1. Capturing Ion-Soild Interactions with MOS structures
Radhey Shyam, Daniel Field, Steven Chambers, W.R. Harrell, James E. Harriss, C.E. Sosolik
Outline of the Talk.
What is MOS?
Why we make the MOS? Ans. To study the Ion Irradiation Effects on Oxides/Insulator
That will be technique used for studying these effects: We will be using Capacitance-Voltage measurements ?
Because it is a capacitor and the charge is stored and it has capacitance but the interesting physics is at Si/SiO2 interface which is the reason we use this technique to figure out ion implantation/damage physics using this technique.
So this is the reason I will be describing this technique in next 4 or 5 slides then our experimental setup and expected outcome.

2. “Devices” to Study Ion-Solid Physics
Our goal?
To use standard electronic devices (diodes, capacitors, tunnel junctions)
Physics of operation is known
Can be fabricated in-house (or even in-situ)
To modify those devices with an ion beam and use the “new” device characteristics to determine the beam-solid interaction

3. Metal-Oxide-Semiconductor(MOS)
The goal is to establish that MOS devices can be used to probe ion-modified oxides.

4. Metal-Oxide-Semiconductor(MOS)
Ion Beam
Oxide
Semiconductor

5. Metal-Oxide-Semiconductor(MOS)
Ion Beam
Oxide
Semiconductor
Metal
Oxide
Semiconductor
Encapsulation of the ion irradiation effects in a finished device

6. The Experiment Irradiate the oxide
Device fabrication
Si wafer: P-type; <100>; resistivity 1-10 Ω-cm.
Grow an oxide (1900 Ǻ of SiO2) on the front side.
Make Ohmic contacts on wafer back side.
Deposit top metal contact on one of the control devices.
Irradiate the oxide
alkalis ions: Na+ ; 100 eV - 10 keV (kinetic energy).
HCIs (at CUEBIT).
Deposit top metal contact (capture irradiation effects)
Characterize the device (Capacitance-Voltage measurements)
Take bare Si wafers
RCA clean for 5 min. with ultrasonic agitation
Etch Si surface with dilute 1% HF for 2 min
Triple rinse with DI water
Second RCA clean for 5 min. for removing organics on surface
Second etching with dilute HF
Rinse with DI water
Blow N2 to dry surface.
Growth of SiO2 via wet Oxidation for 25 minutes.
Remove SiO2 from the backside of wafer with HF .
Evaporate the 0.5 micron Al for back contact and anneal at 450 C for 30 minutes to make ohmic contact
Finally, Evaporate the top Al contact for the control devices.
We exposed our samples for one minute at beam current 15.82nA( nA for 60 and 66/100 th seconds)
Which corresponds to 5.917*10^12 ions for our whole exposed area of cm^2=PI()*(0.635/2)*(0.635/2)

7. Beam Line & Manipulator

8. Standardized Terminology for Oxide Charges Associated with Thermally Oxidized Silicon Bruce E. Deal*
Q = Net effective charge per unit area at the Si-SiO2 interface (C/cm2).
N = Net number of charges per unit area at the Si-SiO2 interface (no/cm2).
Thus, Q/q =N where q is the electronic charge.
Qf is the fixed charge density and it is +ve ,located within a very thin (< 1nm) transition layer of non-stoichiometric layer of silicon labeled as SiOx at the boundary Si and SiO2.
Qot is the oxide trapped charge density. It can be +ve or –ve (usually –ve) and located in traps distributed throughout the oxide layer. Only a small, often negligible amount of it is introduced by processing and it is fixed except unusual conditions.
Qit is he interface trapped charge density residing in trapping levels Nit (traps cm-2) . The Nit are located at the oxide-silicon interface and have energy levels within the forbidden energy gap. They are distributed with density Dit (traps cm-2 eV-1) .
Qm is the mobile charge density and usually results from alkali-metal ions (mainly sodium and potassium) .The ions are charged and affect the flatband voltage. They are sufficient mobile to drift in the oxide and their mobility increases rapidly with temperature.

9. Energy-band diagram at thermal equilibrium for an ideal MOS system.
At thermal equilibrium, Fermi energy is constant throughout the system. In other words, the Fermi level is constant throughout all three materials: the metal, oxide and the silicon.
2. The Fermi levels in the different materials are equalized by the transfer of the negative charge from materials with higher Fermi level ( lower work functions) across the interfaces to materials with lower Fermi levels (higher work functions).
3. Energy-band diagram at thermal equilibrium for an ideal MOS system composed of the materials indicated in Figure 2.1. The oxide and Si-SiO2 interface are assumed to be ideal and free of charges.
Charge is stored on the either side of oxide.
V_tot=V_ox + V_Si
SiO2 has two barrier heights
Electons from Metal to Silicon see 3.15 eV
Electrons from Silicon to Metal see 3.10 eV
So the electrons from Silicon move into the metal, making Si less p-type, so the conduction band bend towards the Fermi level to make it less p-ype or more n-type as n-type Si the conduction band is nearer to the Fermi level.
Fig.[2]

10. Energy-band diagram of the MOS system under flat-band conditions
V’FB= MS = M – S
This FB voltage does not include oxide charges.
Energy-band diagram of the MOS system of Figure 2.2 under flat-band conditions. An external voltage equal to VFB = -0.8 V(which works in opposing to built-in voltage on MOS capacitor) is applied between the metal and the silicon to achieve this condition, which does not correspond to thermal equilibrium.
Electrons are transferred from Metal to Silicon and the result is the sheet of “+” charge on metal surface and “-” space charge region in Silicon due to Ionized Acceptors.“
Charge is transferred through external IC path not through oxide layer.
At thermal equilibrium voltage ,metal and semiconductor form two plates of capacitor that is charged to voltage diff. between the metal and semiconductor work functions.
S depends on the semiconductor doping.
S =  + (EC – EF)FB
Fig. [2]

11. The Effects of fixed charge oxide charge density on the MOS system
Change in Flatband Voltage for Surface Charge density ~ 10^10 ions/cm^2 for our device of diameter 1cm will be order of 2.38 V .
As our As grown MOS devices having Si wafers of Resistivity of P-type <100> having resistivity of order of 1-10 ohms-cm and oxide thickness of 1900 A SiO2 and C_ox=2.055*10^-8 Farads.
for an arbitrary charge distribution,

12. What is Flatband Votage=

14. Getting at “Ion Beam Physics” with C-V
ΔVFB – the shift in flat band voltage
Calculate using typical doses (beam currents and times)
Dosing the oxide with 5.917*1012 ions /cm2 (15.80 nA for 60 seconds)
Calculated Dosage from ΔVFB =2.08*1013 ions/ cm2
Linked to the probability of implantation of Ions into the oxide. We found this probability to be
Vary this probability as a function of ion species and energy of the ions (follow it with ΔVFB)
C-V plot characteristics may also give information on irradiation damage of the oxide (to be determined).
ΔVFB = Total Charge / Oxide Capacitance
Total Charge = No. of ions * Electron charge = 5.917*10^12*1.602*10^-19=9.479*10^-7 =This is the total charge of Na ions in our exposed area of diameter=0.635 cm
Total Charge in our device=(Area of the device / Area of total exposed area)* Total Charge in exposed area=( /0.3167)*9.479*10^-7
Oxide Capacitance=2.055*10^-10 F
ΔVFB =(9.479* /0.3167)/(2.055)*10^3=

15. References:
1.Goodstein D.M.,Dahl E.B.,Dirubio C.A., and B.H. Cooper,” Trapping of Ions at Metal Surfaces “,Physical Review Letters, Vol. 78,pp
2. Device Electronics for Integrated Circuits Muller & Kamins ,Third Edition.
3. Kohn M, Solid State Electronics,1971,pp , “Ionic Contamination and Transport of Mobile Ions in MOS Structures.”
4. Heatwave Standard Ø.250" Na source with coaxial heater
5. Keithley Semiconductor Characterization System
6. Agiliant E4980 A
7. Micromanipulator Probe Station