Dry and wet etching

ENGINEERING TECHNOLOGY OF NON CONVENTIONAL PROCESSES

1. ETCHING By : Meet shah (51 ) Mechanical (SFI)

2. Contents Etching Wet etching Dry etching

3. After a thin film is deposited, it is usually etched to remove unwanted materials and leave only the desired pattern on the wafer. The process is done many times. In addition to deposited films, sometimes we also need to etch the Si wafer to create trenches. The masking layer may be photoresist, SiO 2 or Si 3 N 4. The etch is usually done until another layer of a different material is reached. etching

4. Etching can be done “wet” or “dry ” Wet etching uses liquid etchants. Wafer is immersed in the liquid. Process is mostly chemical. Wet etching is not used much in VLSI wafer fabrication. Introduction

5. Dry etching Uses gas phase etchants in a plasma. The process is a combination of chemical and physical action. Process is often called “plasma etching”. This is the normal process used in most VLSI fabrication. The ideal etch produces vertical sidewalls. In reality, the etch occurs both vertically and laterally .

6. There is undercutting, non vertical sidewalls, and some etching of the Si. The photoresist may have rounded tops and non-vertical sidewalls. The etch rate of the photoresist is not zero and the mask is etched to some extent. This leads to more undercutting. Etch selectivity is the ratio of the etch rates of different materials in the process.

7. Etch selectivity is the ratio of the etch rates of different materials in the process. If the etch rate of the mask and of the underlying substrate is near zero, and the etch rate of the film is high, we get high selectivity. This is the normally desired situation If the etch rate of the mask or the substrate is high, the selectivity is poor Selectivity of 25 – 50 are reasonable. Materials usually have differing etch rates due to chemical processes rather than physical processes.

8. Etch directionality is a measure of the etch rate in different directions (usually vertical versus lateral)

9. We consider two processes :- “wet” etching “dry” etching Wet process is well-established, simple, and inexpensive. The need for smaller feature sizes could only be met with plasma etching. Plasma etching is used almost exclusively today. Types of etching

10. In wet process by immersing the wafer in these chemicals, exposed areas could be etched and washed away. For SiO 2 , HF was used. Wet etches work through chemical processes to produce a water soluble byproduct. Wet etching

11. In some cases, the etch works by first oxidizing the surface and then dissolving the oxide An etch for Si involves a mixture of nitric acid and HF The nitric acid (HNO 3 ) decomposes to form nitrogen dioxide (NO 2 ) The SiO2 is removed by the previous reaction The overall reaction is

12. Buffers are often added to keep the etchants at maximum strength over use and time Ammonium fluoride (NH 4 F) is often used with HF to help prevent depletion of the F ions This is called Basic Oxide Etch (BOE) or Buffered HF (BHF) The ammonium fluoride reduces the etch rate of photoresist and helps eliminate the lifting of the resist during oxide etching Acetic acid (CH 3 COOH) is often added to the nitric acid/HF Si etch to limit the dissociation of the nitric acid

13. Wet etches can be very selective because they depend on chemistry The selectivity is given by Material “1” is the film being etched and material”2” is either the mask or the material below the film being etched If S>>1, we say the etch has good selectivity for material 1 over material 2

14. Most wet etches etch isotropically. The exception is an etch that depends on the crystallographic orientation. Example—some etches etch <111> Si slower than <100> Si. Etch bias is the amount of undercutting of the mask. If we assume that the selectivity for the oxide over both the mask and the substrate is infinite, we can define the etch depth as “d” and the bias as “b”.

15. We often deliberately build in some over etching into the process. This is to account for the fact that the films are not perfectly uniform the etch is not perfectly uniform The over etch time is usually calculated from the known uncertainties in film thickness and etch rates It is important to be sure that no area is under-etched; we can tolerate some over-etching

16. Plasma etching has (for the most part) replaced wet etching There are two reasons: Very reactive ion species are created in the plasma that give rise to very active etching Plasma etching can be very anisotropic (because the electric field directs the ions) dry etching (plasma etching)

17. Plasma systems can be designed so that either reactive chemical components dominate or ionic components dominate. Often, systems that mix the two are used The etch rate of the mixed system may be much faster than the sum of the individual etch rates. A basic plasma system is shown in the next slide

18. The main species involved in etching are Reactive neutral chemical species Ions The reactive neutral species (free radicals in many cases) are primarily responsible for the chemical component The ions are responsible for the physical component The two can work independently or synergistically Dry Etching Mechanisms

19. Dry Etching

20. If the area of the electrodes is the same (symmetric system) we get the solid curve of 10-8. The sheaths are the regions near each electrode where the voltage drops occur (the dark regions of the plasma). The sheaths form to slow down the electron loss so that it equals the ion loss per RF cycle. In this case, the average RF current is zero.

21. The heavy ions respond to the average voltage The light electrons respond to the instantaneous voltage The electrons cross the sheath only during a short period in the cycle when the sheath thickness is minimum During most of the cycle, most of the electrons are turned back at the sheath edge The sheaths are thus deficient in electrons They are thus dark because of a lack of light-emitting electron-ion collisions

22. For etching photoresist, we use O 2. For other materials we use species containing halides such as Cl 2 , CF 4 , and HBr. Sometimes H 2 , O 2 , and Ar may be added. The high-energy electrons cause a variety of reactions. The plasma contains free electrons ionized molecules neutral molecules ionized fragments Free radicals dry etching

23. In CF 4 plasmas, there are Free electrons CF 4 CF 3 CF 3 + F CF and F are free radicals and are very reactive Typically, there will be 10 15 /cc neutral species and 10 8 -10 12 /cc ions and electrons

24. Features of this system Low gas pressure . High electric field ionizes some of the gas. Energy is supplied by 13.56 MHz RF generator. A bias develops between the plasma and the electrodes because the electrons are much more mobile than the ions (the plasma is biased positive with respect to the electrodes).

25. Advantages of Dry Etching over Wet Etching Eliminates handling of dangerous acids and solvents. Uses small amounts of chemicals. Anisotropic etch profiles. High resolution and cleanliness. Less undercutting. Better process control.

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