INTEGRATED CIRCUITSDr. Esam Yosry

Lec. #2

Ch

ip F

ab

rica

tion Silicon Ingots

Wafers

Chip Fabrication Steps (FEOL, BEOL)

Processing Categories

Processes

Ch

ip F

ab

rica

tion

S

tep

s Once the wafers are prepared, many

process steps are necessary to produce the desired semiconductor integrated circuit.

In general, the steps can be grouped into two major parts: Front-end-of-line (FEOL)

processingcovers everything (but not including

the deposition of metal interconnect layers)

Back-end-of-line (BEOL) processingindividual devices (transistors,

capacitors, resistors, etc.) get interconnected with wiring on the wafer

Processing

In semiconductor device fabrication, the various processing steps fall into four general categories:

Deposition Removal Patterning Modification of electrical properties

Processing (1/4)

Deposition is any process that grows, coats, or otherwise transfers a material onto the wafer.

Available technologies include physical vapor deposition (PVD)

chemical vapor deposition (CVD) electrochemical deposition (ECD)

molecular beam epitaxy (MBE)

atomic layer deposition (ALD)

Processing (2/4)

Removal is any process that removes material from the wafer

examples include etch processes (either wet or dry) and chemical-mechanical planarization (CMP).

Processing (3/4)

Patterning is the shaping or altering of deposited materials, and is generally referred to as lithography

For example, in conventional lithography, the wafer is coated with a chemical called a photoresist; then, a machine called a stepper focuses, aligns, and moves a mask, exposing select portions of the wafer below to short wavelength light; the exposed regions are washed away by a developer solution. After etching or other processing, the remaining photoresist is removed by plasma ashing.

Processing (4/4)

Modification of electrical properties has historically entailed doping transistor sources and drains (originally by diffusion furnaces and later by ion implantation). These doping processes are followed by furnace annealing or, in advanced devices, by rapid thermal annealing (RTA); annealing serves to activate the implanted dopants.

Modern chips have up to eleven metal levels produced in over 300 sequenced processing steps.

Ch

ip F

ab

rica

tion

C

ycl

e

Pro

cess

es

Oxidation

Diffusion

Ion Implantation

Deposition

Etching

Lithography

All theseprocesses areneeded to buildup different layersin a siliconstructure

Oxidation of Silicon (Sio2)

Oxidation proceeds by the diffusion of an oxidant (molecular H2O or O2) through the existing oxide to the Si/SiO2 interface.

The molecules react with Si to form SiO2.

Si (solid) + O2 (vapor) SiO2 (solid) Dry OxideSi (solid) + H2O (vapor) SiO2 (solid) + 2H2 Wet Oxide

Thermal Oxidation at 1000oC - 1300oC

Oxidation of Silicon (Sio2)

The oxidation reaction occurs at the Si/SiO2 interface. Therefore, as the oxide grows, silicon is consumed and the interface moves into the silicon.

SiO2

SiO2 Surface

Original Si Interface

Silicon Substrate

X0 0.5 X0

Oxidation Equation

)(020 tBAxx

B

Axx ii

2

x0 = oxide thickness B = Parabolic Rate Constant

xi = initial oxide thickness B/A = Linear Rate ConstantAs temperature increases

-A goes down

-B goes up

-B/A goes up

X0

tx AB

0

Btx 20

time

Deal & Grove model: describes the growth kinetics of oxide films with thicknesses >300Å.

Oxidation Rates (Wet)

Table 1. RATE CONSTANTS FOR WET OXIDATION OF SILICON

Oxidation A Parabolic Rate Constant Linear rate constant temperature B B/A (°C) (m) (m2/hr) (m/hr)

1200 0.05 0.720 14.40

1100 0.11 0.510 4.64

1000 0.226 0.287 1.27

920 0.50 0.203 0.406

Oxidation Rates (Dry)

Table 2. RATE CONSTANTS FOR DRY OXIDATION OF SILICON

Oxidation A Parabolic Rate Constant Linear rate constant temperature B B/A (°C) (m) (m2/hr) (m/hr)

1200 0.040 0.045 1.12

1100 0.090 0.027 0.301000 0.165 0.0117 0.071920 0.235 0.0049 0.0208800 0.370 0.0011 0.0030

We note that the values of both B and B/A for wet processes is much greater than for dry processes, leading to higher oxidation rates for the wet case.

Oxidation Example

)(020 tBAxx

B

Axx ii

2

Problem: A silicon wafer has 0.5 micron thick layer of SiO2. We now wish to grow additional oxide at 1100C in oxygen for 5 hrs. What is the final oxide thickness (x0)?

Answer: From Table 2, A = .09, B = .027. xi = 0.5 micron. First calculate .

This represents the equivalent time that we would have had to oxidize the wafer at 1100C to grow 0.5 micron. The final oxide thickness is calculated using the quadratic formula.

= (.25+.045)/.027 = 10.9 hours

= .027x(10.9 +5) = 0.429

x0 = 0.5 [-.09 + (.092 +4X.429)1/2] = 0.61 microns

Measurement of Oxide Thickness

The optical interference method is a simple technique to measure oxide thickness from <100Å to more than 1 m.

The method is based on the interference that occurs between light reflected from the air /SiO2 interface and the Si/SiO2 interface.

Silicon

SiO2

light

Xo

Oxidation Furnace

Oxidation is carried out in a 3-zone resistance-heated furnace (1000oC0.1oC).

The furnace consists of a quartz tube inside a ceramic sheath (heat diffuser).

The quartz tube is filled with oxidant gas in a laminar flow regime.

Inert gas is required for annealing.

Oxid

ati

on

Ch

art

s These are Xo (oxide thickness in microns)

vs. t(oxidation time) at different T (oxidation temperatures from say 700 to 1300oC). Charts for dry, wet, or steam oxidation exist.

The thickness and time are drawn on log scales from 0.01 to 10 on the thickness axis and from 0.1 hr to 100 hr on the time axis.

Knowing the oxidation ambient (dry,wet,..), the oxidation temperature T, and the oxidation time t, we can read Xo directly.

Oxidation Charts (Dry)

If the initial oxide thickness is zero, reading the value of Xo is a direct matter.

Oxidation Charts (Wet)

Oxidation Example By Charts

Problem: A silicon wafer has 0.5 micron thick layer of SiO2. We now wish to grow additional oxide at 1100C in oxygen for 5 hrs. What is the final oxide thickness (x0)?

Answer: For an initial oxide thickness exists

1- find the corresponding oxidation time (toi) at the oxidation temperature. 12 hrs

2- add the actual oxidation time (t). 12 + 5 = 17 hrs

3- read the oxide thickness at time (t+toi). 0.6 µm Remember to add times not

thickness'!.

Many thanks to Prof. Hany Fikry and Prof Wael Fikry for their useful materials that help me to prepare this presentation.

Thanks