Diffusion and Ion Imp Systems

7/29/2019 Diffusion and Ion Imp Systems

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102 Integrated Circuits

.. t IInrJ.-LiquidOz tc~ sourceN2 BBrJ (bJ~~1i~~~~~1~~~~~~

Silicon wafert t tBzH6 Oz Nz(Gaseoussource)~

~

(a) ~

~

Fig.4.15.Diffusionofborondopants: - Fig.4.16.Borondiffusion:cBSG(0) gaseous source; (b) liquid source. glass layer on silicon wafer.This process is the chemical vapour deposition (CVD) of a glassy layer on the silion surface which is

a mixture of silica glass (Si02) and boron glass.{BzO3) is caned borosilica glass (BSG). The BSG glassy layer,shown in Fig. 4.16, is a viscousliqllid at the diffusion temperatures and the boron atoms can move aroundrelatively easily. Furthennore, the boron concentration in the BSG is such that the silicon surface will beslturated with boron at the solid solUbility limit throughout the time of the diffusion process as long as BSGremains present. This is a constant source (erfc) diffusion. It is often called a deposition diffusion. This diffusion

. step is referred as predeposition sJep in which the dopant atoms deposit into the surface regions (say 0.3 IJlIldepth} of the silicon wafers. The BSG is preferable because it protects the silicon atoms from pitting orevaporating and acts as a "getter"for undesirable impurities in the silicon. It is etched off before next diffusionas discussed below.

The predeposition step, is followed by a second djffusion process in which the exterual dopant source(BSG) is removed such that no additional dopants enter the silicon. During this diffusion process the dopantsthat are alreadyin the silicon move further in and are thus redistributed. The junction depth increases, and atthe same time tl1e surface concentration decreases. This type of diffusion is called drive-in, or redistribution,or limited-source (Gaussian diffw;ion). The impurity profile for such type of diffusion is already disCl!Ssed.

The two-step diffusion combination of deposition diffusi'1 (predeposition step) followed by a drive-indiffusion is often used to produce the base region of transistors. "

)tqron Diffusion using BBr3 (B()ron Tribromide) Source. This is a liquid source of boron. In thiscase a controlled flow of carrier gas (N2fis bubbled through boron tribromide, as shown in Fig. 4.15 (b), whichwith oxygen again produces boron trioxide (BSG) at the surface of the wafers as per following reaction:

4BBr3+3b2-7 BP3+ 2Bi2 .Thereafterthereaction is as discussed above.

-' 4.4.3. Diffusion of n-Type ImpurityFor phosphorus diffusion such compOllnds as PH3 (phosphine) and POCl3 (phosphorus oxychloride)

can be use


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104 Integrated CircuitsThe four-point probe can be used to measure the sheet resistance of various types of diffused layers,

epitaxiallayerscand that of silicon wafers for the measurement of the resistivity.The sheet resistance values of diffused layers generally fall in the range from I-ohm/square upto about

1000 ohm/square. The transistor base diffused layer has a sheet resistance of typically about 200-0hm/square,and the n+ emitter diffused layer has down in the range of around 2-ohm/square.4.5. ION IMPLANTATION

Ion implantation is an alternative to a de osition di usion and is used to produce a shallow surfacere ion of opant atoms deposIte into a silicon wafer. This technology has ma e significant in roads mtodiffusion techno ogy in several areas.!! ISprocessa beam of impurity ions is accelerated to kinetic energiesin the range of several tens ofkV and is directed to the surtace 01 the SIlIcon. As the impurity atoms enter thecrystal, they give up their energy to the lattice in collisions and finally come to rest at some average penetrationdepth, called the projected range expressed in J.ll11.Depending on the impurity and its implantation energy,the range in.a given semiconductor may vary from a few hundred angstroms to about 1 J.ll11.ypical distributionof impurity about the projected range is approximately Gaussian. By performing several implantations at~ifferent energies, it is possible to synthesize a desired impurity distribution, for example an uniformly dopedregion.4.5.1. Ion Implantation SystemA typical ion implantation system is shown in Fig. 4.18.Mass separatorhaving electromagnet

Targetcham berFig. 4.18. Ion implantationsystem.

A gas containing the desired .impurity is ionized withill the ion source. The ions are generated andrepelled from their source in a diverging beam that is focussed before it passes through a mass seperator thatdirects only the ions of the desired species through a narrow aperature. A second lens focuses this resolvedbeanl which then passes through an accelerator that brings the ions to their required eilergy before they strikethe target and become implanted in the exposed areas of the silicon wafers. The accelerating voltages may befrom 20 kY. to as much as 250 kV. In some ion implanters, the mass separation occurs after the ions areaccelerated to high energy. Because the ion beam is small, means are provided for scanning it unifonnly acrossthe wafers. For this purpose the focussed ion beam is scanned electrostatically over the surface of the waferin the targeT chamber. .l~

Repetitive scanning in a raster pattern provides exceptionally unifOffil doping of the wafer surface. Thetarget chamber commonly includes automatic wafer handling facilities to speed up the process of implantingmany wafers per hour.4.5.2. Properties of Ion ImplantationThe depth of penetration of any particular type of ion will increase with increasing accelerating voltage.

The penetration depth will generally be in the range of 0.1 to 1.0 J.ll11.able 4.1 shows various projected ranges,Rp , for various typical accelerating voltages for boron and phosphorus ions in silicon.

lighterionsIon beamFirst electrical(ense Silicon.wafers


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Monolithic IC Processes 105Table 4.1. Projected Ranges(Rp) for Boron and Phosphorus Ions in Silicon

Impurity Distribution of Implanted Ions. The distribution of the implanted ions as a function of distancex from the silicon surface will be a Gaussian distribution, given by

.[

(x -Rp?](x);::;Npexp 2. 2Mp

distance into substrate from surface...(4.13)

here xRp ;::; projected range

Mp ;::; straggle (standarddeviation) of the projected rangeNp ;::; peak concentrationof implanted ions.

An ion implantation impurity profile is shown in Fig. 4.19. The peak impJanted ion concentration isrelated to the implantation dosageQ byN;::; Q 04(

Q){iitMp - . Mp ,- .. .(4.14)

,.~

t~

HX-Rp)21.-N(x)=Npexp l2t.R~ J

xz

0 JI ~-R -L':.R,Rp R- L':.Rp p p+ P x-Fig. 4.19. Ion implantation impurity profile.

The implanta]:iondosage Q is the number of implanted ions per unit of surface area as given by suchunits as ions/cm2.The ion density drops off rapidly from the peak'value with distance as measurea from Rpin either direction. Note that the Gaussian implanted ion profile will be tmncatedat x";::;O.

Annealing after Implantation. After the ions have been implanted they are lodged principally ininterstitial positions in the silicon crystal structure, and the surface region into which the implantation hastaken place will be heavily damaged by the impact of the high-energy ions. The disarray of silicon atoms inthesurface region isoften tothe extent thatthisregionisnolongercrystalline instmctu~but, rather, amorphous.To restore this surface region back to a well-orderedcrystalline state and to allow the implanted ions to gointo substitutional sites in the crystal structure,the wafer must be subjected to an annealing~process. Theannealing process usually involves the heating of the wafers to sonie elevated temperature, often in the rangeof 1000Cfor a suitable length of time such as 30minutes.

.

Energy Rp'ofboron Rp of phosphorus(kV) :7(lm) (lm)20 ... 0.067 0.026100 ... 0.30 0.123200 ... 0.52 '0.254300 ... 0.70 0.386

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Diffusion and Ion Imp Systems