View source for Engineering:Etching (microfabrication)

{{short description|Technique in microfabrication used to remove material and create structures}}
[[File:Wet etching tanks at LAAS 0465.jpg|thumb|250px|Etching tanks used to perform [[Chemistry:Piranha solution|Piranha]], hydrofluoric acid or RCA clean on 4-inch wafer batches at LAAS technological facility in Toulouse, France]]
'''Etching''' is used in [[Physics:Microfabrication|microfabrication]] to chemically remove layers from the surface of a [[Engineering:Wafer (electronics)|wafer]] during manufacturing. Etching is a critically important process module in fabrication, and every wafer undergoes many etching steps before it is complete.

For many etch steps, part of the wafer is protected from the etchant by a "masking" material which resists etching.  In some cases, the masking material is a [[Physics:Photoresist|photoresist]] which has been patterned using [[Engineering:Photolithography|photolithography]]. Other situations require a more durable mask, such as [[Chemistry:Silicon nitride|silicon nitride]].

== Etching media and technology ==
The two fundamental types of etchants are [[Physics:Liquid|liquid]]-phase ("wet") and [[Physics:Plasma|plasma]]-phase ("dry").  Each of these exists in several varieties.

[[File:Etching, simplified animation of etchant action on a copper sheet with mask.gif|Etching, simplified animation of etchant action on a copper sheet with mask]]

=== Wet etching ===
[[Image:1886VE10-HD.jpg|thumb|Radiation hardened [[Engineering:Die (integrated circuit)|die]] of the 1886VE10 microcontroller prior to metalization etching]]
[[Image:1886VE10-Si-HD.jpg|thumb|Radiation hardened [[Engineering:Die (integrated circuit)|die]] of the 1886VE10 microcontroller after a metalization etching process has been used]]

The first etching processes used [[Physics:Liquid|liquid]]-phase ("wet") etchants. This process is now largely outdated but was used up until the late 1980s when it was superseded by dry plasma etching.<ref>{{Cite book |last=Shubham |first=Kumar |url=https://www.worldcat.org/oclc/1246513110 |title=Integrated circuit fabrication |date=2021 |others=Ankaj Gupta |isbn=978-1-000-39644-7 |location=Abingdon, Oxon |oclc=1246513110}}</ref>{{Rp|page=147}} The wafer can be immersed in a bath of etchant, which must be agitated to achieve good process control. For instance, buffered hydrofluoric acid (BHF) is used commonly to etch [[Chemistry:Silicon dioxide|silicon dioxide]] over a [[Chemistry:Silicon|silicon]] substrate.

Different specialized etchants can be used to characterize the surface etched.

Wet etchants are usually isotropic, which leads to a large bias when etching thick films. They also require the disposal of large amounts of toxic waste. For these reasons, they are seldom used in state-of-the-art processes. However, the [[Chemistry:Photographic developer|photographic developer]] used for [[Physics:Photoresist|photoresist]] resembles wet etching.

As an alternative to immersion, single wafer machines use the Bernoulli principle to employ a gas (usually, pure [[Chemistry:Nitrogen|nitrogen]]) to cushion and protect one side of the wafer while etchant is applied to the other side. It can be done to either the front side or back side. The etch chemistry is dispensed on the top side when in the machine and the bottom side is not affected. This etching method is particularly effective just before "backend" processing (BEOL), where wafers are normally very much thinner after [[Engineering:Wafer backgrinding|wafer backgrinding]], and very sensitive to thermal or mechanical stress. Etching a thin layer of even a few micrometres will remove microcracks produced during backgrinding resulting in the wafer having dramatically increased strength and flexibility without breaking.

=== Anisotropic wet etching (Orientation dependent etching) ===
[[File:Anisotropic wet etching.svg|thumb|An anisotropic wet etch on a silicon wafer creates a cavity with a trapezoidal cross-section.  The bottom of the cavity is a {100} plane (see Miller indices), and the sides are {111} planes.  The blue material is an etch mask, and the green material is silicon.]]
Some wet etchants etch [[Earth:Crystal|crystal]]line materials at very different rates depending upon which crystal face is exposed.  In single-crystal materials (e.g. silicon wafers), this effect can allow very high anisotropy, as shown in the figure. The term "crystallographic etching" is synonymous with "anisotropic etching along crystal planes".

However, for some non-crystal materials like glass, there are unconventional ways to etch in an anisotropic manner.<ref name="X. Mu ">X. Mu, ''et al''. Laminar Flow used as "Liquid Etching Mask" in Wet Chemical Etching to Generate Glass Microstructures with an Improved Aspect Ratio. ''Lab on a Chip'', '''2009''', 9: 1994-1996.</ref> The authors employ multistream laminar flow that contains etching non-etching solutions to fabricate a glass groove. The etching solution at the center is flanked by non-etching solutions and the area contacting etching solutions is limited by the surrounding non-etching solutions. The etching direction is thereby mainly vertical to the glass surface. The [[Physics:Scanning electron microscope|scanning electron microscopy]] (SEM) images demonstrate the breaking of the conventional theoretical limit of aspect ratio (width/height=0.5) and contribute a two-fold improvement (width/height=1).

Several anisotropic wet etchants are available for silicon, all of them hot aqueous caustics.  For instance, [[Chemistry:Potassium hydroxide|potassium hydroxide]] (KOH) displays an etch rate selectivity 400 times higher in <100> crystal directions than in <111> directions.  EDP (an [[Chemistry:Aqueous|aqueous]] solution of ethylene diamine and pyrocatechol), displays a <100>/<111> selectivity of 17X, does not etch silicon dioxide as KOH does, and also displays high selectivity between lightly doped and heavily boron-doped (p-type) silicon. Use of these etchants on wafers that already contain [[Engineering:CMOS|CMOS]] [[Engineering:Integrated circuit|integrated circuit]]s requires protecting the circuitry. KOH may introduce mobile [[Chemistry:Potassium|potassium]] ions into [[Chemistry:Silicon dioxide|silicon dioxide]], and EDP is highly corrosive and [[Biology:Carcinogen|carcinogenic]], so care is required in their use. [[Chemistry:Tetramethylammonium hydroxide|Tetramethylammonium hydroxide]] (TMAH) presents a safer alternative than EDP, with a 37X selectivity between {100} and {111} planes in silicon.

Etching a (100) silicon surface through a rectangular hole in a masking material,  like a hole in a layer of silicon nitride, creates a pit with flat sloping  {111}-oriented sidewalls and a flat (100)-oriented bottom. The {111}-oriented sidewalls have an angle to the surface of the wafer of:

::<math>\arctan\sqrt{2}=54.7^\circ</math>

If the etching is continued "to completion", i.e. until the flat bottom disappears, the pit becomes a trench with a V-shaped cross-section. If the original rectangle was a perfect square, the pit when etched to completion displays a pyramidal shape.

The undercut, ''δ'', under an edge of the masking material is given by:

::<math>\delta = \frac{\sqrt{6} D}{S}=\frac{\sqrt{6} R_{100}T}{R_{100}/R_{111}}=\sqrt{6}TR_{111}</math>,

where ''R''<sub>xxx</sub> is the etch rate in the <xxx> direction, ''T'' is the etch time, ''D'' is the etch depth and ''S'' is the anisotropy of the material and etchant.

Different etchants have different anisotropies. Below is a table of common anisotropic etchants for silicon:

{| style="text-align:center; margin: 1em auto 1em auto" class=wikitable
!Etchant
!Operating temp (°C)
!R<sub>100</sub> (μm/min)
!S=R<sub>100</sub>/R<sub>111</sub>
!Mask materials
|-
|[[Chemistry:Ethylenediamine pyrocatechol|Ethylenediamine pyrocatechol]]<br>(EDP)<ref>{{cite journal
 |last1=Finne
 |first1=R.M.
 |last2=Klein
 |first2=D.L.
 |title=A Water-Amine-Complexing Agent System for Etching Silicon
 |journal=Journal of the Electrochemical Society
 |volume=114
 |issue=9
 |year=1967
 |pages=965–70
 |doi=10.1149/1.2426793
|bibcode=1967JElS..114..965F
 }}</ref>
|110
|0.47
|17
|[[Chemistry:Silicon dioxide|SiO<sub>2</sub>]], [[Chemistry:Silicon nitride|Si<sub>3</sub>N<sub>4</sub>]], [[Chemistry:Gold|Au]], [[Chemistry:Chromium|Cr]], [[Chemistry:Silver|Ag]], [[Chemistry:Copper|Cu]]
|-
|[[Chemistry:Potassium hydroxide|Potassium hydroxide]]/[[Chemistry:Isopropyl alcohol|Isopropyl alcohol]]<br>(KOH/IPA)
|50
|1.0
|400
|Si<sub>3</sub>N<sub>4</sub>, SiO<sub>2</sub> (etches at 2.8&nbsp;nm/min)
|-
|[[Chemistry:Tetramethylammonium hydroxide|Tetramethylammonium hydroxide]]<br>(TMAH)<ref>{{cite journal
 |last1=Shikida
 |first1=M.
 |last2=Sato
 |first2=K.
 |last3=Tokoro
 |first3=K.
 |last4=Uchikawa
 |first4=D.
 |title=Surface morphology of anisotropically etched single-crystal silicon
 |journal=Journal of Micromechanics and Microengineering
 |volume=10
 |issue=4
 |year=2000
 |page=522
 |doi=10.1088/0960-1317/10/4/306
|bibcode=2000JMiMi..10..522S
 |s2cid=250804151
 }}</ref>
|80
|0.6
|37
|Si<sub>3</sub>N<sub>4</sub>, SiO<sub>2</sub>
|}

=== Plasma etching ===
[[File:Photolithography etching process.svg|thumb|140px|Simplified illustration of dry etching using positive photoresist during a photolithography process in semiconductor microfabrication. Note: Not to scale.]]
Modern [[Engineering:Very Large Scale Integration|very large scale integration]] (VLSI) processes avoid wet etching, and use ''[[Engineering:Plasma etching|plasma etching]]'' instead. [[Engineering:Plasma etcher|Plasma etcher]]s can operate in several modes by adjusting the parameters of the plasma. Ordinary plasma etching operates between 0.1 and 5 [[Torr]]. (This unit of pressure, commonly used in vacuum engineering, equals approximately 133.3 [[Pascal (unit)|pascal]]s.) The plasma produces energetic [[Biology:Free radical|free radical]]s, neutrally [[Physics:Electric charge|charge]]d, that react at the surface of the wafer. Since neutral particles attack the wafer from all angles, this process is isotropic.

Plasma etching can be isotropic, i.e., exhibiting a lateral undercut rate on a patterned surface approximately the same as its downward etch rate, or can be anisotropic, i.e., exhibiting a smaller lateral undercut rate than its downward etch rate. Such anisotropy is maximized in deep reactive ion etching (DRIE). The use of the term anisotropy for plasma etching should not be conflated with the use of the same term when referring to orientation-dependent etching.

The source gas for the plasma usually contains small molecules rich in [[Chemistry:Chlorine|chlorine]] or [[Chemistry:Fluorine|fluorine]].  For instance, [[Chemistry:Carbon tetrachloride|carbon tetrachloride]] (CCl<sub>4</sub>) etches silicon and [[Chemistry:Aluminium|aluminium]], and trifluoromethane etches [[Chemistry:Silicon dioxide|silicon dioxide]] and [[Chemistry:Silicon nitride|silicon nitride]].  A plasma containing [[Chemistry:Oxygen|oxygen]] is used to oxidize ("[[Engineering:Plasma ashing|ash]]") photoresist and facilitate its removal.

''Ion milling'', or ''sputter etching'', uses lower pressures, often as low as 10<sup>−4</sup> Torr (10 mPa). It bombards the wafer with energetic ions of [[Chemistry:Noble gas|noble gas]]es, often [[Chemistry:Argon|Ar]]<sup>+</sup>, which knock atoms from the substrate by transferring [[Physics:Momentum|momentum]].  Because the etching is performed by ions, which approach the wafer approximately from one direction, this process is highly anisotropic. On the other hand, it tends to display poor selectivity. [[Engineering:Reactive-ion etching|Reactive-ion etching]] (RIE) operates under conditions intermediate between sputter and plasma etching (between 10<sup>−3</sup> and 10<sup>−1</sup> Torr). [[Engineering:Deep reactive-ion etching|Deep reactive-ion etching]] (DRIE) modifies the RIE technique to produce deep, narrow features.

== Orientation-Dependent Etching ==
* KOH pellets dissolved in water (self-heating) 
* Etch Rate {110} > {100} >> {111} 
** KOH has a slower etching orientation for the {111} planes 
** You cannot use this KOH photoresist as an etching mask, because the oxide attacks too slowly, so this resist will not survive
* Photoresist can be used as an etching mask, and the best photoresist for etching is nitride
* For example, the etch rate of Si in KOH Depends on Crystallographic Plane
* At low temperatures there is high selectivity (etching rate is slower), at high temperatures the selectivity will drop (higher etching rate) 

By increasing the temperature, the etch rate increases, but the selectivity decreases. There is a trade-off between etch rate and etch selectivity.

== Figures of merit ==
If the etch is intended to make a cavity in a material, the depth of the cavity may be controlled approximately using the etching time and the known etch rate. More often, though, etching must entirely remove the top layer of a multilayer structure, without damaging the underlying or masking layers. The etching system's ability to do this depends on the ratio of etch rates in the two materials (''selectivity'').

Some etches undercut the masking layer and form cavities with sloping sidewalls. The distance of undercutting is called ''bias''. Etchants with large bias are called ''isotropic'', because they erode the substrate equally in all directions. Modern processes greatly prefer ''[[Anisotropy|anisotropic]]'' etches, because they produce sharp, well-controlled features.

{| border
|-
| Selectivity
| [[File:Etch selectivity.png|140px]]
| Blue: layer to remain
# A poorly selective etch removes the top layer but also attacks the underlying material.
# A highly selective etch leaves the underlying material unharmed.
|-
| Isotropy
| [[File:Etch anisotropy.png|140px]]
| Red: masking layer; yellow: layer to be removed
# A perfectly isotropic etch produces round sidewalls. 
# A perfectly anisotropic etch produces vertical sidewalls.
|}


== Common etch processes used in microfabrication ==
{| class="wikitable"
|+ Etchants for common microfabrication materials 
|-
!Material to be etched !! Wet etchants !! Plasma etchants
|-
| [[Chemistry:Aluminium|Aluminium]] (Al)
| 80% [[Chemistry:Phosphoric acid|phosphoric acid]] (H<sub>3</sub>PO<sub>4</sub>) + 5% [[Chemistry:Acetic acid|acetic acid]] <br />+ 5% [[Chemistry:Nitric acid|nitric acid]] (HNO<sub>3</sub>) + 10% water (H<sub>2</sub>O) at 35–45&nbsp;°C<ref name=Wolf>{{cite book |title =Silicon Processing for the VLSI Era: Volume 1 - Process Technology |last = Wolf |first = S. |author2=R.N. Tauber |year=1986 |pages=531–534 |publisher=Lattice Press|isbn=978-0-9616721-3-3}}</ref>
| [[Chemistry:Chlorine|Cl<sub>2</sub>]], [[Chemistry:Carbon tetrachloride|CCl<sub>4</sub>]], [[Chemistry:Silicon tetrachloride|SiCl<sub>4</sub>]], [[Chemistry:Boron trichloride|BCl<sub>3</sub>]]<ref name=WolfDry>{{cite book |title =Silicon Processing for the VLSI Era: Volume 1 - Process Technology |last = Wolf |first = S. |author2=R.N. Tauber |year=1986 |publisher=Lattice Press |isbn=978-0-9616721-3-3 |page=546}}</ref>
|-
| [[Chemistry:Indium tin oxide|Indium tin oxide]] [ITO] (In<sub>2</sub>O<sub>3</sub>:SnO<sub>2</sub>) || [[Chemistry:Hydrochloric acid|Hydrochloric acid]] (HCl) + nitric acid (HNO<sub>3</sub>) + water (H<sub>2</sub>O) (1:0.1:1) at 40&nbsp;°C<ref name=LC>{{cite book |last = Bahadur |first=Birendra |title =Liquid Crystals: Applications and Uses vol.1 |year=1990 |publisher=World Scientific |isbn=978-981-02-2975-7 |page = 183}}</ref>
|
|-
| [[Chemistry:Chromium|Chromium]] (Cr)
|
* "Chrome etch": [[Chemistry:Ceric ammonium nitrate|ceric ammonium nitrate]] ((NH<sub>4</sub>)<sub>2</sub>Ce(NO<sub>3</sub>)<sub>6</sub>) + nitric acid (HNO<sub>3</sub>)<ref name=CRCCr>{{cite book |title=CRC Handbook of Metal Etchants |url=https://archive.org/details/crchandbookmetal00walk |url-access=limited |last=Walker |first=Perrin |author2=William H. Tarn |year=1991 |pages=[https://archive.org/details/crchandbookmetal00walk/page/n299 287]–291 |publisher=CRC-Press |isbn=978-0-8493-3623-2}}</ref>
* Hydrochloric acid (HCl)<ref name=CRCCr/> 
|
|-valign="top"
|
Gallium Arsenide (GaAs)
|
* Citric Acid diluted (C<sub>6</sub>H<sub>8</sub>O<sub>7</sub> : H<sub>2</sub>O, 1 : 1 ) + Hydrogen Peroxide (H<sub>2</sub>O<sub>2</sub>)+ Water (H<sub>2</sub>O) 
|
*[[Chemistry:Chlorine|Cl<sub>2</sub>]], CCl<sub>4</sub>, [[Chemistry:Silicon tetrachloride|SiCl<sub>4</sub>]], [[Chemistry:Boron trichloride|BCl<sub>3</sub>]], [[Chemistry:Dichlorodifluoromethane|CCl<sub>2</sub>F<sub>2</sub>]]
|-
| [[Chemistry:Gold|Gold]] (Au) 
| 
* [[Chemistry:Aqua regia|Aqua regia]] 
* Iodine solution 
|-
| [[Chemistry:Molybdenum|Molybdenum]] (Mo) || || [[Chemistry:Tetrafluoromethane|CF<sub>4</sub>]]<ref name=WolfDry/>
|-
| Organic residues and photoresist
| Piranha etch: [[Chemistry:Sulfuric acid|sulfuric acid]] (H<sub>2</sub>SO<sub>4</sub>) + [[Chemistry:Hydrogen peroxide|hydrogen peroxide]] (H<sub>2</sub>O<sub>2</sub>)
| [[Chemistry:Oxygen|O<sub>2</sub>]] ([[Engineering:Plasma ashing|ashing]])
|-
| [[Chemistry:Platinum|Platinum]] (Pt) || Aqua regia ||
|-
| [[Chemistry:Silicon|Silicon]] (Si)
| 
* Nitric acid (HNO<sub>3</sub>) + [[Chemistry:Hydrofluoric acid|hydrofluoric acid]] (HF)<ref name=Wolf/>
* [[Chemistry:Potassium hydroxide|Potassium hydroxide]] (KOH)
* [[Chemistry:Ethylenediamine pyrocatechol|Ethylenediamine pyrocatechol]] (EDP)
* [[Chemistry:Tetramethylammonium hydroxide|Tetramethylammonium hydroxide]] (TMAH) 
| 
* CF<sub>4</sub>, [[Chemistry:Sulfur hexafluoride|SF<sub>6</sub>]], [[Chemistry:Nitrogen trifluoride|NF<sub>3</sub>]]<ref name=WolfDry/>
* Cl<sub>2</sub>, [[Chemistry:Dichlorodifluoromethane|CCl<sub>2</sub>F<sub>2</sub>]]<ref name=WolfDry/>
|-
| [[Chemistry:Silicon dioxide|Silicon dioxide]] (SiO<sub>2</sub>)
|
* Hydrofluoric acid (HF)<ref name=Wolf/>
* Buffered oxide etch [BOE]: [[Chemistry:Ammonium fluoride|ammonium fluoride]] (NH<sub>4</sub>F) and [[Chemistry:Hydrofluoric acid|hydrofluoric acid]] (HF)<ref name=Wolf/>
| CF<sub>4</sub>, SF<sub>6</sub>, NF<sub>3</sub><ref name=WolfDry/>
|-
| [[Chemistry:Silicon nitride|Silicon nitride]] (Si<sub>3</sub>N<sub>4</sub>)
| 
* 85% Phosphoric acid (H<sub>3</sub>PO<sub>4</sub>) at 180&nbsp;°C<ref name=Wolf/> (Requires SiO<sub>2</sub> etch mask) 
| CF<sub>4</sub>, SF<sub>6</sub>, NF<sub>3</sub>,<ref name=WolfDry/> CHF<sub>3</sub>
|-
| [[Chemistry:Tantalum|Tantalum]] (Ta) || || CF<sub>4</sub><ref name=WolfDry/>
|-
| [[Chemistry:Titanium|Titanium]] (Ti) || Hydrofluoric acid (HF)<ref name=Wolf/> || BCl<sub>3</sub><ref name=EtchMicro>{{cite book |last=Kohler |first=Michael |title=Etching in Microsystem Technology |year=1999 |publisher=John Wiley & Son Ltd |isbn=978-3-527-29561-6 |page=329}}</ref>
|-
| [[Chemistry:Titanium nitride|Titanium nitride]] (TiN) || 
* Nitric acid (HNO<sub>3</sub>) + hydrofluoric acid (HF) 
* SC1
* Buffered HF (bHF)   
|-
| [[Chemistry:Tungsten|Tungsten]] (W) ||
* Nitric acid (HNO<sub>3</sub>) + hydrofluoric acid (HF) 
* Hydrogen Peroxide (H<sub>2</sub>O<sub>2</sub>)
|
*[[Chemistry:Tetrafluoromethane|CF<sub>4</sub>]]<ref name=WolfDry/>
*[[Chemistry:Sulfur hexafluoride|SF<sub>6</sub>]]{{Citation needed|date=February 2008}}
|-
|}

==See also==
*[[Engineering:Chemical-mechanical planarization|Chemical-Mechanical Polishing]]
*Ingot sawing
*[[Chemistry:Metal assisted chemical etching|Metal assisted chemical etching]]
*[[Engineering:Lift-off (microtechnology)|Lift-off (microtechnology)]]

== References ==
* {{cite book |last=Jaeger |first=Richard C. |title=Introduction to Microelectronic Fabrication |edition=2nd |year=2002 |publisher=Prentice Hall |location=Upper Saddle River |isbn=978-0-201-44494-0 |chapter=Lithography}}
* ''Ibid, "Processes for MicroElectroMechanical Systems (MEMS)"''

=== Inline references ===
{{reflist}}

== External links ==

[[Category:Semiconductor technology]]
[[Category:Semiconductor device fabrication]]
[[Category:Microtechnology]]

{{Sourceattribution|Etching (microfabrication)}}