Semiconductor doping - Inside Minesinside.mines.edu/impl/lectures/lecture4-Doping12.pdf · Diffusion doping (in fact most doping) is typically done in two steps: (Almost all doping

PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory

Semiconductor doping

• Two Levels of Masks - photoresist, alignment • Etch and oxidation to isolate – thermal oxide, deposited oxide, wet etching, dry etching, isolation schemes • Doping - diffusion/ion implantation • Metallization - Materials deposition, PVD, CVD

Si solar Cell

PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory What’s a metal, a semiconductor?

IV

How do we “dope” a semiconductor


Electrons and holes

Valence Band

Conduction Band

Ev

ED Ec

EA


Sheet Resistance, what is it?

L W t

R = ρ L/Wt We can rearrange to get a film dependent quantity called the Sheet Resistance Rs = ρ/t =R / (L/W)

Notice L/W is unit less, but gives us the number of “squares” in the length of the bar. The units of Rs are ohms, but they are often given as Ω / .

What is the Resistance of this bar of material with resistivity ρ?


Sheet Resistance - Four Point Probe

If Probe spacing is: • Larger than film thickness • Smaller than distance to edge of film • Probe points are “small”

Rs=4.53 V/I and

ρ=Rst where t is thickness

Using a four point approach is a standard technique for eliminating the effects of contact resistance


How do we get the doping?

Rs and t give us ρ, which gives us doping (but we must know t)


Another way to get doping - from C-V of a diode

Formation of a p-n junction

Formation of a Schottky junction


1/C2 vs V

Assumes an abrupt junction - Schottky, p+n or n+p

v

C-2

Slope gives carrier Concentration

x-intercept give Vbi

What if the line isn’t straight?


How about the thickness of our Oxide?

Again, C = εA/W, so we should have another way to measure W. In practice, we must be careful about what C we use.

Corresponds to oxide thickness

What about trapped charge?


Inversion in an MOS structure

accumulation (negative bias)

no bias

inversion (positive bias)

PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory What about I-V Characteristics?

Forward biased pn junction: Probability that carriers are over the barrier is like a Boltzmann factor

But, there is also an electric field pushing carriers back so at V = 0 there should be no current.

We can write this in a simpler form as:

What about when light is shining on the device?

Note, there is a sign difference with respect to the capacitance analysis

PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory How can we tell the carrier type

Thermovoltage

Hall Effect •  carrier type •  mobility •  sheet concentration

Hot Probe

e e e e e e e e

V


Other methods of getting at the carriers

•  SIMS •  RBS – Rutherford Backscattering •  Polaron profiler •  Spreading Resistance •  ...


Doping - reminder

Goal of Doping: Substitution of atoms with excess or deficiency of valence electrons e.g. B or P substituting for Si

Diffusion doping (in fact most doping) is typically done in two steps: (Almost all doping is now ion implantation)

Predeposition - Use a source to create the desired dose

Drive in - Source at surface removed, additional diffusion to get desired distribution (in ion implantation the anneal also removes damage and activates the dopant).


Generic Predeposition Process

Deliver Dopants to Partially Masked Substrates •  Diffusion (Hot) •  Ion Implantation (Cold)

Structure:

Mask: Oxide, Nitride, Photoresist

Silicon

Dopants


Dopant delivery Options for Diffusion

Gas Source: •  Nasty Gases: AsH3, PH3, B2H6 •  Very similar to Deal –Grove Oxidation

Liquid Source: •  SOG: Spin-On Glass •  Doped SiO2 dissolved in solvents •  Apply exactly like Photoresist

Solid Source: •  Glass Discs (B2O3, P2O5) •  Close-space Sublimation •  Vapors sublime/diffuse/react

CB

Co

Cs

Ci

δ

xj

Which is Best?


Drive-in - estimating the profile

Fick’s law - You need the PDE, but you also need the boundary conditions!

C(z,0) = 0, Z ≠ 0 dC(0,t)/dz = 0

C(∞,t) = 0

Solution: ⎟⎟⎠

⎞⎜⎜⎝

⎛

= Dt-z

T eDtQ 4

2

t)C(z,π

We can model the drive in step from our homework, here after a P predep with p8545 we had a sheet resistance of 12Ω/�� and depth of 1.1µm. This gave a carrier concentration of 5x1019/cm3 and a surface concentration of 5.5x1015/cm2

Characteristic Length Scale - Diffusion Length

€

C(z,t)dz =QT0

∞

∫ = constant


What about the diffusion Coefficient?

Use first three terms in Fair’s vacancy model.

−−⎥⎦

⎤⎢⎣

⎡++= 2

2

DnnD

nnDD

ii

o

From Campbell table 3.2 (1100C=1373K)

Do = 3.9cm2/s e-(3.66/k1373) = 1.43 x 10-13cm2/s D- = 4.4cm2/s e-(4.0/k1373) = 9.13 x 10-15cm2/s D2- = 44cm2/s e-(4.37/k1373) = 4.00 x 10-15cm2/s

D = 1.56 x 10-13cm2/s

I told you to assume n~ni ~1019/cm3

Is this reasonable?


Simulations Suprem-IV is a process simulation tool developed at Stanford University


nanoHub TCAD tools

https://nanohub.org/tools


Suprem simulation of boron predep and drive-in

Boron Diffusion

0

5

10

15

20

0 2 4

Depth in microns

Log1

0(B

oron

)

Boron Predep 1100C30 min.

Boron drivein 1100C30 min.

Boron drivein 1100C60 min.

Boron drivein 1100C60 min 200 angstromoxide cap

Boron Diffusion

18.0

18.5

19.0

19.5

20.0

20.5

21.0

0 0.5 1 1.5 2 2.5

Depth in microns

Log1

0(B

oron

)

Boron predep in gas at 5 x 1020/cm3 concentration followed by drive-ins.

Effect of oxide cap on profile near the surface

Why 5x1020/cm3? 1)  Damage threshold 2)  Solubility limit 3) B partial pressure 1)  Dimensional argument

PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory Solid Solubility, what is it?

5x1020/cm3

1100C


Oxide is an effective anti-diffusion barrier for Si VLSI?

1)  For boron but not for phosphorus 2)  For phosphorus but not for boron 3)  It works well for both 4)  It depends

PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory Final Topic on Diffusion: Oxide

How fast do dopants diffuse through oxide? Diffusivity important, Solubility important

Consider Do of Boron Si prefactor 0.37cm2/s Activation Energy 3.46eV SiO2 prefactor 0.0003cm2/s Activation Energy 3.53eV

Now Do of Phosphorous Si prefactor 3.9 cm2/s Activation Energy 3.66eV SiO2 prefactor 0.19 cm2/s Activation Energy 4.03eV

• Oxide is often used as a diffusion mask- how thick does it need to be? • Oxide is used for isolation - does it isolate? What is the thermal load? • Oxide is also a gate dielectric with heavily B doped polysilicon gates - diffusion through gate is an issue

Silicon

Oxide

Metal Doped polysiliconM

O

S


Suprem-IV Wet Oxide then Diffusion

Effect of oxide cap on profile near the surface

Substrate is P doped at 1 x 1014/cm3, Wet oxide growth at atmospheric pressure for 60 minutes at 1000C, Boron predep from 30 minutes at 1100C in gas with a concentration of 5 x 1020/cc.

Oxide antidiffusion barrier

0

5

10

15

20

0 1 2 3 4

Depth in microns

Log1

0(Bo

ron) 60 min wet O2 at 1000C,

30 min boron predep at1100C30 minute boron predep at1100C


Simulation of predep and drive-in to find junction depth

1000°C P predep in p-type wafer doped at 1x1017/cm3. 1100°C drive in. How long to get a 4.0µm deep junction?

Documents

Semiconductor doping - Inside Minesinside.mines.edu/impl/lectures/lecture4-Doping12.pdf · Diffusion doping (in fact most doping) is typically done in two steps: (Almost all doping