6 Diffusion.pdf

ECE 559 - Dew 137Winter 2015

6 Diffusion of Dopants Diffusion is the systematic motion of particles in response to

concentration gradients. It is a thermally activated process D = D0 exp(-Ea/kT)

Since very high dopant concentration gradients exist in IC's, any high temperature step (oxidation, epitaxy, implant annealing) will cause diffusion. This must be considered carefully because correct operation of the transistors is dependent on the localization of dopants. This establishes a thermal budget for a process sequence.

Diffusion doping is a method used to introduce dopants into the semiconductor. (Ion implantation is an alternate method.)

Diffusion doping is done in tube furnaces like those used for oxidation (900-1100C).

Two stages are involved: predeposition (to introduce dopants) and drive-in (to more evenly distribute them over a larger volume).


6.1 Predeposition The semiconductor surface is exposed through a patterned

oxide mask to a compound containing the dopant which, at high temperature, is absorbed into the semiconductor to near the solid solubility limit.

This creates a shallow, non-uniform, high concentration doped region near the surface.

In general, predeposition proceeds in two steps. First, a heavily doped silicate glass (the glaze) is formed on top of the Si (local source deposition). The second step is a high temperature anneal (soak) to allow the dopants to diffuse deeper into the semiconductor.

After soaking, the glaze must be removed by an HF dip. Dopant sources to form the glaze can be solid, liquid or gases.


6.1 Predeposition Classically, dopant sources were gases: arsine (AsH3),

phosphine (PH3), diborane (B2H6). These are introduced into the tube furnace in flowing N2 and O2. The oxygen reacts with the gases to form dopant-oxides which then react at high temperature with Si to form silicate glasses. Adjusting the dopant gas:oxygen ratio controls the dopant concentration in the glaze.

Gas sources have good control qualities, but care must be taken to avoid nonuniformities due to turbulence and depletion. (For example, wafers can't be stacked vertically very closely together.)

All of the main gas sources are extremely toxic, even at ppm levels. They are also pyrophoric and explosive.


6.1 Predeposition Liquid sources such as pockle (POCl3) or boron tribromide

(BBr3) are much safer and easier to handle, but are still flammable and release toxic Br2 or Cl2 gas in air.

POCl3 and BBr3 have relatively low boiling points (105C and 90C), so their vapour pressure is straightforward to control. Nitrogen bubbles through the heated liquid, picking up the dopant source vapour which is then mixed with flowing oxygen and oxidized before deposition onto the heated wafer. The dopant concentration in the resulting glaze is controlled by the liquid temperature and the oxygen flow rate.

Georgia Inst. Of Technology ECE 4752


6.1 Predeposition Even safer than liquid sources are the solid sources which only

evolve dopant gases at high temperature. In addition, the only gases generated tend to be non-toxic (except for As2O3).

Usually solid sources are disks (pucks) interspersed among the wafers (2 wafers for each puck), so doping uniformity is very good, even with closely packed wafers. Temperature controls the dopant vapour pressure and dosing rate on the wafer.

The phosphorous source is silicon pyrophosphate (SiP2O7) in an inert ceramic matrix. This compound decomposes above 900C to release P2O5 vapour.

www.nanolab.ucla.edu


6.1 Predeposition B2O3 vapour is generated from a BN source (800-1100C) which

has been activated before use by an oxygen anneal (900 -975C) to form a surface B2O3 layer. (A variation is to add hydrogen and oxygen during local source deposition. The resulting H2O reacts with B2O3 to produce HBO2 (metaboric acid) which is much more volatile.)

Arsenic doping is done using aluminum arsenate (AlAsO4) which decomposes near 800C to generate arsenic trioxide gas (which is toxic).

An alternative technique for local source predeposition (before soaking) is to use spin-on glass sources. These are applied in the same manner as photoresist, and once the organic solvents are driven off, a doped oxide remains. Oxide doping levels are determined by the manufacturer, but a wide range of doping levels (1016-1020 cm-3) of a wide variety of materials is available.

Since there is no vapour diffusion involved, wafers can be packed densely. A slight overpressure of nitrogen keeps contaminants out during soaking and (at least for P and B) no toxic gases are produced.


6.2 Diffusion Modeling In order to predict doping levels and profiles for a given set of doping

or heat treatment conditions, a good mathematical model of diffusion is required.

C(x,t) is the concentration of a particular dopant at depth x and time t Fick's (First) Law:

Where D is the diffusivity and J is the flux The continuity requirement gives Fick's 2nd Law:

Numerical solutions to this equation can be obtained using process simulators such as SUPREM.

To make analytic solutions possible, D is often assumed to be independent of x. (In practice, D varies with C and, hence, x.)

This gives the classic diffusion equation:

xCDJ

xCD

xtC

2

2

xCD

tC


6.2 Diffusion Modeling For predeposition, an analytic solution of this can be obtained by

assuming the surface concentration is fixed at the solid solubility limit, and given boundary condition C(0,t) = Css, C(,t) = 0

where erfc is the complementary error function

The total dose of dopants introduced is given by

The characteristic diffusion length is given by The metallurgical junction, xj, occurs when C(xj,t) = CB (the

background doping level of the wafer).

Dt2xerfcCt)C(x, SS

)dxexp(-x21erfc(x) Xo2

DtC2Q SS

Dt


6.2 Diffusion Modeling


6.2 Diffusion Modeling For drive-in, if the initial profile is assumed to be highly localized at

the surface, then the solution to the diffusion equation is gaussian: Boundary conditions: C(0,0) , C(,t)=0,

The localization assumption is reasonable if D2t2 > 3D1t1 where 1 refers to predeposition and 2 refers to drive-in conditions

For multiple high temperature steps, use(Dt)eff = D1t1 + D2t2 +

For variable temperatures, use

Q,dxt)C(x,o

4Dtx-exp

DtQt)C(x,

2

dtDDt



p

p

n+


6.3 Diffusion Effects

Lateral Diffusion at a Mask Edge: Diffusion occurs isotropically. Lateral diffusion is 75-85% of vertical. Since dopants at the mask edge are

diffusing into a greater volume, a reduction in surface concentration occurs there.

predeposition drive-in

line source - isotropic



Emitter Push: When diffusing phosphorous to

form the n+ emitter within a p base, the high phosphorous doping increases stress in the Si lattice which enhances the diffusivity of boron atoms.

This makes it more difficult to form a thin base (needed for high and high frequency operation).

Built in electric fields complicate this effect.

Emitter push can be reduced by using As as the n-dopant for the emitter as it doesn't stress the lattice as much.



High Concentration Effects: D usually increases with dopant

concentration (mostly as a result of increased stress).

This causes a more box-like concentration profile at high doping levels.

For anomalous reasons, P doping profiles can have a very long low concentration tail.

These nonlinearities can only be modeled numerically.

x

C

x

C

Box profile

Long tail


6.3 Measurement of doped areas Averaged doping levels can be inferred from

two or four point probe measurements. Depth profiling can be determined by beveling the

surface at a shallow angle Dopant depth profiles can be measured

using dynamic secondary ion mass spectrometry (SIMS) ppm or less sensitivity

Lateral extent of doped regions can be measured using scanning capacitance microscopy or four point probe mapping

P. Ghigna, U. di Pavia

wikipedia.org


6.4 Diffusion Masks

A masking material is needed to prevent dopants from reaching the wafer except where desired.

Thermal oxide (sometimes nitride) is usually used for Si, but is not suitable for GaAs as Ga will readily diffuse into the oxide.

The diffusion length in the mask must be less than the mask thickness. (Higher temperatures or longer times will require thicker masks.)

6 Diffusion.pdf

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