Microelectronics Technology Mustafa Arikan University of Iceland Contact

Microelectronics Technology Mustafa Arikan University of Iceland

Contact info l Mustafa Arıkan (Musti) l arikan@raunvis. hi. is ; mustafa. arikan@gmail. com l Tel : 525 -4751 (Ingvarsson Lab. , VR-III) l Office hours ? ? ?

In this course… l Two parts: Semiconductor processing (from raw material to microelectronic components) l Semicondcutor characterization methods (physical & electrical-optical) l l Lectures & Labs Two lectures on 07. 02. 2008 and 07. 03. 2008 l Two labs in two groups on 14. 02, 21. 02 and 14. 03, 21. 03. 2008 l

Goal of this lectures… l Overview of the fundamentals of microelectronics technology l l Fast & quick The tools we employ to produce and characterize electronic components Complexity and beauty of the technology Desired outcome l l l Understanding of whole process Big picture Different approaches

What is microelectronics? What is it about? Microelectronics is a subfield of electronics l study and manufacture of electronic components which are very small (i. e. transistors, diodes…) l Semiconductors , metals, organic & plastic l

Real small…and impressive…

But very complex sometime…

What takes to achieve it?

What takes to achieve it?

Different approaches

The basics of semiconductor device fabrication l Proper material for the purpose l Geometry l Material growth and removal (over and over again) by the help of lithography

Simple example : MESFET l Metal-Semiconductor Field Effect Transistor

MESFET fabrication & The idea of lithography l A real device from substrate to final form l MESFET is relatively simple but not all the devices can be fabricated this easily l Inverter fabrication

CMOS Inverter

Fabrication of a cmos inverter : Silicon technology l Includes many steps l Many different tools & technologies Crystal (substrate) growth l Oxidation l Diffusion & implantation l Material growth (metal evaporation, sputtering, vapor deposition, epitaxy) l Lithography & etching l

We need a substrate ! l How do we get single crystalline Si? l Czochralski l l Majority of the wafers Floating zone (high purity) High purity – low oxygen & carbon impurity l More complex w. r. t. Czochralski l l Bridgman Easy (melting & cooling) l Low quality l l Drip melting, strain annealing and others

Czochralski growth

Ingot by Czochralski method

Czochralski growth l Typically used for Silicon but also l l l l Single crystal semiconductors (Si, Ge, Ga. As) 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) l Thermally grown oxide (Oxidation) l Photoresist (Lithography & etching) l

Oxidation l l l One of the two main advantages of Si Ge is superior to Si (mobility, power consumption) Si. Ge (MOSFET channel), Gd 2 O 3 Dry oxidation : Si + O 2 Si. O 2 Wet oxidation : Si + 2 H 2 O Si. O 2 + 2 H 2 oxygen must diffuse through the oxide to react at the Si/Si. O 2 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 l Many wafers on the boat (a quartz rack) at the same time l Variants : RTO l

Oxidation : dry vs. wet Dry (molecular oxygen) : better oxide but slow (gate oxide) l Wet (steam – water vapor) : fast but porous (isolation) l Deal-Grove model : thickness vs. time - theory l

Oxidation l Thickness vs. time – practice : Charts !

Oxidation

Lithography & Pattern Transfer Used for pattern transfer into metals, oxides and semiconductors l Thin film deposition and lithography (including photo and e-beam, wet etching and lift-off) are the most frequently used method in our labs l 2 types of resists: l 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. l 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 l

Lithography & Pattern Transfer l Black areas (PR) are the openings after development of PR

Lithography & Pattern Transfer l How do we perform this “lithography” thing? l l l l 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)

Lithography & Pattern Transfer l Baking l spinner

Lithography & Pattern Transfer l Expose l Develop

Lithography & Pattern Transfer : Uses of lithography l Etching Processes: open windows in oxides for diffusion, masks for ion implantation, etching, metal contact to the semiconductor, or interconnect.

Lithography & Pattern Transfer l Lift off Processes: Metalization

Lithography & Pattern Transfer l Issues with photolithography Resolution : feature size (~0. 5 micron usually) l Shorter wavelength = better resolution l Registration : alignment of different layers on the same wafer (~ 1/3 of the resolution or 0. 06 micron) l Throughput : effective cost and time l Resist thickness ~ 1/spin speed l

Lithography & Pattern Transfer l Photolithography systems

Lithography & Pattern Transfer Contact Resist is in contact with the mask: 1: 1 magnification l Inepensive, relatively high resolution (~ 0. 5 micron), contact with the mask (scratches, particles and dirt are imaged in the wafer) l Proximity Resist is almost but not in contact with the mask: 1: 1 magnification l Inexpensive, low resolution (~ 1 -2 micron), diffraction effects limit accuracy of pattern transfer. Less repeatable than contact methods, l Projection Mask image is projected a distance from the mask and de-magnified to a smaller image: 1: 4 -1: 10 magnification l 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 l

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

Lithography & Pattern Transfer : some examples l Pictures for good and bad lithography

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

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

Diffusion & Implantation

Diffusion & Implantation

Diffusion & Implantation

Diffusion & Implantation l What is diffusion? l l Commonly used for l l l Bipolar technology (base, emitters) FET (source, drain) Use when l l 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) Ion implantation damage is not acceptable Deep junctions are needed Cheap & easy solutions are seeked Don’t use for l l Ultra-shallow junctions Forming channel in MOSFET

Diffusion & Implantation : Types of diffusion l l l Instertital Vacancy Interstitialcy Kick-out Dissociative

Diffusion & Implantation l Diffusion equation (derived from Fick’s Law): Different solution for different approximations l Best solution for an experimentalist: Charts (again!) l

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

Diffusion & Implantation l Ion implantation : l l 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.

Diffusion & Implantation

Diffusion & Implantation l Advantages l l l 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 l l l 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 (As. H 3), and phosphine (PH 3). expensive

Diffusion & Implantation

Diffusion & Implantation

Fabrication of a CMOS Inverter

Fabrication of a CMOS Inverter

Fabrication of a CMOS Inverter l Poly-Si deposition (LPCVD) l Let’s look at film deposition

Film deposition & growth l Physical deposition l l Chemical vapor deposition l l Thermal evaporation E-beam evaporation Sputtering CVD LPCVD PECVD Epitaxial growth l l l 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. l The vacuum allows vapor particles to travel directly to the target object (substrate), where they condense back to a solid state. l Advantages l l l Disadvantages l l High purity (good for Schottky contacts), simple, easy & cheap, fast, low vacuum (10 -4) Poor alloy formation, step coverage problems, low throughput (low vacuum), relatively non-uniform deposition, non-smooth surfaces, short mean free path (~60 m), high temperatures. Two basic forms: l Thermally assisted l 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". l The dislodged atoms will have substantial kinetic energies, and some will fly to the substrate to be coated and stick there. l

Sputtering l Advantages l l l 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 l l 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 Si. O 2 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. l a chemical reaction takes place between the source gases. l The product of that reaction is a solid material with condenses on all surfaces inside the reactor. l

Chemical Vapor Deposition l Various different types of CVD but mainly 4 categories l l Atmospheric Pressure (APCVD) l Advantages: High deposition rates, simple, high throughput l Disadvantages: Poor uniformity, purity is less than LPCVD l Thick oxides Low Pressure (LPCVD, 0. 2 – 20 Torr) l Poly-silicon deposition, dielectric layer and doped dielectric deposition. l Advantages: Excellent uniformity, purity l Disadvantages: Lower (but reasonable) deposition rates than APCVD Metal Organic (MOCVD) alternative for MBE l Advantages. : Highly flexible (semiconductors, metals, dielectrics) l Disadvantages: Highly toxic, very expensive source material, environmental disposal costs are high. Plasma Enhanced (PECVD) l dielectric coating such as silicon nitride l Advantages. : Uses low temperatures necessary for rear end processing. l Disadvantages: Plasma damage typically results

Epitaxy l l l We can grow* crystalline semiconductors by raising the temperature to allow more surface migration and by using a crystalline substrate (Si, Ga. As, In. P 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 l l l 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 l 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 Torr Low vacuum 760 to 25 Torr Medium vacuum 25 to 1× 10 -3 Torr High vacuum 1× 10 -3 to 1× 10 -9 Torr Ultra high vacuum 1× 10 -9 to 1× 10 -12 Torr Extremely high vacuum <1× 10 -12 Torr Outer Space 1× 10 -6 to <3× 10 -17 Torr Perfect vacuum 0 Torr

Vacuum pumps l Rough & medium vacuum l l Piston pumps (particle problems) Rotary vane pumps (cheap) Dry pumps High vacuum & UHV l l Diffusion (oil contamination) Turbo Cryo Ion (low pumping speed & capacity)

Transfer pumps l Rotary pump (mechanical)

Vacuum pumps l Turbomolecular pumps

Fabrication of a CMOS inverter

Fabrication of a CMOS inverter

Fabrication of a CMOS inverter

Inverter – After few steps