Fall 2008 EE 410/510: Microfabrication and … 4 Wet Etching.pdfFall 2008 EE 410/510:...

Fall 2008 EE 410/510:Microfabrication and Semiconductor Processes

M W 12:45 PM – 2:20 PMEB 239 Engineering Bldg.

Instructor: John D. Williams, Ph.D.Assistant Professor of Electrical and Computer Engineering

Associate Director of the Nano and Micro Devices CenterUniversity of Alabama in Huntsville

406 Optics BuildingHuntsville, AL 35899Phone: (256) 824-2898

Fax: (256) 824-2898email: [email protected]

Nonreferenced images taken from Madou: Fundamentals of Microfabrication 2nd ed. CRC Press 2002

10/16/2009JDW, Electrical and Computer Engineering,

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Thin Film Wet Etches: SiO2, Si3N4

• Si3N4 Etching– 1% HF etches at about 60 nm/min– 10% HF etches at about 0.500 nm/min– Buffered HF (BHF) etches at 0.5 – 1 nm/min– H3PO4 etches at 10 nm/min at 180oC

• Selective to oxides and silicon• SiO2 etching

– BHF etches at 100 ‐250 nm/min– HF etches extremely fast

• Etch rates vary depending on film quality


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Common Wet Etches for Metals• Titanium

– 1% HF etches at 30 nm/min

• Copper

– HNO3 5% in H20 works well

– NH4OH/H2O2 1:1

• Nickel


• Aluminum– HCl 5% in H20 works well

– H3PO4 /HNO3/HAc

– NaOH 5% in H20 works well

• Chromium

– Chrome etch (commercial)• KMnO4 : NaOH : H2O

– 5% HCl in H20 works

– HCl in H2O2 1:1

• Gold

– KI, I2, H20

• Silver

– HNO3

– NH4OH/H2O2 1:1


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Standard Silicon Clean• Step 1 ‐ Piranha/SPM

– 4:1 H2SO4 (40%):H2O2 (30%) @ 90 C for 15 min– Removes organic contaminants

• Step 2 ‐ DI water rinse• Step 3 ‐ DHF

– HF (2%) for 30 sec• Step 4 ‐ DI water rinse• Step 5 (SC‐1/APM)

– 1:1:5 NH4OH (29%):H2O2 (30%) H2O at 70 C for 10 min

– removes particulate contaminants– desorbs trace metals (Au, Ag, Cu, Ni, etc.)

• Step 6 ‐ DI water rinse• Step 7 ‐ SC‐2

– 1:1:5 HCl (30%):H2O2 (30%):H2O at 70 C for 10 min– dissolves alkali ions and hydroxides of Al3+, Fe3+,

Mg3+– desorbs by complexing residual metals

• Step 8 ‐ DI water rinse• Step 9 ‐ Spin rinse dry

• Prepares silicon for use as a substrate

• Performed immediately before LPCVD or oxidation processes to remove contaminants

• SC1– Etches silicon and leaves a thin oxide

surface.– Oxide surface combined with alkaline pH

value of SC1 solution produces negative charges on the surface.

– Electrostatic repulsion between the removed particle and the oxide surface will prevent particle redeposition.

– Some materials such as alkalis, Fe, Al, Mg remain insoluble

• SC2– Removes metals, hydroxides, and

complexing residuals from the surface.

• Can be followed by a quick HF rince to remove all oxide from the surface prior to step 8


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Etching Mechanisms for Bulk Micromachining

SF6


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Silicon Wafer Crystal Orientation

Crystal orientation effects the direction of an anisotropic etch


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Isotropic vs. Anisotropic Wet Bulk Etching of Silicon

• Cr/Au can be used as a mask material, however pinholes in PVD films will lead to mask etched pits under mask

•SixNy has a near infinite selectivity to most silicon etchants. Stochiometric nitrides are often too stressy for membranes, so engineers have developed a low stress LPVCD nitride that is commonly used. PECVD nitrides have pinholes in the deposited film.


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Isotropic Etches

• Also note that the low selectivity of SiO2 to HF etches make these isotropic etches a viable glass etchant as well.

• Etch rates are approx. 1/20 of the silicon etch rate, but prove quite useful glass etching applications.

• Surface roughness of glass etched under these conditions is 1‐5 um.

• Etches used in silicon primarily for generating bowl shaped structures.

• Good for:– Electrostatic isolation– Microfluidics– Coupled with vias at base of

hole, these shapes are often used for DNA sensing


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Anisotropic Etching


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Determination of CommonSilicon Etch Angle

Flat [110]

• Proper alignment leads to {111} sidewalls, (100) bottom, <110> directed edges and <211> directed ribs

• Consider the unit cube and the off‐normal angle α of the intersection of a (111) sidewall and a (110) cross‐secting plane

L = a* 22

tan α = La

L

a

(110)

α

L = a* 22

α =arctan = 35.26°or

54.74° for the complementary angle(111)


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Overview of Anisotropic <100> Silicon Etching


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Vertical Wet Etches in Silicon• {100} planes perpendicular to the wafer

surface (at a 45° angle with the wafer flat i.e.the {110} direction)

• Anisotropic wet etching along these planes provides vertically etched structures

• Manufacturing errors of approximately 1‐2o

are prominent in flat orientation

• Thus pre‐etches must be performed to identify exact crystal axis prior to device patterning

• Inexact orientation of the pattern has significant effects on deep etches

Flat [110]

(100) planes


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Wagon Wheel Map of Si Anisotropic Etching

Tabata, “Anisotropy and Selectivity Control of TMAH”, Proc. Of MEMS 98, Heidelberg, Germany, 229-233

<110>

<100>

<111>

Simulated wheel for 1 um wide beams after several minutes of etching

Q. D. Nguyen, Electrochemistry in anisotropic etching of silicon in alkaline solutions – A kinematic wave analysis, Ph.D. thesis, University of Twente, Enschede, The Netherlands (2007).


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Creating Complex Shapes: Corner Compensation

• Geometrical limitations of the etch must be considered when creating complex shapes

• Image on the right provides three different attempts to compensate for angular dependence when etching convex structures.

• Variations in the lithographic geometry at the corner result in rounding or flattening of the silicon sidewall.

Lithographic Pattern Etched Silicon

Circular mask shape etches as an octagonal hole (ref. on next slide)

Cross Structures etch as deformed polygons (Bruce Gale Univ. Utah MEMS Lectures)

Gupta et al.: “Corner Compensationin Silicon Micro Machining for Sensor Fabrication”


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Simple Approach to Corner Compensation

http://www.paper.edu.cn/download_feature_paper.php?serial_number=AgilentF-03

WEI FAN (1) ; DACHENG ZHANG (1) ; Institute of Microelectronics, Peking University, Beijing 100871, CHINA ????

http://www.paper.edu.cn/download_feature_paper.php?serial_number=AgilentF-03


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Electrochemical Etch Stops


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Undercutting and Release of Wet Etched Devices

http://pkukmweb.ukm.my/~imen/Notes/MEMS%20Fabrication_Seminar.pdf

N-doped silicon layers provide thickness to silicon cantilevers

http://pkukmweb.ukm.my/~imen/Notes/MEMS Fabrication_Seminar.pdf

Fig. 3 Fabrication flow of the spiral inductor; (a) after the COMS process, (b) oxide layer etched by an anisotropic dry etching, (c) silicon substrate etched by an anisotropic wet etching with TMAH.

High Q-factor CMOS-MEMS inductorChing-Liang Dai, Jin-Yu Hong and Mao-Chen Liu

Department of Mechanical Engineering,National Chung Hsing University,

Taichung, 402 Taiwan, R.O.C.


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Wet Bulk Micromachining of InP• Indium phosphide (InP) is a direct

bandgap III–V semiconductor • Important for optoelectronics. • InGaAsP and InGaAlAs are grown on InP to

form heterostructures that provide the basis for lasers, photodetectors, transistors, waveguides, gratings, lenses and mirrors

• Anisotropic etching of this material is crucial for the development of micro‐opto‐electromechanical systems

• InGaAs mask used• 3HCl:1H3PO4 at 16 ◦C ± 0.05 ◦C in light

without agitation.• Approx.. 15 um/min etch rate. Mask

thickness used to was roughly 0.1 um• Directional dependence based on time in

etch. This is important because secondary facets define corners.

J. Micromech. Microeng. 14 (2004) 1205–1214


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3‐D Micro channels using KOH• Combination of DRIE and KOH Etching• Dry etch trench• Fill with KOH and etch out diamond

shape• Fill Diamond with TMAH oxide

(LPCVD)• Supply appropriate mask geometry to

provide inlets from top surface

Therkstra et.al, “Etching Technology for Microchannels,” IEEE MEMS, 1997, 147-152.


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Porous Silicon• Electrochemically etched silicon

– Pores : nm μm sized– Pore nucleation – many models [1,2]– Suggests defects near surface play

primary role• Discovered in 1956 by Arthur Uhlir at Bell

Labs, during electropolishing experiments

Lehmann, J. Electrochem. Soc., 140, 2836-43, 1993

http://www.tf.uni-kiel.de/matwis/amat/poren/eccv.html

http://www.tf.uni-kiel.de/matwis/amat/poren/eccv.html


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Thin Film Etches: SiO2, Si3N4

• Si3N4 Etching– 1% HF etches at about 60 nm/min– 10% HF etches at about 0.500 nm/min– Buffered HF (BHF) etches at 0.5 – 1 nm/min– H3PO4 etches at 10 nm/min at 180oC

• Selective to oxides and silicon• SiO2 etching

– BHF etches at 100 ‐250 nm/min– HF etches extremely fast

• Etch rates vary depending on film quality


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Standard Silicon Clean• Step 1 ‐ Piranha/SPM

– 4:1 H2SO4 (40%):H2O2 (30%) @ 90 C for 15 min– Removes organic contaminants

• Step 2 ‐ DI water rinse• Step 3 ‐ DHF

– HF (2%) for 30 sec• Step 4 ‐ DI water rinse• Step 5 (SC‐1/APM)

– 1:1:5 NH4OH (29%):H2O2 (30%) H2O at 70 C for 10 min

– removes particulate contaminants– desorbs trace metals (Au, Ag, Cu, Ni, etc.)

• Step 6 ‐ DI water rinse• Step 7 ‐ SC‐2

– 1:1:5 HCl (30%):H2O2 (30%):H2O at 70 C for 10 min– dissolves alkali ions and hydroxides of Al3+, Fe3+,

Mg3+– desorbs by complexing residual metals

• Step 8 ‐ DI water rinse• Step 9 ‐ Spin rinse dry

• Prepares silicon for use as a substrate

• Performed immediately before LPCVD or oxidation processes to remove contaminants

• SC1– Etches silicon and leaves a thin oxide

surface.– Oxide surface combined with alkaline pH

value of SC1 solution produces negative charges on the surface.

– Electrostatic repulsion between the removed particle and the oxide surface will prevent particle redeposition.

– Some materials such as alkalis, Fe, Al, Mg remain insoluble

• SC2– Removes metals, hydroxides, and

complexing residuals from the surface.

• Can be followed by a quick HF rince to remove all oxide from the surface prior to step 8


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Common Metal Etches• Titanium

– 1% HF etches at 30 nm/min

• Copper


– NH4OH/H2O2 1:1

• Nickel


• Aluminum– HCl 5% in H20 works well

– H3PO4 /HNO3/HAc

– NaOH 5% in H20 works well

• Chromium

– Chrome etch (commercial)• KMnO4 : NaOH : H2O

– 5% HCl in H20 works

– HCl in H2O2 1:1

• Gold

– KI, I2, H20

• Silver

– HNO3

– NH4OH/H2O2 1:1

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