between the different materials (PR and the target material, hard mask material and the target material) Selectivity = Etch Rate of Material 1 Etch Rate of Material 2 7

8 Anisotropy & Uniformity Anisotropy: degree of anisotropy, A f Uniformity: Measuring the thickness at certain points before and after the etch process. Uniformity = undercut B = 1 2 h 0 A 1 Maximum etch rate - Minimum etch rate Maximum etch rate + Minimum etch rate A f f f 8

9 Wet Etching Placing wafer in solution that attacks the film, not the mask. 9

10 Problem of Wet Etching Isotropic Phenomenon (Wet Etching) Anisotropic Phenomenon (Dry Etching) 10

11 Dry Etching Gaseous form can obtain highly anisotropic etching profiles, mainly avoid the undercutting problem of wet etching. Common used dry etching techniques: Plasma systems Ion milling Reactive ion etching 11

12 Plasma Systems Use RF excitation to ionize a variety of source gases in vacuum system. The plasma contains fluorine or chlorine ions to etch Si, SiO2, SiN4, organics and metals. RF power operates at 13.56MHz, why? FCC assigned frequency for industrial and scientific purposes 12

13 What is Plasma Processing? Plasma is one of the four fundamental states of matter. The matter in plasma contains ions, free radicals and byproducts. The free radicals and byproducts decrease the activation energy in a chemical reaction, resulting in material removal. 13

14 Sputtering (Ion Milling) Use energetic noble gas ions such as Argon (Ar + ) to bombard the wafer surface. Etching occurs by physically knocking atoms off the surface. Highly anisotropic etching. Poor selectivity. 14

15 Reactive Ion Etching (RIE) Combines the plasma and sputter (ion milling) etching processes. Plasma systems are used to ionize reactive gases, the ions are accelerated to bombard the surface. Etching occurs through: 1. Chemical reaction. 2. Physical momentum transfer from the etching species. 15

16 Reactive Ion Etching (Continue) RIE chamber: strong electronmagnetic field makes gas transfer into plasma state Plasma state generates: (1) Ions. (2) Activated neutrals. 16

17 Two Power Sources: ICP & RF Inductively coupled plasma (ICP): A 2.54 GHz plasma source producing electromagnetic induction. Generate high density of ions. RF: A MHz RF powered magnetic field to create directional electric fields to achieve more anisotropic etch profiles 17

18 Features of Dry Etching 18

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20 Comparison of Wet & Dry Etching (> 10nm) 20

21 LAB: Oxford Plasmalab 100 Loading Dock Gas flow Chamber 21

22 LAB: Oxford Plasmalab 100 Temperature: -150 o C to +400 o C Gases: Cl 2, SiCl 4, BCl 3, SF 6, Ar, O 2, H 2 and N 2 ICP and RF Power: Up to 300W 22

23 LAB: Al Etching Recipe Step 1. Step 2. Step 3. Step 4. Pressure (mtorr) RIE (W) ICP (W) Time (sec) Gas 1 (scmm) Ar: 50 BCL3: 30 BCL3: 30 Ar: 50 Gas 2 (scmm) CL2: 15 CL2: 8 Temperature(Celsius) Comment Purge the chamber Al2O3 etching Al etching Purge the chamber S1805 Al2O3 Al Native SiO2 Silicon Wafer ~500nm several nm ~200nm several nm ~200nm Al Step 2 Native SiO2 Step 3 Silicon Wafer Native SiO2 Silicon Wafer several nm ~200nm 23

24 S1805 Residual Removal Some S1805 left on the sample surface Removal Lift-off: Use Remover-PG to soak the substrate. Further Etching: O2 to remove organics. (potentially further oxidizes the Al) 24

25 Wire-grid Polarizer (100nm) Parallel Perpendicular Al SiO2 25

26 Review Etching and its terminologies Wet & Dry etching Plasma systems Ion milling Reactive ion etching Lab 26

27 Reference [1] etching.pdf [2] 450L11-Etching%20especially%20Plasma%20Etching.pdf [3] 27

28 Questions?? 28