ch. 4 basics of device fabrication reference: s. m. sze...

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Photonics Laboratory

Ch. 4 Basics of device fabrication

Reference: S. M. Sze, Semiconductor Devices, ISBN 0-471-87424-8

: We will understand the common techniques for growing single crystals. (a) The starting

materials (e.g., silicon dioxide for a silicon wafer) are chemically processed to form a high-purity

polycrystalline semiconductor from which single crystals are grown. (b) The single-crystal ingots

are shaped to define the diameter of the material and sawed into wafers. (c) These wafers are

etched and polished to provide smooth, specular surfaces on which devices will be made.

A technology closely related to crystal growth involves the growth of single-crystal

semiconductor layers upon a single-crystal semiconductor substrate. This is called epitaxy.

(1) Crystal growth from the melt

There are basically two techniques for crystal growth from the melt: the Czochralski technique

and the Bridgman technique. A substantial percentage (~90%) of the silicon crystals for the

semiconductor industry are prepare by the Czochralski technique; virtually all the silicon used for

fabricating integrated circuits is prepared by this technique. Most gallium arsenide, on the other

hand, is grown by the Bridgman technique. However, the Czochralski technique is becoming more

popular for the growth of large-diameter gallium arsenide.

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(2) Starting materials

The starting material for silicon is a relatively pure form of sand (SiO2) called quartzite. This is

placed in a furnace with various forms of carbon (coal, coke, and wood chips). This process

produces metallurgical-grade silicon with a purity of about 98%.

).()()()()( 2 gasCOgasSiOsolidSisolidSiOsolidSiC

Next, the silicon is pulverized and treated with hydrogen chloride (HCl) to form

trichlorosilane (SiHCl3):

).()()(3)( 23

300

gasHgasSiHClsolidHClsolidSiCo

The trichlorosilane is a liquid at room temperature (boiling point 32oC). Fractional

distillation of the liquid removes the unwanted impurities. The purified SiHCl3 is then

used in a hydrogen reduction reaction to prepare the electronic-grade silicon (EGS):

).(3)()()( 23 gasHClsolidSigasHgasSiHCl

This reaction takes place in a reactor containing a resistance-heated silicon rod, which

serves as the nucleation point for the deposition of silicon. The EGS, a polycrystalline

material of high purity, is the raw material used to prepare device quality, single-crystal

silicon. Pure EGS generally has impurity concentrations in the parts-per-billion

range.

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(3) The Czochralski technique

The Czochralski technique for silicon crystal growth uses an apparatus called a puller. The puller

has three main components: (1) a furnace, which includes a fused-dilica (SiO2) crucible, a

graphite susceptor, a rotation mechanism, a heating element, and a power supply; (2) a crystal-

pulling mechanism, which includes a seed holder and a rotation mechanism; and (3) an ambient

control, which includes a gas source, a flow control, and a exhaust system.

In the crystal-growing process, polycrystalline silicon is placed in the crucible and the furnace is

heated above the melting temperature of silicon. A suitably oriented seed crystal (e.e., <111>) is

suspended over the crucible in a seed holder. The seed is inserted into the melt. Part of it melts,

but the tip of the remaining seed crystal still touches the liquid surface. It is then slowly

withdrawn. Progressive freezing at the solid-liquid interface yields a large single crystal. A typical

pull rate is a few millimeters per minute.

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(4) Unitary diagrams

These diagrams show the phase change in a single element as a function of temperature and

pressure.

The common point, referred to as a triple point, is invariant for the system and defines the

temperature and pressure at which solid, liquid, and gaseous phases are all in equilibrium with one

another.

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(5) Binary diagrams

These phase diagrams show the relationship between two components as a funciton of

temperature. The second variable, pressure, is usually set at 1 atm. In this way a relatively

complex three-dimensional representation is avoided.

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(6) The lever rule

At any temperature, the equilibrium composition of the two single phases that make up a two-

phase region may be determined as follows. Consider a melt of initial composition CM (the

percentage weight of B in the melt). Let this melt be cooled from some temperature T1 to a

tempeerature T2, corresponding to a point in the two-phase region:

WL=weight of liquid at this temperature

Ws=weight of solid (in the β phase, for this example)

CL, Cs=composition of the liquid and solid, respectively (percentage amount of B by weight)

Then WLCL and WsCs are the weights of B in the liquid and solid, respectively. But total weight of

B is (WL+Ws)CM. Hence by the conservation of matter

.rulelever theasknown is sregion.Thi phase- twothe

of boundaries the toordinate C thefrom measured lines two theoflength theare s and l where M

s

l

CC

CC

W

W

Ms

LM

L

s

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(7) The phase rule

The correct interpretation of phase diagrams is greatly helped by knowledge of the phase rule.

This rule, which is based on thermodynamic consideration, states that for any system in thermal

equilibrium, the sum of the number of phases P and the number of degrees of freedom F is related

to the number of components C by

P + F = C + 2.

Here the degrees of freedom are the number of variables that can be independently changed while

still preserving a specific phase.

Ex) For a single-component diagram of the type shown in Fig. 2.1

P + F = 3

(a) For water in its liquid phase (P=1), F=2; we have 2 degrees of freedom, i.e., both pressure and

temperature can be independently changed, and still maintain water in this liquid phase.

(b) Along 0B, however, P=2 (liquid and vapor) so that we have 1 degree of freedom (F=1). Now

either pressure or temperature (but not both) can be independently varied while this two-phase

coexistence is still preserved.

(c) At 0, P=3, and F=0. i.e., there is a unique temperature-pressure combination where water

coexists in all three phases.

For a binary phase diagram, F + P = C + 2 = 4.

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(8) Distribution of dopant

In crystal growth, a known amount of dopant is added to the melt to obtain the desired doping

concentration in the grown crystal. For silicon, boron and phosphorus are the most common

dopants for p- and n-type materials, respectively.

As a crystal is pulled from the melt, the doping concentration incorporated into the crystal (solid)

is usually different from the doping concentration of the melt (liquid) at the interface. The ratio of

these two concentrations is defined as the equilibrium segregation coefficient k0:

l

s

c

ck 0

where Cs and Cl are respectively the equilibrium concentrations of the dopant in the solid and

liquid near the interface.

Consider a crystal being grown from a melt having an initial weight M0 with an initial doping

concentration C0 in the melt (i.e., the weight of the dopant per 1 gram melt). A given point of

growth when a crystal of weight M has been grown, the amount of dopant remaining in the melt

(by weight) is S.

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1

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dMk

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dMCdS where Cs and Cl are respectively the equilibrium

concentrations of the dopant in the solid and

liquid near the interface.

Consider a crystal being grown from a melt

having an initial weight M0 with an initial

doping concentration C0 in the melt (i.e., the

weight of the dopant per 1 gram melt). A given

point of growth when a crystal of weight M has

been grown, the amount of dopant remaining in

the melt (by weight) is S.

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Problem 1) A silicon ingot, which should contain 1016 boron atoms/cm3, is to be grown by the

Czochralski technique. What concentration of boron atoms should be in the melt to give the

required concentration in the ingot? If the initial load of silicon in the crucible is 60 kg, how many

grams boron (atomic weight 10.8) should be added?

(9) Wafer shaping and material characterization

After a crystal is grown, (a) the first shaping operation is to remove the seed and the other end of

the ingot, which is last to solidify. (b) The next operation is to grind the surface so that the

diameter of the material is defined. (c) After that, one or more flat regions are ground along the

length of the ingot. These regions, or flats, mark the specific crystal orientation of the ingot and

the conductivity type of the material. The largest flat, the primary flat, allows a mechanical locator

in automatic processing equipment to position the wafer and to orient devices relative to the

crystal in a specific manner. Other smaller flats, called secondary flats, are ground to identify the

orientation and conductivity type of the crystal.

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1) Once these operations have been done, the ingot is ready to be sliced by diamond saw into

wafers. Slicing determines four wafer parameters: surface orientation, thickness, taper (i.e., wafer

thickness variations from one end to another), and bow (i.e., surface curvature of the wafer,

measured from the center of the wafer to its edge).

2) After slicing, both sides of the wafer are lapped using a mixture of Al2O3 and glycerine to

produce a typical flatness uniformity within 2μm. The lapping operation usually leaves the surface

and edges of the wafer damaged and contaminated. The damaged and contaminated regions can

be removed by chemical etching. The final step of wafer shaping is polishing. Its purpose is to

provide a smooth, specular surface where device features can be defined by lithographic

processes.

3) The oxygen and carbon concentrations are substantially higher in Czochralski crystals than in

float zone crystals due to the dissolution of the silica crucible (for oxygen) and transport to the

melt from the graphite susceptor (carbon) during crystal growth. Especially, the precipitates of

oxygen due to the solubility effect, can be used for gettering. Gettering is a general term meaning

a process that removes harmful impurities or defects from the region in a wafer where devices are

fabricated.

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Table 2 Table 3

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(10) Vapor-phase epitaxy

Epitaxial processes are differentiated from the melt growth processes in that the epitaxial layer

can be grown at a temperature substantially below the melting point (typically 30 to 50% lower).

Four silicon sources have been used for vapor phase epitaxial growth. They are silicon

tetrachloride (SiCl4), dichlorosiliane (SiH2Cl2), trichlorosilane (SiHCl3), and silane (SiH4).

Silicon tetrachloride has been the most studied and has the widest industrial use. The typical

reaction temperature is 1200oC.

)(2)()(

)(4)()(2)(

24

24

gasSiClsolidSigasSiCl

gasHClsolidSigasHgasSiCl

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The dopant is introduced at the same time as the silicon tetrachloride during epitaxial growth.

Gaseous diborane (B2H6) is used as the p-type dopant, while phosphine (PH3) and arsine

(AsH3) are used as n-type dopants. Gas mixtures are ordinarily used with hydrogen as the diluent

to allow reasonable control of flow rates for the desired doping concentration. The dopant

chemistry shows dopant being adsorbed on the surface, decomposing, and being incorporated into

the growing layer.

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(11) Molecular-beam epitaxy (MBE)

MBE is an epitaxial process involving the reaction of one or more thermal beams of atoms or

molecules with a crystalline surface under ultrahigh vacuum conditions(~10-10Torr). MBE can

achieve precise control in both chemical compositions and doping profiles. Single crystal

multilayer structures with dimensions of the order of atomic layers can be made using MBE.

.T asely approximat

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32).(Moxygen for s-cmmolecules/ 103.6 is ratet impingemen the

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Torr 10at hr 6

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Another important parameter is the mean free path. During their motion, the molecules suffer

collisions among themselves. The average distance traversed by all the molecules between successive

collisions with each other is defined as the mean free path. A molecule having a diameter d and a

velocity v will move a distance vδt in the time δt. The molecule suffers a collision with another

molecule if its center is anywhere within the distance d of the center of another molecule. Therefore, it

sweeps out (without collision) a cylinder of diameter 2d. The volume of the cylinder is

e. temeraturroomat )A3.7 ofdiameter molecular t (equivalen moleculesair for cm ) (

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Problem 2. Assume an effusion oven geometry of area A=5 cm2 and a distance L between the top of

the oven and the gallium arsenide substrate of 10 cm. Calculate the MBE growth rate for the effusion

oven filled with gallium arsenide at 900oC.

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12) Thermal oxidation

Semiconductors can be oxidized by various methods. These include thermal oxidation,

electrochemical anodization, and plasma reaction. Among these methods thermal oxidation is by far

the most important for silicon devices. It is the key process in modern silicon integrated-circuit

technology. The silicon-silicon dioxide interface moves into the silicon during the oxidation process.

).(2)()(2)(

)()(

:aporor water voxygen in silicon ofoxidation thermal thedescribe reactions chemical following The

222

22

gasHsolidSiOgasOHsolidSi

solidSiOgasOSi(solid)

Problem 3) If a silicon oxide layer of thickness x is grown from the thermal oxidation, what is the

thickness of silicon being consumed?

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13) Metallization

Metallization refers to the formation of metal films used for interconnections, ohmic contacts, and

rectifying metal-semiconductor contacts. Metal films can be formed by various methods, the most

important being physical vapor deposition and chemical vapor deposition.

We shall now consider the thickness of the film deposited from an evaporation source.

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.cos

stream, vapor the

ofdirection the to anglean at inclined is normal whosesurface aon dS2 area small aat arrives material theIf

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:law cosine by thegiven is unit timeper

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2

2

r

dSd

dm

dm

A

dm

dm

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source. plane for the ](L/H)[1

1

)(

and sourcepoint for the (L/H)][1

1

4)(4

surface, receiving plane-parallel a and source theoft arrangemen practical a devices, discretefor ,Especially

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(cm/s), is unit timeper deposited film theof thickness theand )(g/cm density a has material theAssuming

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ml

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mdm

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mdm

dd

dd

d

d

d

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Problem 4) A silicon wafer is placed at a perpendicular distance of 30cm from a plane source. If the

total deposited mass is 1 g and the density is 2.7 g/cm3, what is the film thickness at L=0? If the

variation in film thickness must be less than 10%, how large can the wafer be?

Aluminum Metallization

Aluminum and its alloy are used extensively for metallization in integrated circuits. Because

aluminum and its alloys have low resistivities (i.e., 2.7μΩ-cm for Al and up to 3.5 μΩ-cm its alloys),

these metals satisfy the requirements of low resistance. However, the use of aluminum in integrated

circuits at shallow junctions often creates problems such as spiking and eletromigration. Hence, during

annealing silicon will dissolve into the aluminum. The amount of silicon dissolved will depend not

only on the solubility at the annealing temperature but also on the volume of aluminum to be saturated

with silicon.

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.2

is consumed be uldsilicon wo which depth to theA, areacontact over theuniformly

place n takesconsumptio theIf re. temperatuannealing at the aluminumin silicon of solubility theis S where

,)(2

is consumedsilicon of volume thesilicon, and aluminum of densities theare and Assuming SilA

Si

Al

Si

Al

SA

HZDtb

SHZDtVol

Problem 5) For T=500oC, t=30min, ZL=16μm2, Z=5μm, and H=1μm. Find the depth b, assuming

uniform dissolution.

ch. 4 basics of device fabrication reference: s. m. sze...

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