Dry Etch Index Basics Plasma RIE Operation Oxide

Dry Etch: Index + Basics. Plasma, RIE + Operation + Oxide, Nitride, Al etches + Issues: De veil, ESD, Faceting, Trenching + Summary 10/27/2020 2

Dry Etch: Overview + Similar to Sputtering in some respects + Plasma used + Etch can be started or stopped very quickly + Some sputtering occurs (mechanical removal) + Directional removal (anisotropic) + Loss of selectivity + Need to control with sputtering energy + control of directional removal 10/27/2020 3

Plasma Etch Overall Mechanism + Etching species (radicals) generated in Plasma + Diffusion to the surface + Adsorption + Reaction + Desorption (product has to be volatile) + Diffusion to gas phase 10/27/2020 4

Plasma Etch Basics + Usually forms a ‘veil’ + Helps in directional etch + Creates problem in removing! (De veil) + Wet de-veil is better than dry de-veil (selectivity and cleaning are very good) + Cu cannot be etched + Just plasma etching is chemical (like wet etch) + Just sputtering is physical (wafer is like sputter target) + Reactive Ion Etching is physical + chemical (RIE) + Reasonable selectivity and anisotropic 10/27/2020 5

Dry Etch: Plasma: Operation + Similar to RF sputter (but not the same) + esp. In plasma etching, no sputtering + plasma is used to produce highly reactive radicals and ions + input and output gas flow pattern determines etch uniformity + Loading (micro loading), PPC + Rise in temperature (exothermic reactions, heat generated by plasma, low specific heat capacity of gases) + variation in etch rate (temp changes ==> reaction rate change) 10/27/2020 6

Dry etch: RIE: Operation + RIE closer to RF sputter (but not the same) + plasma is used to produce highly reactive radicals and ions + some sputtering also occurs, + more directional etch + loss of selectivity (photo resist may erode faster!) + photo resist ‘hardening’ with plasma (before etch), may help (low power, short time plasma hardening) + lower pressure than plasma etch ==> directional path for ions 10/27/2020 7

Dry etch: RIE: Operation + However, RIE is NOT the same as RF sputter + Instead of Ar ions, other chemical plasma is used + primarily a chemical etching. Some sputtering to enhance directional etch + mainly ions participate in the reactions + ions come with lot of energy. Substrate temp variation, impurity concentration (PSG), crystalline nature are not that important. + Loading effects are less (since ions are accelerated) 10/27/2020 8

Dry etch: RIE: Issues + Increase in wafer temp + slower etch rate (lower pressure ==> lower concentration) + lower selectivity 10/27/2020 9

Oxide Etch + Fluorine based + Forms (volatile) + Intermediate species/ reactions + Many more species have been proposed / observed + CF 4 (on its own) will not etch Si, but F species (in plasma) will etch 10/27/2020 10

Oxide Etch + Add Oxygen to increase etch rate by increasing F concentration + When oxygen concentration is very high, dilution of F decreases rate 10/27/2020 11

Oxide Etch + Need to be selective with respect to silicon (FEOL) + Add hydrogen and reduce F conc. + H+ + e- + F => HF + Si etch rate decreases a lot, Si. O 2 rate decreases a little + ==> controlled selectivity 10/27/2020 12

Oxide Etch: Control + More C ==> polymerization + More F ==> etching + Add H to increase C/F ratio ==> polymerization + Add O to decrease C/F ratio ==> more etching + Use C 2 F 4 instead of CF 4 (for example) to change C/F ratio + OR C 2 F 6 or C 4 F 10. . . 10/27/2020 13

Nitride etch + Fluorine based / Oxygen added + Intermediates includes O+, O-, O 2+, O 2 -, COF, OH, O, OF + Oxygen helps in reducing carbon contamination, and increasing F + In oxide etch, some oxygen is provided by oxide; in nitride etch, it is supplied + For example. . 10/27/2020 14

Al etch + Al. F 3 is not volatile ==> No fluorine based compound + Use chlorine based compounds + Al has some Si and Cu added + Si. Cl 4 and Al. Cl 3 are volatile (relatively) + Cu or Cu. Cl 2 are not volatile + sputtered during RIE + Cu. Cl 2 washed in DI water rinse + Pure CCl 4, BCl 3, additional Cl 2 + CCl 4 ==> chloro carbon polymer + BCl 3 ==> potential contamination from Boron 10/27/2020 15

Al etch + Residual chlorine (absorbing moisture) will etch Al and Cu + Expose to Fluorine plasma (replace Chlorine with Fluorine) + Fluorine compounds less likely to absorb moisture + eg. HF does not completely dissociate in water. 10/27/2020 16

Photo resist etch + Plasma etcher + Oxygen based + Forms CO, CO 2, H 2 O (gaseous) Sample image + RIE 10/27/2020 17

Dry Etch: Veil + Polymers (carbon/fluorine based) form on the side wall + will not dissolve in plasma chemistry + will be removed by ion collision (sputtering) + usually glancing angle sputter for side wall ==> no removal + Anisotripic etch 10/27/2020 18

Dry Etch: De-Veil + After dry etch (via etch for example), veil has to be removed + De-veil + Wet clean (de veil) + dry clean : Ashing(etch tool can be used) + Wet clean is better +(selective dissolution) + Sometimes ashing + wet clean ULVAC dry etch tool + De veil should NOT attack other materials! 10/27/2020 19

Dry Etch: De-Veil ©EKC + Effect of deveil + Remember: Marketing! +IPA = Iso

Dry Etch: De-Veil ©EKC + Ti. N is liner + TEOS based insulator (oxide/dielectric / IMD/ ILD) + Al. Si. Cu = Cu and Si doped Al 10/27/2020 21

Dry Etch: Extra + Confinement of Plasma: ESC-Electro. Static Chuck + ©Lam Research Inc

ESD Issues + Plasma ==> electrical charge + large metal area getting reduced to small metal lines +==> charge accumulation and potential discharge + ==> Gate oxide integrity + ==> Need protective diodes * + Especially for aluminum metal lines (many parallel lines) + DR limits the maximum length of lines (or the area to be precise) 10/27/2020 23

Other Issues + Faceting + What you want 10/27/2020 + What you get 24

Other Issues + Trenching + What you want 10/27/2020 +Glancing angles sputter + What you get 25

Dry Etch Summary + Better control for anisotripic etch + Problems like photoresist adhesion etc do not arise + suitable for sub-micron + Contamination control is easier (no metal contaminant) + Running cost lower + Easier to start and stop etch + Results are more reproducible (wafer to wafer) + Inline monitoring can be done (spectroscopy, interferometry) + need little over etch + ==> lower defectivity because of over etch + Not the complete story: center/edge difference exists 10/27/2020 26

Dry Etch Summary + Lower selectivity (especially for photo resist) + Higher investment for the equipment + Usually single water process (lower throughput) + De-veil is an added step + Microloading is present (as in wet etch) + need PPC + Not all materials can be etched: Cu is an important example + ESD 10/27/2020 27