Pros & Cons of Etching Techniques in Microelectronics: Wet Chemical, Plasma, Reactive Ion , Study notes of Electrical and Electronics Engineering

Download Pros & Cons of Etching Techniques in Microelectronics: Wet Chemical, Plasma, Reactive Ion and more Study notes Electrical and Electronics Engineering in PDF only on Docsity! ECE 6450 - Dr. Alan DoolittleGeorgia Tech Lecture 11 Etching Techniques Reading: Chapter 11 ECE 6450 - Dr. Alan DoolittleGeorgia Tech Characterized by: 1.) Etch rate (A/minute) 2.) Selectivity: S=etch rate material 1 / etch rate material 2 is said to have a selectivity of “S” for material 1 over material 2. RateEtch Vertical RateEtch Lateral1−=A 3.) Anisotropy: 4.) Under cut: If 0.8 um lines result from an etch using 1 um photoresist lines as a mask, it is said that the process bias is 0.1 um for that particular etch. Etching Techniques ECE 6450 - Dr. Alan DoolittleGeorgia Tech Process requires: 1.) Movement of etchant species toward the wafer surface 2.) Reaction at the surface 3.) Movement of reactant products away from the surface Any one of the above three steps can be the etch rate limiting step 2 important Wet Etch Concepts: A.) Buffering the solution to maintain constant etch rate with time: Consider etching of SiO2 OHSiFHHFSiO 2622 26 ++→+ But as the HF is depleted (used up) from the solution the etch rate would change. Thus, a Buffering solution is added that controls the HF concentration as: HFNHFNH +⇔ 34 The HF concentration remains “saturated”. As HF is consumed etching SiO2, the above reaction replaces the HF, keeping the etch rate constant. Wet Chemical Etching: ECE 6450 - Dr. Alan DoolittleGeorgia Tech Wet Chemical Etching: B.) Oxidation/Reduction Reactions Many chemical etchants use oxidation reactions to oxidize the surface followed by (simultaneously) reduction reactions to “reduce” the oxidized material (moving it into solution). Sometimes the chemicals used to oxidize and reduce are diluted in water or other solutions (acetic acid, ethylene glycol, etc…) Example: Use oxidizing agents such as nitric acid (HNO3)to oxidize material (Si) with HF to remove the oxide. Acetic acid (HC2H3O2) is used to dilute the solution. If the solution is to be 10% HF, 40% nitric acid and 50% acetic acid, what is the etch rate? Two determine etch rate 1.) Draw a line from 10% HF parallel to the side of the triangle counterclockwise to the HF side, 2.) Draw a line from 40% nitric side parallel to the side of the triangle counterclockwise to the nitric side 3.) Draw a line that goes from the 50% Acetic point to the intersection of the previous two lines ====> less than 7.6 um/minute (lowest value on the plot) For Si, regions exist where the reduction reaction is so slow, the surface is very planar and ends up being “polished “ after the etch. ECE 6450 - Dr. Alan DoolittleGeorgia Tech Plasma Etching: Consider a reaction of CF4 and Silicon. It is desired to replace C with Si to form a volatile* Si gas SiFx. This requires the breaking of C-F (supplying 105 kcal/mole) and Si-Si (supplying 42.2 kcal/mole) bonds and the formation of Si-F bonds (“consuming” 130 kcal/mole). ===> CF4 will not etch Si directly due to excessive energy requirements. We can aid this process to get the CF4 to etch Si, by: A.) Pre-breaking the C-F bonds via the plasma, lowering the net energy required at the surface B.) Pre-breaking the surface Si-Si bonds via the ion bombardment, lowering the net energy required at the surface If both of these are done, it only take 17 kcal/mole to form Si-F bonds. Consider what happens to the Carbon? Without the addition of a “scavenging gas” (discussed in a moment), the C merely exchanges with the Si until complete C coverage occurs and the reaction stops (assuming no ion bombardment, see below). No Scavenging gas: Carbon layers the surface to prevent further etching Extra “scavenging gas added to remove C from surface *A volatile gas is a gas with a high enough vapor pressure to be pumped away ECE 6450 - Dr. Alan DoolittleGeorgia Tech Problems experienced in practice: ECE 6450 - Dr. Alan DoolittleGeorgia Tech DecreasedIncreasedDecreasedC-Rich IncreasedDecreasedIncreasedF-Rich (O2 added) Selectivity Si over SiO2AnisotropyEtch RateMaterial = Si Condition UnchangedIncreased Decreased (only slightly since HF etches SiO2) C-Rich (H2 added) DecreasedDecreasedIncreasedF-Rich Selectivity SiO2 over SiAnisotropyEtch RateMaterial = SiO2 Condition Summary of Plasma Etch Chemistry using CF4 to etch Si Effect of reactor loading Depletion of the reactant gas by increased surface area is sometimes a problem. (Lab conditions and multi-wafer systems). The etch rate can be determined as, where Ro is the empty chamber etch rate, A is area of wafers loading into the reactor, and k is constant that can be reduced by increasing gas flows at constant pressure kA RoR = = 1 ECE 6450 - Dr. Alan DoolittleGeorgia Tech Emission Spectroscopy: Observing the intensity of individual plasma lines resulting from molecular recombination events. These events can be very weak so sensitive equipment is required. Interferometry: Requires large unpatterned areas. Plasma Process Monitoring Reactive Ion Etching Designed to give better control of Selectivity and Anisotropy independently: Characteristics: 1.) Lower operating pressures result in higher anisotropy (longer mean free path allows more directed acceleration of ions) 2.) A DC bias enhances ion bombardment energy, resulting in some sputtering and chemical catalyst effect. 3.) A sidewall polymerization gas (BCl3, CCl4 etc...) is added to enhance anisotropy. The enhanced sputtering features of RIE insure “mostly” or “only” sidewall polymerization. More damage occurs: Many III-V processes can not tolerate this damage. Some unintentional deposition of the polymerization gas can result in defects if conditions are not optimized.