INTEGRATED CIRCUITS Dr. Esam Yosry Lec. #7

Etching  Introduction  Etching  Wet Etching  Dry Etching  Plasma Etching  Wet vs. Dry Etching  Physical vs. Chemical Processes

Introduction ( Chip Fabrication Cycle)

Introduction ( Processes)  Oxidation  Diffusion  Ion Implantation  Deposition  Etching  Lithography  Deposition  Removal  Patterning  Modification of electrical properties

Etching We want to transfer the photoresist pattern to the underlying material. Removal of material by chemical or physical means is called etching. Process must be selective to the material you are removing.

Etching

Want faster etches so process is done quicker. Too fast process will be difficult to control. Ratio of etch rates of other materials referenced to the material you are etching. Lateral extent of the etch under the resist mask.

Etching

Usually selectivity and directionality are the first order issues. Selectivity comes from chemistry process. Directionality comes from physical process. Modern etching try to optimize both.

Etching Two main types of etching: Wet and Dry (plasma). Plasma etching dominates today. Etching of thin films and sometimes the silicon substrate are very common process steps.

Wet Etching Wafers submerged in specific chemical baths. Purpose is to selectively remove materials (Si, SiO 2, Al) that are not covered by a Photoresist. Etch Rate is the rate at which the material is removed (measured in Å/min). Isotropic Etch proceeds in all directions at the same rate. Anisotropic Etch is preferential in one direction.

Wet Etching Resist Mask Insulator Film Isotropic Anisotropic Semiconductor hfhf Mask Film Substrate AnisotropicIsotropic

Wet Etching Oxide, Poly, and Metal layers are to be etched through a developed PR window. Wet etchant acids : HNO 3, H 3 PO 4 and HF acids. Wet etching is isotropic (only in large windows).

Wet Etching

Wet Etching Substrate Selectivity Etch Stop We want the etchant to remove SiO 2 and stop at Si. Stringer When material is cleared from a planar region - residual material remains at steps. Mask SiO 2 Si FILM II FILM I SUBSTRATE Prior to Etch FILM I II SUBSTRATE Etched to “Endpoint” “Residue”

Wet Etching Wet Etching Silicon Dioxide Solution of Hydrofluoric Acid is normally used. Reaction is: SiO 2 + 6HF  H 2 + SiF 6 + 2H 2 O Etch Rate ~ 1000Å/min at 25°C. Wet Etching Aluminum Phosphoric Acid is used to etch Aluminum. The reaction forms Al 2 O 3 which dissolves in water. Etch rate = 2000Å/min at 25°C

Dry Etching Removing by exposing the mask to bombardment of ions. The dry etching process typically etches directionally or anisotropically. Dry etching is useful for materials which are chemically resistant and could not be wet etched. Dry etchant : Plasma and Reactive Ion Etching. Dry etching is anisotropic (for small windows).

Dry Etching Plasma Etching It is a faster and simpler chemical process. More directional (anisotropic). Both chemical and ionic species play a role. Reactant ions in the RF induced plasma diffuse to the wafer. Reaction products are volatile: CF 4 for Si  SiF 4 (volatile) CHF 3 for SiO 2  SiF 4 +CO 2 + HF

Dry Etching Plasma Etching

Plasma Etching Mechanics Three mechanisms:  Chemical etching (isotropic, selective).  Physical etching (anisotropic, less selective).  Ion-enhanced etching (anisotropic, selective).

Plasma Etching

Wet vs. Dry Etching Characteristic Dry Etching Wet Etching cost highlow control easynot easy etch rate slowfast selectivity moderatehigh resolution goodfair anisotropy gooddifficult waste disposal problemlowhigh

Physical vs. Chemical Processes

Thanks Many thanks to Prof. Hany Fikry and Prof Wael Fikry for their useful materials that help me to prepare this presentation.