lecture 19 - wet etching · • selectivity of better than 100:1 can be obtained between si and sio...

Lecture 19 EE 441 Spring 2009: TadigadapaEtching Etching is the removal of a material in the exposed areas using:

– Wet Chemicals & Solutions– Gases– Plasma– Milling

For Example:– Wet etching has high selectivity, lowWet etching has high selectivity, low

physical damage, and low anisotropy– Ion milling has low selectivity, high

damage and high anisotropyAnisotropy is given by:

L

RRA −=1

Etching Metrics:1. Etch Rate (μm/s)

RL is the lateral etch rateRv is the vertical etch rate

vR(μ )2. Etch Rate Uniformity (%

of etch rate)3. Selectivity (Ratio of etch

t f t t i l ) vFor Perfect Anisotropic etch A=1For Perfect Isotropic etch A=0

rate of two materials)4. Undercut (lateral

distance per side)

Lecture 19 EE 441 Spring 2009: Tadigadapa

Wet EtchingW t t hi i t f th• Wet etching consists of three processes:– Movement of etchant of species to the surface of the wafer

A chemical reaction with the exposed surface– A chemical reaction with the exposed surface– Movement of the reaction products away from the surface

• The slowest of these processes is the “Rate Limiting Step”The slowest of these processes is the Rate Limiting Step . It is also assumed that the chemical reaction produces soluble products which can be moved away!

• In general, agitation is necessary to maintain uniformity in etch rate which aids the movement of the material.

• If the etch rate produces gaseous products, these bubbles can stick to the surface of the wafer and cause non-uniformity in etch. This pattern is most pronounced at pattern edges.


Wet Etching MethodsSi l di i t h t ( ith ti ti )• Simple dip in an etchant (with magnetic stirrer)

• Can result in extreme non-uniformities due to non-uniform temperaturestemperatures

• Etch time control• Changing concentration of etch chemicalChanging concentration of etch chemical• A simple variation is to combine this process with

ultrasonic agitation but can cause cavitation!• A more elaborate set-up is spray etching. The wafer is

sprayed with the correct concentration and temperature t h t hil f i l l t t d Thietchant while wafer is slowly rotated. This ensures very

high process control but creates enormous waste chemicals!

• May also be useful in single-sided etching.


Wet Etching of Some MaterialsR b i d f th t h t t b f l• Remember: in order for the etch step to be successful we need good selectivity between photoresist and the material etch rate in a specific etchant!p

• In some cases: we first transfer the pattern into a material of high selectivity – then use this material as the mask.

• Most commonly etched materials are: SiO2, Si3N4, Al, and Polysilicon and Silicides (Si compounds with transition metals)transition metals).

• SiO2: Wet etching of silicon dioxide is accomplished in a dilute solution of Hydrofluoric Acid (HF).dilute solution of Hydrofluoric Acid (HF).

• Dilutions in water of H2O:HF::6:1, 10:1 & 20:1 are commonly used.

• Etch rate of thermal oxide in 6:1 HF solution is about about 1200Å/min.


• In general the etch rate of SiO2 follows the following trend:Wet Etching

In general the etch rate of SiO2 follows the following trend:

Thermal Oxide Deposited Oxide Doped Oxide PECVD Oxide

Increasing Etch Rate

• Selectivity of better than 100:1 can be obtained between Si and SiO2. HF is an isotropic oxide etch.

(LPCVD)p

PSG/BSG

2 p• The reaction pathway depends upon the ionic strength, the

solution pH, and the composition of etchant solution.SiO + 6HF H + SiF + H OSiO2 + 6HF → H2 + SiF6 + H2O

• Since the reaction consumes HF, the reaction rate will decrease with time. To avoid this HF is buffered with NH4F which maintains a constant concentration of HF through

NH4F ↔ NH3 + HF• Buffering controls the pH of the etchant and minimizesBuffering controls the pH of the etchant and minimizes

photoresist attack!


Wet Etching of Silicon Nitride• A 20:1 BHF (Buffered HF) or BOE (Buffered Oxide Etch)• A 20:1 BHF (Buffered HF) or BOE (Buffered Oxide Etch)

etches thermal oxide at ~ 300Å/min but the etch rate for silicon nitride is only ~10Å/min!P ti l t h t f Si N hi d i h h i• Practical etch rates for Si3N4 are achieved using phosphoric acid (H3PO4) @ 140-200°C.

• Typical selectivities in phosphoric etch are 10:1 for nitride over oxide and 30:1 for nitride over Si!

• If nitride is exposed to high-temperature oxidation step, then a BHF dip must precede the nitride etch step.BHF dip must precede the nitride etch step.

• Wet Etch of Aluminum: • Common aluminum etchant is a mixture of 20% Acetic acid

(CH COOH) 77% Ph h i id (H PO ) d 3% Nit i(CH3COOH), 77% Phosphoric acid (H3PO4) and 3% Nitric Acid (HNO3) by volume.

• Or NaOH (Sodium Hydroxide) @ 60-90°C can also etch aluminum.


Wet EtchingWet etching is a purely chemical process:process:• Lack of anisotropy• Poor process controlp• Particle contamination• Commonly used in the

fabrication ofSili I t h C

• Isotropic Wet Etching of Silicon:

fabrication of Microelectromechanical Systems (MEMS).

Silicon Iso-etch Curves

• Most techniques use strong oxidants to chemically oxidize the Si and HF is used to etch the oxide. A common etchant solution is HF and Nitric acid in watersolution is HF and Nitric acid in water.

• Si + HNO3 + 6HF → H2SiF6 + HNO2 +H2 +H2O• Acetic acid is used as a diluent than water. The maximumAcetic acid is used as a diluent than water. The maximum

etch rate for this solution ~ 470μm/min i.e. a hole in wafer can be etched in 90 seconds!!


Anisotropic Etching of Silicon• For example: to isolate devices it may be necessary to make• For example: to isolate devices it may be necessary to make

0.25μm wide several microns deep etch into the wafer.• Some etchants etch certain crystal directions much faster. These y

etches produce sharp facets with well controlled angles in single crystal substrates.Anisotropic etchants of Si:• Anisotropic etchants of Si:

Etchant Etch Rate Etch Rate Ratio (against 100)

KOH/H2O/IPA @ 80-85°C 1.4μm/min 400 for (110) 600 for (111)600 for (111)

Ethylene Diamine Pyrocatechol (water) @ 115°C

1.2μm/min 35 for (111)

Tetramethyl Ammonium Hydroxide @ 90 °C

1μm/min 12.5-50 for (111)


Anisotropic Etching of Silicon


For a (100) oriented Si afer the Primar flat is oriented along

Anisotropic Etching of SiliconFor a (100) oriented Si wafer, the Primary flat is oriented along the (110) direction.

The {111} planes will be along the 110 direction at an angle of

<111><100>

The {111} planes will be along the 110 direction at an angle of 54.74 to the {100} plane.

54 74[001] (100)

Wm54.74

<111> <110>

W0

z

Then[100]

0Then

zWW m 20 −=[010]


Anisotropic Etching of Silicon

lecture 19 - wet etching · • selectivity of better than 100:1 can be obtained between si and sio...

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