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1 Silicon Wafer Manufacture Packaging Epitaxial Growth Oxidation Photo- lithography Etching Diffusion (Ion Implantation) Metallization Fabricati on Processes for VLSI Devices Chip Fabrication Processes 2 Silicon Wafer Preparation 3 Silicon Wafer Preparation 4 Czochralski (CZ) Growth Method CZ is more co mmon method to grow silicon crystal today because it is capable of producing large diameter crystals, from which large diameter wafer can be cut. Lecture # 4
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Lecture 4 Crystal Growth

Apr 09, 2018

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Page 1: Lecture 4 Crystal Growth

8/8/2019 Lecture 4 Crystal Growth

http://slidepdf.com/reader/full/lecture-4-crystal-growth 1/7

1

Silicon Wafer

Manufacture

Packaging

Epitaxial

GrowthOxidation

Photo-

lithography

Etching

Diffusion (IonImplantation)

Metallization

Fabrication Processes for VLSI Devices

Chip Fabrication Processes

2

Silicon Wafer Preparation

3

Silicon Wafer Preparation

4

Czochralski (CZ) Growth Method

• CZ is more commonmethod to grow

silicon crystal todaybecause it is capableof producing largediameter crystals,from which largediameter wafer canbe cut.

Lecture # 4

Page 2: Lecture 4 Crystal Growth

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5

Modern CZ Crystal Growth

• The raw Si used for crystalgrowth is purified from SiO2

(sand) through refining, fractionaldistillation and CVD.

• The raw material contains < 1 ppbimpurities except for O (» 1018

cm-3) and C (» 1016 cm-3)

• Essentially all Si wafers used forICs today come from Czochralskigrown crystals. Polysilicon materialis melted, held at close to 1415°C, and a single crystal seed isused to start the crystal growth.

• Pull rate, melt temperature androtation rate are all importantcontrol parameters.

6

CZ crystal growth (cont.)

• Sequence of

photographs anddrawings illustratingCZ crystal growth.The charge ismelted,

• the seed isinserted, the neckregion is grown at ahigh rate to removedislocations andfinally the growth is

• slowed down to

produce a uniformcrystal.

7

300-mm (12 in.) and 400 mm (16 in.) Czochralski-grownsilicon ingots. (Photo courtesy of Sin-Etsu Handotai Co.,

Tokyo.)8

Wafer Slicing

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9

Drawback of the CZ method

• The only significant drawbackto the CZ method is that thesilicon is contained in liquidform in a crucible duringgrowth and as a result,impurities from the crucibleare incorporated.

• in the growing crystal. Oxygenand carbon are the two mostsignificant contaminants.

• These impurities are notalways a drawback, however.

Oxygen in particular can bevery useful in mechanicallystrengthening the siliconcrystal and in providing ameans for gettering otherunwanted impurities duringdevice fabrication.

10

Modeling Crystal Growth: relationship between pull

rate and crystal diameter.

• Freezing occurs between

isotherms X1 and X2.

• Heat balance: latent heat of

crystallization + heat conducted

from melt to crystal = heat

conducted away.

11

A= Cross-sectional area

12

2πrdx=radiation surface

area of an incremental

Length. σ=Boltzman constant

KS=thermal conductvity of the soild

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V pmax = maximum crystal pull rate is inversely proportional to the square root of the

crystal radius. 14

15

Modeling Dopant Behavior During CZ Crystal

Growth

• •Dopants are addedto the melt to

provide a controlled• n or p doping level

in the wafers.

• However, the dopantincorporationprocess iscomplicated by

dopant segregation.

Ko is the segregation coefficient.CS

and CL are the impurity concentration

just on the either side of the solid/

liquid interface.

16

VO=initial volume

IO=number of impurities

CL=impurity concentration

n the melt.

Lecture # 4

All values are below 1,which means that duringGrowth the dopants areRejected into the melt.

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17f= Vs/Vo, a fraction of melt that has solidified 18

Doping concentration versus position along the grown CZ

crystal for common dopants in silicon.

Consider three cases: If K<1If K>1if K~1

19

Curves for growth from the melt showing the doping

concentration in a solid as a function of the fraction

solidified.20

Float Zone Growth Technique

• CZ wafers are contaminated by O2 and Cfrom the crucible or graphite heaters.

• This limits the resistivity to ~ 20 Ωcm,while intrinsic Si is 230 kΩcm.

• Extremely high purity Si wafers aremade using float zone growth.

• FZ does not use a crucible or carbonheaters.

• * More expensive.

• * Carrier concentrations down to 10 11

atoms/cm 3 have been achieved.• * High purity needed for power

thyristors and rectifiers.

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21

Float-zone process. (a) Schematic setup. (b) Simple model for

doping evaluation. 22

Float Zone Growth Technique

• A seed crystal is brought into contact withthe top of the rod.

• In an inert atmosphere, an RF coil is slowlypassed along the length of the rod, startingat seed contact.

• The field set up by the coil induces eddycurrents in the rod, leading to jouleheating, and so melts the rod in the vicinityof the coil.

• The "floating" melt zone is about 2 cmwide..

• The seed crystal touches the melt zone andis pulled away, along with a solidifying Siboule following the seed.

• The seed crystal determines the crystalorientation of the boule.

• Limited to about a 4" wafer, as the meltzone will collapse.

• It is only held together by surface tension(and RF levitation).

23

FZ Growth Considerations

• Segregation and evaporation of impurities in the melt zone helppurify the Si further.

• * Recall, if k < 1, then more dopants/impurities in the liquid thanin the solid.

• * Thus, the impurities generally stay in the melt zone, and don'tsolidify in the boule.

• * You can "purify" FZ wafers further by successively passing thecoil along the boule. The impurities then segregate towards theend of the boule.

• Thermal instability in the melt zone can cause microvariations incomposition and doping.

• Difficult in making a uniform dopant concentration.24

Doping in FZ Growth

• Gas Doping:• Dopants are introduced in gaseous form during FZ growth.• * PH 3 (Phosphine), AsCl3--> n-dopant

• * B 2H6 (Diborane), BCl3 -> p-dopant• * Good uniformity along the length of the boule.

• Pill Doping:• Drill a small hole in the top of the EGS rod, and insert the dopant.

• * If the dopant has a small segregation coefficient, most of it will becarried with the melt as it passes the length of the boule.

• * Resulting in only a small non-uniformity.

• * Ga and In doping work well this way.

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25

Wafer Lapping and Etching

26

Wafer Polishing and Cleaning