Etching
- 1. ETCHING NEHA SHARMA ASSISTANT PROFESSOR DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING COLLEGE JHALAWAR
- 2. CONTENTS ■ WET ETCHING ■ DRY ETCHING ■ PROPERTIES OF ETCHING ■ Refer: • S.M Sze, VLSI technology. McGraw-hill. • S. K. Ghandhi, VLSI fabrication principles: silicon and gallium arsenide. John Wiley & Sons.
- 3. Etching ■ Etching refers to the removal of material from the wafer surface. ■ The process is usually combined with lithography in order to select specific areas on the wafer from which material is to be removed. ■ Etching represents one way of permanently transferring the mask pattern from the photoresist to the wafer surface.
- 4. Types of etching ■ Dry etching o Occurs in gas phase o Reactive Ion etching o Plasma etching ■ Wet etching o Etching by chemicals in liquid state (etchant) o Oxidation od reduction reactions for removal
- 6. • In wet etching, the wafers are immersed in a tank of the etchant (mix of chemicals), as shown in figure 1. • There is a chemical reaction between the wafer surface and the etchants that helps in material removal. • Either a photoresist layer or a hard mask like oxide or nitride layer is used to protect the rest of the wafer. • The time for etching depends on the amount and type of material that needs to be removed. KOH (potassium hydroxide) is a common etchant used to remove Si. • After etching, the wafers are rinsed, usually in DI water, for removal of etchant and then finally dried.
- 7. Etchants ■ For Si etching, KOH is used or a mixture of nitric acid and hydrofluoric acid (HF). ■ For silicon oxide etching, usually a mixture of HF and ammonium fluoride (NH4F) is used, called as BOE (buffered oxide etchants). ■ For silicon nitride, usually a strong acid like hot phosphoric acid is used at high temperatures (180 ◦C) since it is a very good passivating layer and hard to remove under normal conditions.
- 8. PROPERTIES OF ETCHING AND PROBLEMS (Directivity, Selectivity, Etch rate)
- 9. DIRECTIVITY OF ETCHING • ISOTROPIC: IN ALL DIRECTIONS • ANISOTROPIC: IN ONE DIRECTIONS (IDEALLY)
- 10. Uniformity: is defined as the percentage change in etch rate across the entire etched region Etch Control: Etch rate is defined as the amount of the film etched in a given time • Too short an etch time • The presence of a surface layer that slows the etching process • A lowered temperature or weakened etch solution
- 11. SELECTIVITY
- 12. PROBLEMS ■ OVER ETCHING AND UNDERCUTTING ■ Severe undercutting takes place when 1. Excessive etch time 2. High temperature 3. Strong etchant solution 4. Adhesion between resist and wafer is weak.
- 13. PROBLEMS WITH WET ECTHING 1. Wet etching is used for large pattern sizes, usually larger than 2 µm. 2. It is an isotropic process - sloped sidewalls rather than straight walls. 3. Wet etch has to be combined with subsequent rinse and dry steps. This increases chances of defects or contamination. 4. Hazardous chemicals and conditions are used, so safety is an issue. Safe disposal of chemicals is essential. 5. Undercutting and resist peel off can happen if time is not controlled or etch conditions change during process.
- 14. DRY ETCHING (REACTIVE ION ETCHING, PLASMA ETCHING, ION BEAM MILLING) etchant gases are the primary medium for the removal of material.
- 15. WHAT IS A PLASMA ? ■ FORTH STATE OF MATTER ■ WHEN GAS IS EXPOSED TO VERY HIGH TEMPERATURE OR VERY HIGH VOLTAGE, ITS MOLECULES BREAKS INTO IONS AND THEN PLASMA IS CREATED ■ IT IS AN IONISED GAS. ■ CONDUCTS ELECTRICITY AND CAN PRODUCE MAGNECTIC FIELD.
- 16. PLASMA ETCHING • The chemical etchant is introduced in the gas phase. • For etching silicon oxide, CF4 (tetrafluoromethane) is used. • The chamber is first evacuated before introducing the gas. • Radio frequency (RF) electrodes are then used to generate the plasma that ionizes the gas. • This ionized gas attacks the oxide layer, removing the layer
- 18. Ion beam etching • Ion beam etching is similar to the ion beam milling process that is used for transmission electron microscopy sample preparation. • This is a physical process where ionized inert gas ions (usually Ar) are used to remove material from the wafer. • The process is not selective but it is highly directional
- 19. Reactive Ion Etching (RIE) • It combines the plasma and ion beam etching process • good directionality but low selectivity • process in which chemical etching is accompanied by ionic bombardment • It is accomplished by replacing neutral gas with one or more chemical species in RF sputtering environment. • Plasma is formed by these species. • Substrate/wafer are normal to the plasma flow and rf field, thus highly directional and rapid movement of ionized particles.
- 20. How anisotropic nature /directionality can be increased? • Using chemicals whose reaction products have large ionized components. • increasing voltage drop across cathode, as increasing impingement velocity. • Reduced pressure reduced collision with cathode Why low selectivity? • Ion bombardment • Energetic neutral species isotropic in nature also patriciates.
- 21. Dry Etching Advantages ■ Eliminates handling of dangerous acids and solvents ■ Uses small amounts of chemicals ■ Isotropic or anisotropic etch profiles ■ Directional etching without using the crystal orientation of Si ■ High resolution and cleanliness ■ Less undercutting ■ No unintentional prolongation of etching ■ Better process control ■ Ease of automation Disadvantages ■ Some gases are quite toxic and corrosive ■ Re-deposition of no volatile compounds ■ Need for specialised expensive equipment