Download - Microelectronics Technology Mustafa Arikan University of Iceland.

Microelectronics Technology

Microelectronics Technology

Mustafa Arikan

University of Iceland

Contact info

Mustafa Arıkan (Musti)[email protected] ;

[email protected] : 525-4751 (Ingvarsson Lab., VR-III)Office hours ???

In this course…

Two parts:Semiconductor processing (from raw

material to microelectronic components)Semicondcutor characterization methods

(physical & electrical-optical)Lectures & Labs

Two lectures on 07.02.2008 and 07.03.2008Two labs in two groups on 14.02, 21.02 and

14.03, 21.03.2008

Goal of this lectures…

Overview of the fundamentals of microelectronics technology Fast & quick The tools we employ to produce and characterize

electronic components Complexity and beauty of the technology

Desired outcome Understanding of whole process Big picture Different approaches

What is microelectronics? What is it about?

Microelectronics is a subfield of electronicsstudy and manufacture of electronic

components which are very small (i.e. transistors, diodes…)

Semiconductors , metals, organic & plastic

Real small…and impressive…

But very complex sometime…

What takes to achieve it?

Different approaches

The basics of semiconductor device fabrication

Proper material for the purposeGeometry Material growth and removal (over and

over again) by the help of lithography

Simple example : MESFET

Metal-Semiconductor Field Effect Transistor

MESFET fabrication & The idea of lithography

A real device from substrate to final form MESFET is relatively simple but not all

the devices can be fabricated this easilyInverter fabrication

CMOS Inverter

Fabrication of a cmos inverter : Silicon technology

Includes many stepsMany different tools & technologies

Crystal (substrate) growthOxidationDiffusion & implantationMaterial growth (metal evaporation,

sputtering, vapor deposition, epitaxy)Lithography & etching

We need a substrate !

How do we get single crystalline Si?Czochralski

Majority of the wafersFloating zone (high purity)

High purity – low oxygen & carbon impurityMore complex w.r.t. Czochralski

BridgmanEasy (melting & cooling)Low quality

Drip melting, strain annealing and others

Czochralski growth

Ingot by Czochralski method

Czochralski growth

Typically used for Silicon but also Single crystal semiconductors (Si, Ge, GaAs) Metals (Pd, Pt, Ag, Au) Salts etc…

Requires seed crystal Fast (1-2 mm/min) Oxygen contamination from crucible Uniformity of axial resistivity is poor Segregation problems for dopants

We have Si substrate… Next…

Let’s focus on individual steps and technologies from now on

Oxidation

CVD – LPCVD (chemical vapor deposition (film growth) Thermally grown oxide (Oxidation) Photoresist (Lithography & etching)

Oxidation

One of the two main advantages of Si Ge is superior to Si (mobility, power

consumption) SiGe (MOSFET channel), Gd2O3

Dry oxidation : Si + O2 SiO2

Wet oxidation : Si + 2H2O SiO2 + 2H2

oxygen must diffuse through the oxide to react at the Si/SiO2 interface, so rate depends on the thickness of the oxide and reduces as the oxidation progresses.

Oxidation

thermal oxidation is performed in furnaces at temperatures between 800 and 1200°C

Many wafers on the boat (a quartz rack) at the same time Variants : RTO

Oxidation : dry vs. wet

Dry (molecular oxygen) : better oxide but slow (gate oxide)

Wet (steam – water vapor) : fast but porous (isolation) Deal-Grove model : thickness vs. time - theory

Oxidation

Thickness vs. time – practice : Charts !

Oxidation

Lithography & Pattern Transfer

Used for pattern transfer into metals, oxides and semiconductors Thin film deposition and lithography (including photo and e-beam,

wet etching and lift-off) are the most frequently used method in our labs

2 types of resists: Positive : PR pattern is same as mask. On exposure to light,

light degrades the polymers resulting in the photoresist being more soluble in developers. The PR can be removed in inexpensive solvents such as acetone.

Negative : PR pattern is the inverse of the mask. On exposure to light, light polymerizes the rubbers in the photoresist to strengthen it’s resistance to dissolution in the developer


Black areas (PR) are the openings after development of PR


How do we perform this “lithography” thing? Dehydration bake or pre-bake Adhesion promoter (i.e. HMDS) Apply resist – spinner Soft bake UV-exposure with mask Post-bake Post processing such as development & etching &

lift-off Other processes required by specific needs

(MEMS)


Baking

spinner


Expose

Develop

Lithography & Pattern Transfer : Uses of lithography

Etching Processes: open windows in oxides for diffusion, masks for ion implantation, etching, metal contact to the semiconductor, or interconnect.


Lift off Processes: Metalization


Issues with photolithographyResolution : feature size (~0.5 micron

usually) Shorter wavelength = better resolutionRegistration : alignment of different layers

on the same wafer (~ 1/3 of the resolution or 0.06 micron)

Throughput : effective cost and timeResist thickness ~ 1/spin speed


Photolithography systems


Contact Resist is in contact with the mask: 1:1 magnification Inepensive, relatively high resolution (~ 0.5 micron), contact

with the mask (scratches, particles and dirt are imaged in the wafer)

Proximity Resist is almost but not in contact with the mask: 1:1 magnification Inexpensive, low resolution (~ 1-2micron), diffraction effects

limit accuracy of pattern transfer. Less repeatable than contact methods,

Projection Mask image is projected a distance from the mask and de-magnified to a smaller image: 1:4 -1:10magnification Can be very high resolution (~0.07 um or slightly better), No

mask contact results in almost no mask wear (high production compatible), mask defects or particles on mask are reduced in size on the wafer. Extremely expensive and complicated equipment, Diffraction effects limit accuracy of pattern transfer

Lithography & Pattern Transfer : Light sources

Typically mercury (Hg)- Xenon (Xe) vapor bulbs are used as a light source in visible (>420 nm) and ultraviolet (>250-300 nm and <420 nm) lithography equipment

Lasers are used to increase resolution, and decrease the optical complexity for deep ultraviolet (DUV) lithography systems. Excited dimer (Excimer or Exiplex) pulsed lasers are typically used. These are powerful, extremely expensive to purchase and maintain, optically noisy lasers.

Alternative approaches such as: Nano-imprint, soft, dip-pen, e-beam, FIB, x-ray lithography : Very active research field!

Lithography & Pattern Transfer : some examples

Pictures for good and bad lithography

Oxidation

Chemical vapor deposition CVD – LPCVD (film growth) Thermally grown oxide (Oxidation) Photoresist (Lithography & etching)

Diffusion & Implantation

Dopants for N+ and P+ regions (implantation & diffusion)


What is diffusion? Diffusion is the spontaneous net movement of particles from

an area of high concentration to an area of low concentration (particle penetration from surface into the wafer)

Commonly used for Bipolar technology (base, emitters) FET (source, drain)

Use when Ion implantation damage is not acceptable Deep junctions are needed Cheap & easy solutions are seeked

Don’t use for Ultra-shallow junctions Forming channel in MOSFET

Diffusion & Implantation : Types of diffusion

Instertital Vacancy Interstitialcy Kick-out Dissociative


Diffusion equation (derived from Fick’s Law):

Different solution for different approximations Best solution for an experimentalist: Charts

(again!)


Diffusion depends on: Diffusion time Diffusion constant (diffusivity) Material density Temperature


Ion implantation : Ions (charged atoms or molecules) are created via

an enormous electric field stripping away an electron.

These ions are filtered and accelerated toward a target wafer, where they are buried in the wafer.

The depth of the implantation depends on the acceleration energy (voltage).

The dose is very carefully controlled by integrating the measured ion current.


Advantages Very precise control of the dose and position Independent control of impurity depth and dose Very fast (just few seconds) Complex profiles can be achieved by multiple & sequential

implantations Disadvantages

Very deep and very shallow profiles are difficult Not all the damage can be corrected by annealing. Typically has higher impurity content than diffusion. Often uses extremely toxic gas sources such as arsine

(AsH3), and phosphine (PH3). expensive

Fabrication of a CMOS Inverter

Fabrication of a CMOS Inverter

Poly-Si deposition (LPCVD)Let’s look at film deposition

Film deposition & growth

Physical deposition Thermal evaporation E-beam evaporation Sputtering

Chemical vapor deposition CVD LPCVD PECVD

Epitaxial growth MBE MOCVD CBE

Thermal & E-beam Evaporation

The source material is evaporated in a vacuum. The vapors other than the source material are almost entirely removed before the process begins.

The vacuum allows vapor particles to travel directly to the target object (substrate), where they condense back to a solid state.

Advantages High purity (good for Schottky contacts), simple, easy & cheap, fast,

low vacuum (10-4) Disadvantages

Poor alloy formation, step coverage problems, low throughput (low vacuum), relatively non-uniform deposition, non-smooth surfaces, short mean free path (~60m), high temperatures.

Two basic forms: Thermally assisted E-beam (electron beam by thermionic, field emission or arc)

Thermal & E-beam Evaporation

Thermal

E-beam

Sputtering

A "target" made of the material to be deposited is bombarded by energetic ions which will dislodge atomes of the target, i.e., "sputter them off".

The dislodged atoms will have substantial kinetic energies, and some will fly to the substrate to be coated and stick there.

Sputtering

Advantages The target atoms hit the substrate with an energy large enough so they "get

stuck", but not so large as to liberate substrate atoms. Sputtered layers therefore usually stick well to the substrate (in contrast to other techniques, most notably evaporation

All atoms of the target will become deposited, in pretty much the same composition as in the target. It is thus possible, e.g., to deposit a silicide slightly off the stoichiometric composition

The target atoms hit the substrate coming from all directions. Homogeneous coverage of the substrate is relatively easy to achieve- just

make the substrate holder and the target big enough. The process is also relatively easily scaled to larger size substrates.

Disadvantages Sputtered layers usually have a very bad crystallinity - very small grains full of

defects or even amorphous layers result. Usually some kind of annealing of the layers is necessary to restore acceptable crystal quality.

Sputtering works well for metals or other somewhat conducting materials. It is not easy or simply impossible for insulators. Sputtering SiO2 layers, e.g., has been tried often, but never made it to production (Zn-oxide, tin-oxide etc. are easily achieved however)

Chemical Vapor Deposition

the substrate is placed inside a reactor to which a number of gases are supplied.

a chemical reaction takes place between the source gases.

The product of that reaction is a solid material with condenses on all surfaces inside the reactor.

Chemical Vapor Deposition

Various different types of CVD but mainly 4 categories Atmospheric Pressure (APCVD)

Advantages: High deposition rates, simple, high throughput Disadvantages: Poor uniformity, purity is less than LPCVD Thick oxides

Low Pressure (LPCVD, 0.2 – 20 Torr) Poly-silicon deposition, dielectric layer and doped dielectric deposition. Advantages: Excellent uniformity, purity Disadvantages: Lower (but reasonable) deposition rates than APCVD

Metal Organic (MOCVD) alternative for MBE Advantages.: Highly flexible (semiconductors, metals, dielectrics) Disadvantages: Highly toxic, very expensive source material,

environmental disposal costs are high. Plasma Enhanced (PECVD)

dielectric coating such as silicon nitride Advantages.: Uses low temperatures necessary for rear end processing. Disadvantages: Plasma damage typically results

Epitaxy

We can grow* crystalline semiconductors by raising the temperature to allow more surface migration and by using a crystalline substrate (Si, GaAs, InP wafer, etc…)

Growth, not deposition ! The lattice constant of the epitaxially grown layer needs to

be close to the lattice constant of the substrate wafer. Otherwise the bonds can not stretch far enough and dislocations will result.

Advantages : Very high quality, extremely clean samples,crystallinity, very long mean free path (few hundred meters), precise atomic layer deposition

Disatvantages : UHV system, low deposition rate, very expensive equipment, not suitable for mass production

Different versions: LPE, VPE, MBE,CBE etc…

Expitaxy

Vacuum

A vacuum is a volume of space that is essentially empty of matter such that its gaseous pressure is much less than standard atmospheric pressure.

A perfect vacuum with a gaseous pressure of absolute zero is a philosophical concept that is never observed in practice

quantum theory predicts that no volume of space can be perfectly empty in this way.

The quality of a vacuum is measured in relation to how closely it approaches a perfect vacuum. The residual gas pressure is the primary indicator of quality, and is most commonly measured in units called torr

The average distance between collisions (mean free path)

Vacuum

Vacuum quality is subdivided into ranges according to the technology required to achieve it or measure it. These ranges do not have universally agreed definitions (hence the gaps below), but a typical distribution is as follows:

Atmospheric 760 TorrLow vacuum 760 to 25 TorrMedium vacuum 25 to 1×10-3 TorrHigh vacuum 1×10-3 to 1×10-9 TorrUltra high vacuum 1×10-9 to 1×10-12 TorrExtremely high vacuum <1×10-12 TorrOuter Space 1×10-6 to <3×10-17 TorrPerfect vacuum 0 Torr

Vacuum pumps

Rough & medium vacuum Piston pumps (particle problems) Rotary vane pumps (cheap) Dry pumps

High vacuum & UHV Diffusion (oil contamination) Turbo Cryo Ion (low pumping speed & capacity)

Transfer pumps

Rotary pump (mechanical)

Vacuum pumps

Turbomolecular pumps

Fabrication of a CMOS inverter

Inverter – After few steps