Biomicroelectromechanical Systems- Lecture 10 -13

8/13/2019 Biomicroelectromechanical Systems- Lecture 10 -13

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Biomicroelectromechanical

Systems- Lecture 10 -12

Instructor: Shantanu Bhattacharya



Subtractive TechniquesWet etching:

Wet etching is referred to as etching processes of solid materials in a chemical

solution.

Wet etching in microelectronics are mostly isotropic, independent of

crystalline orientation.

Because of the under-etching effect, isotropic etching has drawbacks in

designing lateral structures.

If the etch solution is well stirred , the isotropic etch front has a spherical

front.



Dry Etching• Physical Dry Etching:

It utilizes beams of ions, electrons or photons to bombard the material surface.The kinetic energy of the ions knocks out atoms from the substrate surface.

The high beam energy then evaporates the knocked out materials.

Limitations:

Slow etch rates

Low selectivity because ions attack all materials.

Trench effects caused by reflected ions.

• Chemical Dry Etching:

Chemical dry etching uses a chemical reaction between etchant gases to attackmaterial surface.

Gaseous products are conditions for chemical dry etching because deposition ofreaction products will stop the etching process.

Chemical dry etching is isotropic. This technique is similar to wet etching andexhibits relatively high selectivity.



Chemical Vapor Deposition reactors

Horizontal Reactor Vertical Reactor



Etching Cantilevers and Diaphragms using Wet

Etching

Borrowed From Madou et. al. 2002.



Recipes of Dry Etchant Gases

Physical Chemical Etching

•Some Dry etching is referred to as a physical-chemical etching

process.•These are RIE (reactive ion etching), Anodic Plasma Etcing (APE),

Magnetically enhanced reactive ion etching (MERIE), Triode

reactive ion etching (TRIE), and transmission coupled plasma

etching (TCPE).



Fluorine etching of Silicon



Anisotropic Wet EtchingBasic etching processes:

• For single crystalline materials such as silicon, etch rates of anisotropic wet etching

depend on crystal orientation.

•In an anisotropic wet etching process, hydroxides react with silicon in the following

steps:

•In the steps of the reaction overall 4 electrons are transferred from each silicon atom to

the conduction band.

•The presence of electrons is important for the etching process.

•Manipulating the avialability of electrons makes a controllable etch stop possible.

•Because of its crystalline structure, silicon atoms in {111} planes haver stronger binding

forces, which make it more difficult to release electrons from this plane. This the etchrates at {111} planes are the slowest.



Basic Silicon Etchants which provide

hydroxide groups

i f i i



SEM images of Anisotropic vs.

Isotropic Etches

Isotropic Etchants Anisotropic Etchants



Trench Profile of Anisotropic Wet

Etching of {100} silicon

A micro-channel is etched in a {100} wafer with KOH solution.

(1)Determine the angle between the channel wall and the front surface.

(2)If the top channel width and the etch rate are 100 microns and 1 micron/ min.

respectively, what is the bottom channel width after 20 mins. of etching?

(3)How long will it take until the etching process stops?



System setup for

Microstereolithography



Mass production with Microsterolithography

The disadvantage of a prototyping approach can be solved with the setup shown

below.Instead of a single beam, the source beam splits into several beams that are guided by

optical fibers.

The number of optical fibers determines the number of devices, which can be

fabricated in parallel.



Thermal Oxidation

•Although silicon dioxide can be deposited with CVD, thermal oxidation is the simplest

technique to create a silicon dioxide layer on silicon.

•In silicon based micro-fludic devices, thermal oxidation can be used for adjusting gaps

such as filter pores or channel width with micrometer accuracy.

•Based on the type of oxidation thermal oxidation can be categorized as dry and wet

oxidation.

•In dry oxidation, pure oxygen reacts with silicon at high temperatures from 800 deg. C –

1200 deg. C.

Si + O2 ------- SiO2

•In wet oxidation, water vapor reacts with Silicon at high temperatures:

Si + H2O ------- SiO2 + 2H2



Thickness of Oxide Layer of Thermal Oxidation

The density of silicon and silicon dioxide are 2,330 Kg/ m3 and 2,200 kg/ m3 , respectively.

Molecular masses of silicon and xoygen are 28.09 kg/kmol and 15.99 kg/kmol, respectively.

Determine the consumed silicon thickness for a silicon dioxide film of thickness ‘d’.

For 1 kmol silicon, one will get 1 kmol silicon dioxide. For the same surface area, the ratio of

the thicknesses is equal to the ratio of volume:



Deposition methods

Metal evaporation process.• Evaporation deposits thin film on a

substrate by sublimating a heated source

material in vacuum.

• The vapor flux from the source coats the

substrate surface.

• Based on the various heating sources the

various evaporation techniques are

vacuum thermal evaporation (VTA), E-

beam evaporation (EBE).

• Alloys can be deposited with materials

with two more material sources.



Sputtering methods

• The sputtering process is carried out with plasmaunder very low pressure (about 5 E-7 Torrs). This

process involves low temperature, which iscontrary to CVD.

• The plasma is made up of positively charged ionsand electrons and can be produced by highvoltage DC sources, RF sources or by fluctuatingmagnetic fields created by inductive coupling.

• The positive ions of the metal in the inert Argongas carrier bombard the surface of the target atsuch a high velocity that the momentum transferon impingement causes the metal ions toevaporate.

• The metal vapor is then led to the substratesurface and is deposited after condensation.



Deposition and Liftoff

These are mostly used for fabricating thin wire like patterns of gold, silver,

Platinum or palladium (mostly metals) for IC interconnects.

Inter-digitated Electrodes Made

with Platinum metal

S Mi i I f



Some more Microscopic Images of

Patterned metals



Doping and Ion Implantation• The ion implantation technique is one of

the most important technique for

microelectronics and adds impurities to

semiconductors such as silicon.• In Ion implantation ionized impurity

atoms accelerated through an

electrostatic field strike the surface of a

wafer.

• The dose can be tightly controlled using

an ion current. Doses from process range

from 1011 cm-2 for very light implants to

1016 cm-2 for low resistance regions such

as contacts, emitters, buried collectors

etc. as in transistors.

• By controlling the electric field, the

penetration depth of the impurity atoms

can also be controlled.

• By controlling the electric field, the

penetration depth of the impurity atoms

can also be controlled.

•BF3 , AsH3 and PH3 are the

commonly used gases

•Separation by implantation

of oxygen (SIMOX), SOI



Drive-in-Diffusion• After ion implantation , dopants are distributed in a layer on the silicon surface.

Subsequent annealing redistributes the dopant atoms.

•This process is based on the diffusion of dopants and is called drive-in-diffusion.

Silicon on Insulator•For applications of MEMS and micro-fluidics, a much thicker SOI layer than that

resulting from SIMOX is needed. Most SOI wafers used in MEMS are fabricated with

bonded etched- back silicon on insulator (BESOI) technique.

•This technique uses two polished silicon wafers with an oxide layer on each. The two

wafers are then bonded together using fusion bonding.

•One wafer is thinned to desired thickness using chemical-mechanical polishing (CMP).

•The major advantages with the SOI fabricated using this technique are:

1. The thickness of SOI is adjustable and allows thicker structural layer of the device.

2. The thickness of Silicon dioxide is adjustable.

3. The SOI layer has the same quality of the bulk substrate, while SOI from SIMOX may

have crystal defects caused by ion implantation.

Mi li h h



Microstereolithography•In many polymeric micromachining techniques discussed above, resins are cured by UV

exposure.

•The polymerization reactions are generated by absorption of photons.

•The most common absorption process used in photolithography is single photon

absorption.

•Concurrently, if the combined energy of two photons matches the transition energy

between the ground and excited state of the material, a non linear phenomena called two

photon absorption occurs.

•The rate of two photon absorption is a square function of the incident light intensity.

Polymerization with single photon absorption:

•Using a directed UV beam, 2-dimensional slides

can be formed on the focusing plane in a liquid

resin.

•Stacking many such slides allow shaping a threedimensional structure.

•Stereo lithography is used for rapid prototyping in

the macro scale.

•Based on the state of the surface of the liquid resin

there are 2 major stereo configurations:

Constrained surface and free surface.

T d i M h d



Transduction Methods

Electrochemical Transduction

• Fundamentals of electrochemistry.

• Potentiometry: The measurment of cell potential

at zero current.• Voltammetry (amperometry): Oxidizing (orreducing) potential is applied between the cellelectrodes and cell current is measured.

•Conductometry (Impedometry): Conductance(reciprocal of resistance) of the cell is measuredby an AC current bridge method.

Cells and electrodes



Cells and electrodes•When a piece of metal (such as Zinc) is inserted

in water or a solution containing ions (such as

Zinc ions) , there is a charge separation in the

boundary.•Small no. of zinc atoms leave the electrode and go as

ions into solution. The electrons are left in the

electrodes.

Example : Zn------ Zn+2 + 2e-

•As this process goes on the electrons within the zinc

electrode keep building and makes it increasingly

difficult for the +ve ions to go freely into solution and it

results in a stable charge bilayer or double layer.

• The degree of charge unbalance

produces an electric potential

between the two phases (solid

and liquid).

•Electrochemistry is all about the

no. of charges that cross thru the

interface between the two

phases.

++

++

+

+

--

--

-

-

S f b h l i l d bl l



Some facts about the electrical double layer•The interfacial potential may be of the order of a few

volts.

•The layer spans over a very small distance typically only

tens of nanometers.•In case of a metal electrode this layer extends only a

very small distance ( a few atomic diameters)

•The electric field across this monolayer of interface is

huge.

Lets say 1 V across 10-8 m would generate an electric

field 100 million V/m.

How can we let the oxidation of metal continue for a little longer?

•A simple way to keep the oxidation process going is to immerse a

piece of zinc in copper sulphate solution in place of pure water.

•The zinc surface immediately becomes coated with a brownishcoating of finely divided metallic copper. The reaction is a simple

oxidation-reduction process.

•However, in this case as well after the copper has totally coated the

surface the diffusion of cu ions across the interface slows down and

the process stops after an extended time.

Zn(s) → Zn2+ + 2e– Cu2+ + 2e– → Cu(s) Zn(s) + Cu2+ → Zn2+ + Cu(s)

How else can we keep the continuity of the



How else can we keep the continuity of the

oxidation process?•If we connect both the electrodes with an

external circuit then there will be an increased

concentration of Zn+2 in the left side and anincreased amount of NO3

-1 .

•Thus we need to some how contact the

solutions so that the excess ions formulated on

both sides can balance and the charge flow can

sustain for a longer time.

•Thus an excess of Zn2+ in the right compartmentcould be alleviated by the drift of these ions into

the left side, or equally well by diffusion of

nitrate ions to the left.

In the simplest cells, the barrier between the

two solutions can be a porous membrane, but

for precise measurements, a more complicated

arrangement, known as a salt bridge, is used.

The salt bridge consists of an intermediate

compartment filled with a concentrated

solution of KCl and fitted with porous barriersat each end.

Sign Conventions



Sign Conventions

Notation for electrochemical cells:

Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s)

Sign Conventions followed:

•The anode is where oxidation occurs, and

the cathode is the site of reduction.

•If electrons flow from the left electrode to

the right electrode (as depicted in the above

cell notation) when the cell operates in its

spontaneous direction.

•"Conventional current flow" is from positive

to negative, which is opposite to the

direction of the electron flow.

It is a Daniel cell if

M1= Zn, M2=Cu

S1= ZnSO4, S2= CuSO4

Other Types of Electrodes



Other Types of ElectrodesIon-Ion Electrodes:

•Some time electrochemical reactions involve only ionic species, such as Fe2+ and

Fe3+

•If neither of the electroactive species is a metal, some other metal must serve as a

conduit for the supply or removal of electrons from the system. Inert metal like

Platinum can be used.

Pt(s) | Fe3+(aq), Fe2+(aq) || ...

Gas Electrodes:

•Some electrode reactions involve a gaseous species such as H2, O2 and Cl2 done

over inert metals acting as electrodes.

Fe2+(aq) → Fe3+ (aq) + e–

Cl –(aq) → ½ Cl2(g) + e–

Insoluble salt Electrodes:•A typical electrode of this kind consists of a silver wire covered with a thin coating

of silver chloride, which is insoluble in water.

... || Cl – (aq) | AgCl (s) | Ag (s) AgCl(s) + e– → Ag(s) + Cl –(aq)

C ll P t ti l d El t ti S i



Cell Potential and Electromotive Series•Some metals are more "active" than others in the sense that a more active metal can

"displace" a less active one from a solution of its salt. For Example: the classical Zn, Cu

reaction.

•Here Zinc being more electroactive displaces copper ions from its salt.

Zn(s) + Cu

2+

→ Zn

2+

+ Cu(s)

•The scale of electroactivity of metals

can be determined based onwhether the Metal is able to displace

Hydrogen from water, acid or Steam.

The sequence is known as activity

series.

Half Cell Potential



Half Cell Potential•Each half-cell has associated with it an

electrode-solution potential difference whose

magnitude depends on the nature of the

particular electrode reaction .

In the cell Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s)

Electron is going from left to right. Therefore,

the EMF is expressed conventionally for +ve to –

ve current flow as:E cell = ΔV = E right – E left

Therefore, E cell = V Cu – V solution + V solution – V Zn

•So, half cell potentials can be measured relatively and absolute values cannot be

determined.

•However, it can be determined with reference to a reference electrode whose

electrochemical potential is defined as zero. The reference electrode in this application has

been universally accepted to be Hydrogen electrode over Pt.

Standard Hydrogen Electrode



Standard Hydrogen Electrode

•Hydrogen electrode, hydrogen gas is bubbled overmetallic platinum immersed in an aqueous solution

containing hydronium cations (H3O+).

• A continuous flow of molecular hydrogen is

maintained around the electrode at 1 atmosphere.

•The concentration of the H+ cations (in reality

H3O+) is 1 mol l-1 (pH = 0).

•The temperature is fixed at 25°C, in order to

respect standard conditions.

• Consequently, the potential of this electrode is

taken as a reference zero although the actual

potential in this configuration is 4.44 + 0.02 volts at

25 deg.C.

Wit Respect to Re erence e ectrode



Wit Respect to Re erence e ectrodeFor any Metal M, Pt | H2(g) | H+ || M2+ (aq) | M(s)

The redox reaction can be written as H2(g) + M2+(aq) → 2H+ + M(s)

E cell = V M –

V solution + V solution –

V Pt

V solution – V Pt = 0 (as defined)

Therefore, E cell = V M – V solution = E 0cell

oxidant

(electron acceptor)

reductant

(elecron donor)E°, volts

Na+ Na(s) –2.71

Zn2+ Zn(s) –.76

Fe2+ Fe(s) –.44

Cd2+ Cd(s) –.40Pb2+ Pb(s) –.126

2 H+ H2(g) 0.000

Hg2Cl2(s) 2Cl –(aq) + 2Hg() +.268

AgCl(s) Ag(s) + Cl –(aq) +.111

Cu2+ Cu(s) +.337

I2(s) 2 I – +.535Fe3+ Fe2+ +.771

Ag+ Ag(s) +.799

O2(g) + 4H+ 2 H2O() +1.23

Cl2(g) 2 Cl – +1.36

Some standard reduction potentials

Find the standard potential of the cell

Zn(s) | Zn2+ || Cu+2| Cu(s)

and predict the direction of electron flow when

the two electrodes are connected.

Solution: The net reaction corresponding

to this cell will be

Cu+2 + Zn→ Cu+ Zn+2

corresponding half-cell potential:

E cell = (Eright -Eleft) v = (Ecu-Eh) – (Ezn-Eh) =

.34-(-.76) = 1.10V



What is the cell reaction of the

galvanic cell?

Pt/ Fe+3 , Fe+2 || Cu2+ | Cu

Fe+3 + e- ------- Fe+2

Cu2+ + 2e-------- Cu

2 Fe+3 + Cu------ Cu2+ + 2Fe+2

Another Important Problem?



Another Important Problem?

Find the standard potential of the cell

Cl – | Ag(s)| AgCl(s) || Cu2+ |Cu(s)

and predict the direction of electron flow when the two electrodes are connected.

Solution: The net reaction corresponding to this cell will be

Half cell reactions:

Cu+2 + 2e-- ------ Cu

2Ag (s) + 2Cl- (aq)------ 2Agcl (s) + 2e-

2 Ag(s) + 2 Cl –(aq) + Cu2+(aq) →2AgCl(s) + Cu(s)

Since this involves the reverse of the AgCl reduction, we must reverse the

corresponding half-cell potential:

E cell = (.337 -.222) v = .115 v

Since this potential is positive, the reaction will proceed to the right



Biomicro-electromechanical

Systems- Lecture 13Instructor: Shantanu Bhattacharya



Work Done By an Electrochemical Cell

• During operation of electrochemical cells, chemical energy istransformed into electrical energy.

• Electrical Energy = E’cell Ctrans

Where E’cell is the differential cell potential and Ctrans is the totalcharge transferred.

Ctrans is also expressed as the no. of moles of electronstransferred multiplied by Faraday constant (96,485 C/mol).The electronic charge being negative the Electrical energy isalso expressed as = -nFE’cell = Wmax (Maximum work done bythe system) = ∆G = (Free energy is the maximum energy thatcan be extracted from the system).

Also , ∆G= -RT ln(K) where K is the equilibrium constant. (thiscomes from the Vannt Hoff Equation)

Van 't Hoff equation (From Thermodynamics)



Van t Hoff equation (From Thermodynamics)

•Van’t Hoff observed for the first time that there is a linear

relationship between the natural log of the rate of any

reaction and the inverse of temperature.

•We know that by Le Chatlier’s principle the rate constant

of any forward reaction is proportional to the product of

the activity of the products raised to their stoichiometric

coefficients and inversely proportional to a similar factor

realized from the reactants.

For a general Chemical reaction

•In the solutions of high ionic strength the activity coefficient is

by and large constant and the activity of the product changes to

concentration





Linear fit between 1/T and ln K.

Slope was observed as ∆H/R and

Intercept was ∆S/R, where, ∆H and

∆S are the change in enthalpy and

entropy respectively due to the

occurrence of the reaction

From the laws of thermodynamics the

Gibs Free energy is given by





Nernst Equation Let us put the values of R, F and T



Nernst Equation∆G = nFE’cell = RTlnK

E’cell = RT lnKnF

Ecell-E0

cell = RT lnK

nF

= E0cell + RT lnK

nF

K comes from Le Chatlier’s principle.

In a reaction R ------- Ox+e-1

K=[aOx]/[aR]

Ecell = E0cell +RT ln [aOx] aOx and aR are

nF [aR] activities, i.e.,

same as conc.

for diluted solution

p ,

assuming that the whole reaction

takes place at 25 deg. C room

temperature.

R = 8.314 J/K.molF= 96480 C/mol

T= 298 K

Therefore,

Ecell = E0cell + .06 log[Ox]

n [R]

N t E ti f h lf ll



Nernst Equation for a half cell

combined to a Reference electrode

++

++

++

--

--

-

-

Reference

Electrode

Liquid Junction

Metal Electrode

M

Mn+ Solution



What is Ionic activity?

• Activity of an ion comes as a result of interactions betweenions both electrostatic and covalent. The activity of an ion isinfluenced by its surroundings. The activity of an ion in acage of water molecules is different from that in the middleof a counter-ion cloud.

• In most electrochemistry the activity of the surroundingnon interfering ions is kept very high so that the target ionsare able to get detected in trace concentrations and also donot easily get affected by their own interaction.

Yi = ionic activity= Activity coefficient [ Concentration]



Ion Selective Electrodes



Ion Selective Electrodes•Ion-selective electrode (ISE) is a transducer which converts the activity of a specific

ion dissolved in a solution into an electrical potential.

•It can specifically select one particular ion over a range of different ions. pH

electrodes are an example of ion selective electrodes.•The basic ISE setup includes a meter

(capable of reading millivolts), a

probe (selective for each analyte of

interest), and various consumables

used for pH or ionic strength

adjustments.•It is based on the measurement of

the potential generated across a

membrane.

•The membrane is usually attached to

the end of a tube that contains an

internal reference electrode.

•This membrane electrode and an external reference electrode are then immersed in

the solution of interest.



Ion Exchange Membrane for ISE

• Glass membranes:

They are made from an ion exchange type of glass (silicate of Chalcogenide).Chalcogenides are group VI elements (Sulphur, Selenium or Tellurium).These are convalently bonded materials and so can think of the entireglass matrix as an infinitely bonded molecule. The glass shows high

selectivity for single charged cations like H+

, Na+

, and Ag+

or some doublecharged metal ions such as Pb2+, and Cd2+.

• Crystalline membranes:

These contain the mono or poly crystallites of a single substance. Only thoseions which can introduce themselves within the crystal. Selectivity ofcrystalline membranes can be for both cation and anion of the membrane-forming substance. An example is the fluoride selective electrode basedon LaF3 crystals.

Some more Ion Selective membranes



Some more Ion Selective membranesIon Exchange Resin membranes:

Ion exchange resins are based on special organic polymer

membranes which contains a specific ion exchange resin. A

resin is a natural extract . For example the resin from trees.

One example could be Valinomycin (organic extract)

(obtained from the several cells of streptomyces). These

molecules are present within cell membranes and are highly

selective to Potassium over sodium ions. Valinomycin

Enzyme Electrodes:

They are not true ion selective electrodes. In such electrodes an enzyme reacts

with a particular substrate and produces another product which can bedetected by a true ion selective electrode. For example the enzyme Glucose

Oxidase oxygenates glucose and breaks it into gluconic acid and hydrogen

peroxide. The hydrogen peroxide is further oxidized by an electrode potential

and generates hydrogen ions which is measured with a pH electrode.

Biomicroelectromechanical Systems- Lecture 10 -13

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