N.CHEUNG EE143, F2005 Homework Assignment # 7 (Due Oct 27, Thursday ,9:30am) Reading Assignment 1) EE143 reader Week #8 : Chapter on Etching by G. Anner, “Planar Processing Primer” 2) EE143 Lecture Notes Problem 1 Etching Profile (a) Poly-Si of the following structure is to be etched using a completely anisotropic dry-etch process, to remove poly-Si at a rate of 0.1 µm /min. However, this etch process has poor selectivities: selectivity to SiO2 is 5; selectivity to photoresist is 2. (a) Sketch the cross-section after 5 minutes of etching. (b) Calculate the angle of the SiO2 sidewalls after 5 minutes of etching. Problem 2 Poly-Si cross-section if mask is also etched The structure shown below is subjected to an etching process to form a poly-Si line with the following etching characteristics Vertical mask etching rate = 0.01 µm/min Vertical poly-Si etching rate = 0.1 µm/min Degree of anisotropy for mask etching , Am = 1 Degree of anisotropy for poly-Si etching , Af= 0 θ = 60° . * The mask and poly-Si have absolute uniform thicknesses. 0.5 umphotoresist0.7umoxidePoly-Si0.1um1 um0.5 umphotoresist0.7umoxidePoly-Si0.1um1 um poly-Si Substrate 5 um 0.5 um X Y1 um mask Θ Θ2 * The mask and poly-Si have absolute uniform etching rates. (i) After the poly-Si has just been cleared at points X and Y, sketch the cross-sections of the poly-Si and mask. Include the original two vertical dash lines at X and Y for reference. (ii) Find the maximum width of the poly-Si line cross-section? (iii) Find the minimum width of the poly-Si line cross-section? (iv) Find the maximum linewidth of the mask cross-section? (v) Find the maximum thickness of the mask cross-section ? Problem 3 Worst-case design considerations A 0.5 µm SiO2 on a Si substrate is to be etched away. It is known there is a ±5% variation of the oxide thickness and a ±5% variation in the oxide etch rate. (a) How much overetch (in % etch time) is required to ensure all oxide on Si is removed? (b)What minimum selectivity of the oxide etch rate to the Si etch rate is required so that a maximum of 5nm (= 0.005 µm) of Si etched with the overetch calculated in part (a). Problem 4 Deep submicron patterning by overetching We would like to fabricate devices with deep-submicron dimensions but our lithography equipment can only pattern resist linewidths down to 0.5 µm at best. For resists with vertical sidewalls, propose a simple process flow which can give 0.2 µm poly-Si linewidth with vertical sidewalls. You cannot use undercut with wet etching because the sidewall will be curved. Problem 5 Sidewall slope of contact holes Taking advantage of photoresist erosion during reactive ion etching of SiO2, we can vary the slope angle of SiO2 contact holes. The following schematic shows the resist and SiO2 cross-sections before (solid lines) and after (dash lines) reactive ion etching. Given: α = 80° VRv= vertical etching rate of resist = 1000 Å/min VRl= lateral etching rate of resist = 500 Å/min VOv= vertical etching rate of oxide = 1000 Å/min VOl= lateral etching rate of oxide = 0 Å/min Resist poly-Si Substrate SiO2 0.5 um Your Process Substrate 0.2um3 ResistSiO2SubstrateαβBefore RIEAfter RIE whenSiO2 is just cleared (i) Use algebra to prove that the slope of SiO2 after etching is a straight line with constant β. (ii) If VRv, VRl, and VOv all have a variation of ±10% from run to run. Find the maximum and minimum values of β. Problem 6 RIE Questions (a) How can we increase the degree of anisotropy in reactive ion etching? (b) Selectivity in reactive ion etching can be increased by using gas mixtures. Quote one example to illustrate the principle. (c) In class and in the reader, we discuss the formation of an inhibitor during RIE can enhance etching anisotropy. However, with excessive inhibitor deposition rate, the sidewall of the etch film can be more tapered than less inhibitor deposition (illustrated in the attached figures). Fill in the intermediate cross-sections to show progression of the etching process .
View Full Document
Unlocking...