Silicon Etching using Chlorine based plasma for
High Selectivity and High Etch Rate.
Abstract : Silicon Etching using Inductively Coupled Plasma (ICP) in Cl2/BCl3 Plasma, under various Source power, Platen power,
Pressure, Flow rates and Temperature are presented. We achieved an Etch Rate (ER) of 350 nm/min and Selectivity (SEL) of 35
with SiO2 hard mask. The experiment was conducted on Oxford Instruments Plasma Lab system 100 ICP 180 at James Watt
Nanofabrication Centre, University of Glasgow.
Introduction : Plasma is an ionized gas with approximately
equal number of positively charged particles (positive ions) and
negatively charged particles (electrons), and different numbers
of excited and unexcited neutral particles [1]. Plasma based dry
etching, enables processing at comparatively low temperature ,
wide area can be processed uniformly ,achieve anisotropic etch
and direction of etching can be controlled [2].
ICP generates high density, low pressure plasma which allows
independent control of ion flux and ion energy[3].In ICP, a coil is
wrapped around a cylindrical dielectric vacuum vessel. When
RF current flows through coil, RF magnetic flux is produced
along the axis of cylinder. This flux induces RF electric field
inside the vessel. Electrons in vacuum vessel are accelerated
by this electric field and dense plasma is produced [1].
Experiment : Consist of analyzing the effects of various
plasma parameters on etching of silicon with SiO2 as hard
mask. Finally deduce an optimized recipe which provides
anisotropic etch with high selectivity and high etch rate.
Finalized recipe was used to etch silicon with SiN and 8um thick
photoresist mask.
Results and Discussion :
Source Power ↑
Plasma density ↑
Ions & Radicals ↑
Thus, Etch rate ↑.
Now, Plasma Density ↑
Etch rate of mask ↑
Thus, Selectivity ↓.
Etchant Radicals ↑
Chemical Reaction ↑
Thus, Etch rate ↑.
Now, Selectivity almost
same Chlorine etch
Silicon fast as
compared to SiO2.
Etchant Radicals ↑
Chemical Reaction ↑
Thus, Etch rate ↑.
Now,
Chemical Reaction ↑
Thus, Selectivity ↑.
Temperature was reduced from 200C to 00C. No change was
observed in etch rate and selectivity, but verticality of etched profile
was improved, as a result of enhanced passivation layer. Helium
back cooling pressure was varied from 0T to 15T. No change in etch
rate and selectivity was found, but at low pressure, verticality of
profile improved.
Finalized Recipe:
Source Power = 2000 Watt, Bias Power=60 Watt, Pressure=25mT,
Temperature=00C,Heluim back cooling=5T, Time=10min.Pictures
below show the silicon etched with SiO2, SiN and photoresist masks
respectively. Etch rate is in nm/min.
Bias Power ↑
Ion Energy ↑
Bias Voltage ↑
Milling Rate ↑
Thus, Etch Rate ↑.
Now, Milling Rate ↑
Etch Rate ↑
Thus, Selectivity ↑.
ER=350,Sel=35
Pressure ↑
No. of Molecules ↑
Ions & Radicals ↑
Thus, Etch rate ↑.
Now,
No. of Molecules ↑
Etch rate ↑
Thus, Selectivity ↑.
University of Glasgow, charity number SC004401
ER=320,SEL=16
ER=300,SEL=1
References:
[1] M. Sugawara , “Plasma Etching ; Fundamentals and Applications”
, Oxford University Press, New York, 1988.
[2]. Stephen M.Rossnagel, Jerome J.Cuomo, William D.Westwood,
Handbook of plasma processing technology, Noyes publication,
August 11, 2013.
[3]. C.Y. Chang and S.M. Sze, ULSI Technology, McGraw-Hill(1996).
Author : Kratika Jakhar (2056073J)
MS.c in Electronics And Electrical Engineering.
Supervisor : Haiping Zhou.