1. The last fabrication step of an accelerometer chip is the removal of silicon from a small region on the backside of t

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1 The Last Fabrication Step Of An Accelerometer Chip Is The Removal Of Silicon From A Small Region On The Backside Of T 1 (118.49 KiB) Viewed 34 times

1. The last fabrication step of an accelerometer chip is the removal of silicon from a small region on the backside of the substrate using bulk micromachining with an anisotropic wet etchant. For this purpose, a small window is opened in a stack of 3 thin films which is used to protect the rest of the wafer backside against the corrosive attack of the anisotropic wet etchant. The stack consists of a bottom thermal oxide layer with a thickness tj = 0.2 um, a middle LPCVD nitride film with a thickness t2 = 0.1 um, and a top layer of PECVD oxynitride whose thickness t3 can have values up to the maximal limit of 2 um dictated by the fabrication process. The internal built-in stress of the bottom thermal oxide layer is 0/=-500 MPa, the stress of the middle LPCVD nitride film is 02 = 1000 MPa and the stress of the oxynitride film is 03 = 500 MPa. It is assumed that the stress levels of any thin films do not depend on (or change throughout) the thickness of the thin film, unless otherwise indicated. (a) Calculate the thickness of the top PECVD oxynitride layer të required in the following cases if it were to provide an overall average stress Total for the entire three-layer stack with a value of: (i) Exactly zero, or (ii) A slightly tensile stress of 250 MPa. (iii) A slightly compressive stress of -250 MPa. What can you comment in this last case? (b) Calculate the thickness of the top PECVD oxynitride layer të required to provide a low compressive overall average stress with a value Orotal = -250 MPa if the stress 03 of the top layer of PECVD oxynitride is no longer independent from the film thickness but depends on it according to the equation 03 = 250t3-1600 (with values resulting in [MPa] if the thickness t3 is given in [um]). Discuss the obtained result with respect to the above given maximal thickness of 2 um which can be practically deposited for the top PECVD oxynitride layer. What would be the value of the overall average stress Ototal if the oxynitride layer in this case were to have the maximally admissible thickness t3 = 2 um?