PPT ON DRY ETCHING

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Dry Etching  Presentation Transcript

1.Dry Etching 2.Question What are

2 basic techniques for transferring resist features into a layer?

3.Wet Etch Processes Use chemical solution to dissolve the materials Byproducts are gases, liquids or solids that are soluble in the etchant solution or evaporate away

4.Wet Etch Profiles

5.Dry Etch Plasma etch Reactive ion etch Difference between Plasma and Reactive Ion Etching In RIE, the rf-driven electrode instead of the grounded electrode holds the wafer. RIE is also called RSE – Reactive Sputter Etching.

6.Comparison of Wet and Dry Etching Profiles

7.Wet and Dry etch .

8.Purpose of Dry Etching To anisotropicaly etch the desired layer. To isotropicaly etch the layer if required to do so

9.Equipment An RIE consists of two electrodes (1 and 4) that create an electric field (3) meant to accelerate ions (2) toward the surface of the samples (5). 10.Anisotropic etch system

11.Equipment description A typical (parallel plate) RIE system consists of a cylindrical vacuum chamber, with a wafer platter situated in the bottom portion of the chamber. The wafer platter is electrically isolated from the rest of the chamber, which is usually grounded. Gas enters through small inlets in the top of the chamber, and exits to the vacuum pump system through the bottom. The types and amount of gas used vary depending upon the etch process; for instance, sulfur hexafluoride is commonly used for etching silicon. Gas pressure is typically maintained in a range between a few millitorr and a few hundred millitorr by adjusting gas flow rates and/or adjusting an exhaust orifice.

12.Method of operation Plasma is initiated in the system by applying a strong RF electromagnetic field to the wafer platter. The field is typically set to a frequency of 13.56 MHz, applied at a few hundred watts. The oscillating electric field ionizes the gas molecules by stripping them of electrons, creating a plasma.

13.In each cycle of the field, the electrons are electrically accelerated up and down in the chamber, sometimes striking both the upper wall of the chamber and the wafer platter. At the same time, the much more massive ions move relatively little in response to the RF electric field. When electrons are absorbed into the chamber walls they are simply fed out to ground and do not alter the electronic state of the system. However, electrons absorbed into the wafer platter cause the platter to build up charge due to its DC isolation. This charge build up develops a large negative voltage on the platter, typically around a few hundred volts. The plasma itself develops a slightly positive charge due to the higher concentration of positive ions compared to free electrons.

14.Because of the large voltage difference, positive ions tend to drift toward the wafer platter, where they collide with the samples to be etched. The ions react chemically with the materials on the surface of the samples, but can also knock off (sputter) some material by transferring some of their kinetic energy. Due to the mostly vertical delivery of reactive ions, reactive ion etching can produce very anisotropic etch profiles. Etch conditions in an RIE system depend strongly on the many process parameters, such as pressure, gas flows, and RF power. A modified version of RIE is deep reactive-ion etching, used to excavate deep features.

15.Dry Etch Processes Chemical etchants in gaseous form. Byproducts are volatile and removed from the substrate surface. Plasma generates chemically reactive free radicals that can significantly increase the chemical reaction rate and enhance the chemical etch. Causes ion bombardment of the wafer surface. Physical removal of materials. Breaking of chemical bonds. Accelerates chemical reaction rate.

16.Plasma Etch Sequence .

17.What is Plasma? Plasma is the fourth state of matter. It is a quasi-neutral gas of charged and neutral particles that exhibits collective behavior. It is an ionized gas with equal numbers of positive and negative charges.

18.PLASMA Generation Plasmas are generated by RF power applied between two parallel plate electrodes in a vacuum chamber. (capacitively coupled plasma source). When RF power is applied to the electrodes , a varying electric field is established between them. Ionization: high RF power e- + A ? A+ + 2 e- Cascading of the ionization collisions fills the entire chamber with equal number of electrons and ions i.e. with Plasma.

19.PLASMA Generation In the plasma, electrons and ions are continuously lost and consumed by collisons with electrodes, the chamber wall and recombination collisions. When the generation rate of electrons through ionization collisions is equal to the loss of electrons, the plasma is said to be stabilized.

20.Plasma Etch Processes Dielectric etch Single-crystal silicon etch Polysilicon etch Metal etch Photoresist strip Blanket dry etch processes

21.Etch Mechanisms Damaging mechanism Blocking mechanism Both are related to the ion bombardment Dielectric etch processes use a damaging mechanism Single-crystal silicon, poly and metal etch processes use blocking mechanism

22.Damaging mechanism Closer to the physical etch side To improve anisotropic profile – increase ion bombardment and RF power and decrease pressure Disadvantages – Increased ion bombardment on the inner surfaces of the chamber reduces their life. Can cause plasma-induced device damage This also increases particle contamination

23.Blocking mechanism In this process, Chemical deposition during the etch process protects the sidewall and blocks etching in the horizontal direction. If the hard photoresist is left on the wafer, ion bombardment will sputter some of the PR into the gap. PR coating on the side wall will block the etching in that direction. PR deposited on the bottom will be constantly removed by the bombardment from the plasma and expose the surface of the wafer to the etchants. Closure to the chemical etch side Requires less ion bombardment Disadvantage – The sidewall deposition needs to be cleaned by either dry or wet clean process or both

24.Plasma Etch Chambers Plasma strip or plasma ashing Atomic oxygen free radicals react rapidly with carbon and hydrogen in carbonaceous material to form volatile CO, carbon di oxide and water This removes carbonaceous material (e.g. PR) from the surface

25.Remote Plasma System Designed for isotropic etch

26.Batch RIE System

27.Comparison of etch processes

28.Etch Endpoint For Wet Etch BY time – pre-measured etch rate & etch thickness Visual inspection sensitive to etchant concentration and temperature For plasma etch - optical method .

29.Etchants and safety The chemicals used in etching are corrosive, toxic and some times explosive also. HCl, HNO3 , H2SO4 are all strong acids HF can penetrate skin and cause bone cancer H2O2 is a very strong oxidizing agent KOH is a strong alkali Cl2 , BCl3 , SiF4 and HBr are highly corrosive and toxic. Inhalation of these gases at a high concentration (>1000 ppm) could be fatal 30.