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Savannah

Savannah is a thermal atomic layer deposition (ALD) system. It is a Savannah S200 from Cambridge Nanotech and is categorized as gold contaminated. The system can accommodate pieces up to an 8" wafer.

Picture and Location

 Lab Map Savannah Location

Savannah is located at the end of the first aisle on your left as you enter from the gowning room.

Background

 Atomic layer deposition (ALD) is a type of chemical vapor deposition that utilizes a cycling of different precursor gases to produce monolayers of a particular solid film.  In conventional LPCVD as performed in the SNF furnaces the precursor gases are mixed continuously and are so reactive that the solid films are formed everywhere including occasionally in above the substrate surface.  Conversely, in ALD first one precursor is introduced which will ideally completely and with self-limitation coat the substrate with a single molecular layer.  The second precursor again in a self-limiting process reacts with the first molecular layer at the surface to produce a monolayer of the desire material.  The key is finding a chemistry for the ALD precursors that can react at under the appropriate pressures and thermal energy the system can deliver, but will not desorb from the surface in the reactor conditions.  This "energy window" for proper ALD growth is shown schematically in below (from Atomic Layer Deposition for Nanotechnology by Arthur Sherman):

ALD energy window

 

Because of the self-limiting nature of the sequential precursors, ALD systems can achieve near monolayer-at-a-time growth conditions with extreme uniformity over large areas.  Additionally, the thin films have tremendously conformal coatings even under extreme topology.  The films in general are dense and provide excellent interface preservation.  However, the deposition rate is exceedingly slow (in general on the order of an Angstrom/minute) and this technique is not ideal for films thicker than a few tens of nanometers.

A more detailed introduction to ALD can be found in this presentation (given by Dr. J Provine 11/1/12 at Stanford University):  ALD Introductory Tutorial 2012-11-01

A summary of more in-depth ALD processing information can be found in this presentation (given by Dr. J Provine 11/1/12 at Stanford University): ALD In Depth Tutorial 2012-11-01

 

Process Capabilities

Cleanliness Standard

 The Savannah is classified as Gold contamination level.

 

The ALD processes take place with a chamber temperature in the range of 150C-250C.  Because of this the following materials are not allowed:

1.       Polymers not specifically approved.

2.       Wet samples

3.       Anything with melting points or ignition points below 250C

4.       Plastic, including Teflon

5.       Non-encapsulated particles

6.       Samples small enough to fall into the exhaust line.


A Note on III-V Materials

Traditionally III-V materials in SNF are classified as gold contaminated.  However, because the Savannah operates at low temperatures III-V materials can be used in the savannah under the following conditions (failure to adhere to these rules will result in removal from use of the tool):
1. The III-V materials in question do not violate any of the above restrictions
2. Users wishing to use III-V materials must speak directly to the quality circle about their plans before beginning to use these materials in the system.
3. III-V materials should never touch the inside of the deposition chamber or the tweezers that are used in the chamber.  A clean Si, SiN, or SiO2 substrate must be used as a carrier wafer.  
 

How to make a Carrier Wafer for the Savannah

1. Grow about 100nm SiO2 on a Si <100> wafer.  SiO2 can be deposited, but needs to be on both front and back.
2. Pattern SiO2 with resist.  The pattern should be a larger area than your piece as this pattern defines your pocket.
3. Dry etch SiO2. Note: wet etch does not work because you need to keep the backside oxide.
4. Strip the photoresist.
5a. Wet etch Si wafer with SiO2 as a mask in TMAH heated to about 90C for a few hours.  This makes a recess with depth dependent on your initial mask because TMAH stops on the Si 111 planes.  The chips can sit on the crystal smooth angled sidewalls of the trenches, which are very flat allowing good thermal conduction from the substrate, and hence the substrate heater, to your chip.
5b. You could also try a DRIE of Si to define your pocket.  Here the etch would be vertical and your pocket would have an approximately horizontal floor.
 
Many thanks to Jenny Hu for providing this detailed description of how to make a carrier wafer for the Savannah.


Performance of the Tool

What the Tool CAN do

  • Deposit high quality metal oxides
  • Tightly controlled thickness through monolayer at a time growth.
  • Thin film deposition with angstrom precision.
  • Ultra-conformal deposition.


What the Tool CANNOT do

  • Deposit thick films.  This is because of both the slow deposition rates and the expense of material precursors.
  • Films thicker than 50nm require prior approval from the quality circle.

  • Selective ALD is a difficult issue and current research topic...it should not be expected that materials will deposit on only certain materials.
  • ALD of certain films has not been demonstrated (Au for instance).  If you want to try something not currently available, please consult the quality circle or the ever-expanding ALD literature.


The System

The Savannah system consists of the following components:

  • A set of precursor cannisters
  • High speed ALD valves that control the release of precursors into the manifold for introduction to the reaction chamber
  • A heated manifold the conduct the precursors into the reaction chamber via a carrier gas (we use the house, five-9s purity N2)
  • The reaction chamber with controllable temperature (150-250C) and variable pressure controlled by a pump (.05-1Torr).
  • A heated exhaust line from the chamber including deposition trap
  • A pump to maintain low pressures in the chamber and remove unreacted precursor
  • A electronic control box (called the "e-box") that controls all of the heaters, valves, etc and communicates with the control computer via USB
  • A control computer (a non-networked PC running Windows Vista)

Possible Films

The films available on the Savannah is determined by the precursors installed in the system.  The list of available precursors is maintained on this page and also on a file on the desktop of control computer.  The file on the computer is the final word on what precursors are installed on the system currently.  In the event that this website and the file on the computer disagree - follow the document on the control computer.

Current films available (in stock precursors):

*Please note film deposition rates listed are specified to one significant digit for estimation purposes.  For applications with tight thickness specifications it is recommended the user perform some process characterization to ensure precise depositions.

Film   NOTES Tools Deposition rate (A/cycle) @ temp) Thickness Nonuniformity over 2 100mm wafers)  Precursor 1  Precursor 1 temperature   Precursor 2 
Thermal Al2O3  Well characterized Fiji1, Fiji2, Savannah 1 @ 200C 1% Trimethylaluminum  Unheated   H2O  
Plasma Al2O3   Well characterized  Fiji1, Fiji2 1 @ 200C 0.50% Trimethylaluminum   Unheated   O2 plasma  
Thermal TiO2  Being characterized Fiji1, Fiji2, Savannah 0.4 1% Tetrakis(dimethylamido)titanium(IV)  75C   H2O  
Plasma TiO2  Being characterized Fiji1, Fiji2 0.4 2% Tetrakis(dimethylamido)titanium(IV)   75C   O2 plasma  
Plasma TiN  Well characterized  Fiji1, Fiji2 0.5   Tetrakis(dimethylamido)titanium(IV)  75C   N2  plasma
Plasma WN  In development Fiji1, Fiji2     Bis(ter-butylimino)bis(dimethylamino)tungsten(VI)  60C (TBD)   N2 plasma
Thermal HfO2  Well characterized Fiji1, Fiji2, Savannah 1.0 1% Tetrakis(dimethylamido)hafnium   75C    H2O 
Plasma HfO2   Well characterized Fiji1, Fiji2 1.0 1% Tetrakis(dimethylamido)hafnium(IV)   75C   O2  
Thermal ZrO2   In development  Fiji1, Fiji2, Savannah 0.8 1% Tetrakis(dimethylamido)zirconium(IV)   75C   H2O  
Plasma ZrO2  In development Fiji1, Fiji2     tetrakis(dimethylamido)zirconium(IV)   75C   O2 plasma 
Thermal Ta2O5  In development  Fiji1, Fiji2, Savannah     Tris(ethylmethylamido)tert-butylamido)tantalum (V) 105C H20  
Plasma Ta2O5   In development  Fiji1, Fiji2     Tris(ethylmethylamido)tert-butylamido)tantalum (V) 105C O2 plasma  
Plasma TaN   In development  Fiji1, Fiji2     Tris(ethylmethylamido)tert-butylamido)tantalum (V) 105C H2 plasma    
Plasma SiO2  Being characterized  Fiji1, Fiji2 0.8 1% Tris[dimethylamino]Silane   Unheated   O2 plasma  
Thermal Pt   Being characterized  Fiji1, Fiji2, Savannah 0.4 (plus substrate dependent nucleation)   Trimethyl(methylcyclopentadienyl)platinum(IV)   80C   O2  
Plasma Pt   Being characterized  Fiji1, Fiji2 0.5   Trimethyl(methylcyclopentadienyl)platinum(IV)   85C   O2 plasma  
Thermal Ru   In development  Fiji1, Fiji2, Savannah     Bis(ethylcyclopentadienyl)ruthenium(II)  TBD   O2 and H2  
Plasma NiO   In development  Fiji1, Fiji2     bis(cyclopentadienyl)-nickel   TBD   O2 plasma 
Thermal ZnO In development Savannah     Diethylzinc Unheated   H2O  
Silanization No characterization       APTES (3-Aminopropyl)triethoxysilane    
Indium Oxide No characterization       Cyclopentadienylindium(I)    
GaN No characterization       Trimethylgallium    
Thermal SnO2 Being characterized Fiji2     Tetrakis(dimethylamido)tin Unheated O2 plasma 
Thermal FeO
In development
Fiji2


Ferrocene (BIS(CYCLOPENTADIENYL)IRON) 60C
O2 Plasma
FeO
In development
Fiji2


Fe amidinate (accudep Iron)
TBD

Thermal Y2O3
In development
Fiji2


Tris(Cp)Y
165C
H2O
La2O3
In development
Fiji2


Tris(i-propylcyclopentadienyl)lanthanum
TBD

BaO
In development
Fiji2


Bis(pentamethylcyclopentadienyl)barium, 1,2-dimethoxyethane adduct
TBD

BaO
In development
Fiji2


Bis(n-propyltetramethylcyclopentadienyl)barium
TBD

Co3O4
In development
Fiji2


Bis(cyclopentadienyl)cobalt(II) TBD

Thermal SrO
In development
Fiji2


Bis(1,2,4-tritertiarybutylcyclopentadienyl)strontium
110C



For the list of currently installed precursors consult the document on the Savannah and coral for the most recent changes.

  • If the precursor or film you want is not shown here, please contact the quality circle.  New materials and special runs can be possible as needed and with appropriate planning.

 

Contact List and How to Become a User

Contact List

The following people make up the Tool Quality Circle:

  • Process Staff: Michelle Rincon (mmrincon at snf dot stanford dot edu)
  • Maintenance:  Michelle Rincon (mmrincon at snf dot stanford dot edu)
  • Super-Users:  J Provine (jprovine at stanford dot edu)

 

Training to Become a Tool User

Becoming a Savannah user is a three stage process.

  1. Shadowing.  Contact a current user of the Savannah and request that they introduce you to the system and demonstrate the use of the system for you.  If you do not know a Savannah user, contact the user list (savannah at snf dot stanford dot edu) or check coral for upcoming user reservation.  It is up to the user if they are willing to have you shadow them.  If you have trouble finding someone to shadow, please feel free to contact the quality circle for help.  (Helpful hint:  ask lots of questions.)
  2. Written Quiz.  After shadowing a user, contact the Michelle Rincon or J Provine to take the short written quiz for the Savannah.  The quiz is closed book and all the information should be known to a someone who has studied the online documentation and has gone through a shadowing.
  3. Oral Qualification Exam.  After the written quiz has been passed, you may schedule a final oral qualification exam with a superuser or staff.  At this exam you are expected to demonstrate your ability to run the tool safely and appropriately without any input.  Once this final stage is satisfactorily passed, you will be given full access and utilization for the Savannah.

Operating Procedures

Basic Operation Instructions

Reservations

  1. Make reservation in coral.
  2. At least 24 hours prior to reservation time, make a comment in the maintenance section to request the specific precursors you would like to use for your run.  The precursors are swapped frequently so do not assume that a precursor that is on the tool when you make your reservation will still be there when you are going to be running.  If your reservation is for the weekend, please make the comment by the end of the day Thursdays.

Processing

  1. Enable the system on coral.  Do not operate the system without being enabled on coral.  Wafers found in the system without coral enabled will be removed at the discretion of the enabling user.
  2. Check that the system is in working condition:  all the heaters enabled and within range, the precursor gas lines are open, and the reaction chamber is pumping to below 250mTorr with a carrier flow of 5sccm.  If any of these features are not found to be in range, report the situation on coral.  The base pressure creeps over time as the capacitive pressure sensor drifts with deposited film and over time.  The key is that the pressure is low, stable, and similar to the previous reported value.  Note the base pressure at 5sccm to check with your process is done to ensure nothing has changed during deposition.
  3. Check the status of the system on the control computer.  This is the final note about what precursors are loaded on the tool will be here.
  4. Load your desired recipe:  right click on the recipe in the control software and select from the Windows file list.
  5. Edit your recipe as needed:  see the More on Recipes subsection for more information about writing and editing recipes.
  6. Vent the system by pushing the "VENT" button in the control software
  7. Transfer your wafers into the reaction chamber (Be Careful It is HOT).  Do not use plastic tweezers or wear clear vinyl gloves when working in the reaction chamber because of the possibility of melting these elements and contaminating the chamber.  Teflon coated metal tweezers are allowed.  All tweezers and substrates should be clean as described above.
  8. Close the lid and pump the chamber by pushing the "PUMP" button in the control software
  9. Replace the wire mesh heat guard
  10. When the system has reached base pressure (check that it is similar to what base pressure you found the system in - different by not more than a few mTorr), run your recipe by pushing the "START" button
  11. Check to make sure the pressure pulses of your recipe are in the correct range by monitoring the pressure readout in the control software.  Typical pressure pulses are one hundred mTorr or more above the base pressure.
  12. When the process finishes, check that the base pressure has returned to the initial value.  This checks that the system is not leaking and has returned to its original state.
  13. Vent the chamber and remove your wafers.
  14. Pump down the chamber
  15. Load the standby recipe STANDBY and run it  (please see A Note on Shutdown subsection)
  16. Update the status document on the desktop
  17. Lock the computer monitor screen ("window"-Lock).  NOTE:  do not log off the computer as this will disconnect the control software from the e-box.
  18. Disable the system on coral

If you need to stop your deposition at any time during the process, press the "STOP" button which will leave the heaters and the pump on but stop the recipe.  It is not necessary to close the control program overall.  If you do accidentally log off the computer or close the control program, open the program up again, run it, and load & run the STANDBY program.

Recipes

The recipes are all maintained in the folder C://Savannah/Recipes/.  Standard recipes listed in this document are kept in that folder.  USERS MAY ONLY CHANGE THE NUMBER OF CYCLES FOR STANDARD RECIPES AND WHICH PRECURSOR VALVE IS APPROPRIATE .  If you wish to write you own recipe, please make a folder to save your own recipes into.  To write a recipe:

  1. Load a similar recipe
  2. Edit the recipe by typing in the values, identifying the elements, selecting the commands from option list that pops up when you right click on the recipe box.
  3. Save the recipe to a new file name in your recipe folder.

 

The parameters you can control in a recipe are listed below with comments about each:

Parameter Notes
Heater  Each heater has an unique item number and a value given in degrees Celsius.  Do not overheat precursors (all precursors should be below 85C and some should not be heated at all - consult with quality circle if unsure).  The reaction chamber should only be operated in the range of 150-250C.  The manifold and exhaust lines should be maintained at 150C to avoid any condensation.
Flow This controls the flow of nitrogen carrier gas flowing through the system.  It is defined in sccm.  Typical recipes use a flow of 20sccm, and when in standby the system is lowered to 5sccm flow to reduce N2 usage.  The source is the house nitrogen line. 
 Pulse This command is for pulsing a precursor line.  It requires an ALD valve number and the amount of time you want the valve open in seconds.  The fastest these valves can fire is roughly .015 seconds, so note that even if you define a shorter time that is likely the valve open time you will get.  (NOTE:  when writing a recipe the time in seconds needs a digit to the left of the decimal place; thus you should use "0.015" instead of ".015" for the minimum duration pulse.) 
 goto Used to define loops in the recipes.  This command takes as an input the step to which the recipe should return. The value for this command defines how many times the loop will run.
 stabilize This command is used to hold a recipe until a heater has reached the desired value.  It takes as input a heater ID number and will wait until that heater demonstrates the set temperature with a degree C over a few seconds. 
 wait This command takes as input a value in seconds that you would like the system to wait before proceeding to the next command. (NOTE:  when writing a recipe the time in seconds needs a digit to the left of the decimal place; thus you should use "0.015" instead of ".015" for the minimum duration pulse.)  
 stopvalve This command will close or open the output valve for the reaction chamber depending on a Boolean input.  This command is currently not used in any of the standard recipes, but development is beginning for recipes using this feature.
 line ac out Users should not use this command. It changes the heater voltage on precursor heater wraps.


A Note on Shutdown  

The STANDBY recipe is used to set all the heaters at nominal values and adjust the flow of the N2 carrier gas to a less consumptive value.  All of the heaters are maintained at nominal values to avoid condensation or other history related problems in the system.  The manual valves for all the precursors can be left open with the ALD valve providing sufficient gating on the precursor.  Previously the ammonia regulator needed to be changed, but with the installation of the new ammonia line, nothing needs to be changed for nitride films.  There is no need to adjust anything inside the cabinet when the machine is under normal, "green light" status.

Reservations

The following restrictions are in place for reservations on Savannah:
1) The maximum reservation window is 7 days.
2) The maximum primetime reservation any day is 4hours.
3) The maximum reservations allowed is 24hours.
4) Please make a comment in CORAL stating which precursors you will be using at least 24 hours before your run.

 

Process Monitoring and Machine Qualification

Tool Qualification Run

Frequency

Once a month the standard recipes will be run and results reported to this website. 

Procedure

A freshly RCA cleaned 4" 100 Si wafer will be run for each of the standard recipes for 100 cycles.

A multi-point spectroscopic ellipsometry measurement will be made on the Woollam.

The n and k values will be reported along with thickness and wafer uniformity.  If the values are out of the acceptable range, the system will be put into "Yellow" status and the cause will be explored by the quality circle.

Responsibility

Qualification runs are the responsibility of the quality circle.  They will be completed, including reporting, during the first week of a given month. 

Machine Status States

Red:  The tool is not capable of depositing any films and needs maintenance or verification of proper working conditions.

Yellow:  The tool is capable of depositing some films or films of non-standard quality.  Reasons could include lack of necessary precursor, issues with the machine (pump, heaters, etc), or contamination that needs to be cleaned.

Green:  The tool is in working condition for all available films, and those films were found to be within specifications.

Process Monitoring Results & General Data

 

HfO2 from Tetrakis(dimethylamido)-hafnium(IV)

Savannah

Many thanks to Emily Ross for the data (collected Summer 2012)

 

Deposition Rate: ~1.0 Å/cycle    Standard Non-Uniformity: ~1%

 

Demonstration of Precursor Saturation:

The standard recipe (with a pulse time of 0.20 seconds) was confirmed to saturate the substrate surface, as well as not be too close to the demonstrated thickness drop-off.

hfo2 average thickness vs pulse timehfo2 non uniformity vs pulse time

Purge Time Variation:

Varying the purge time in conjunction with the standard recipe pulse time of 0.20 seconds verified that the standard purge time of 15 seconds had the lowest non-uniformity. The slight decrease in thickness and increase in non-uniformity with longer purge times could indicate that the film is prone to desorption.

hfo2 average thickness vs purge timehfo2 non uniformity vs purge time

 

 

 

 

Background Carrier Gas Flow Rate Variation:

The background carrier gas flow rate was varied to see if the uniformity of the films could be improved further. It was found to have no effect.

hfo2 average thickness vs background flow ratehfo2 non uniformity vs background flow rate

 

Known Chamber Gradient:

The films were consistently thinner towards the input and thicker towards the exhaust. Attempts to remedy this were unsuccessful. However, even with this gradient, the % non-uniformity is around 1%.

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ZrO2 from Tetrakis(dimethylamido)zirconium(IV)

Savannah

Many thanks to Emily Ross for the data (collected summer 2012)

 

Deposition Rate: ~0.8 Å/cycle    Standard Non-Uniformity: ~1%

 

Demonstration of Precursor Saturation:

The original TDMA-Zr pulse time of 0.40 seconds was overly long, resulting in wasted precursor. Due a demonstration of surface saturation at shorter pulse times, the TDMA-Zr pulse time on the standard recipe has been reduced to 0.15 seconds.

zrox thickness vs pulse timenon uniformity vs pulse time

The thickness drop-off is apparent with a TDMA-Zr pulse time of less than 0.03 seconds, in conjunction with a high non-uniformity (~130%). This means that the pulse time of 0.015 seconds was not sufficiently long to saturate the substrate surface. Removing that point shows that even though the substrate surface is saturated, the non-uniformity is significantly higher with pulse times of less than 0.10 seconds.

saturated zrox thickness vs pulse timeSaturated non-uniformity vs pulse time

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Al2ODeposition rate of 1.0A/cycle for standard recipe

(Thanks to Rebecca Schuster, Gaurav Thareja, Alice Wu, Shimeng Yu, and Ju Hyung Nam for their help with data collection.)

A. alumina thickness plot

B. autumn al2o3 deposition rate test

The deposition plot A is from the summer of 2010 and showed a deposition rate of almost 1.1A/cycle.  The deposition plot B is from Autumn 2010 and shows a deposition rate of 1.0A/cycle.  The deposition rate is being tracked by process monitoring shown below.

Al2O3 Process Monitoring

al2o3 cv

The CV measurements were done with capacitors with area of 100um x 100um and there is a strong effect from forming gas anneal.  The MOSCAPs were Al2O3 deposited on Si with a sputtered Al upper electrode (defined by wet etch) and a 325C 30min FGA.  Additionally, from CV curves the dielectric constant can be determined to be ~6.75.

The deposition rate has a weak dependence on chamber temperature.

al2o3 dep rate temperature dependence

Breakdown Voltage of Al2O3 ALD films.

al2o3 breakdown voltage

Leakage Current in Al2O3 ALD films.

al2o3 leakage current

Additional information can be found in the various final reports from EE412 ALD projects.

TiO2 Standard Recipe Results and Monitoring 

TiO2 Deposition rate of .38A/cycle for standard recipe

(Thanks to Jason Lin for his help with data collection.)

titania deposition rate in savannah

The deposition rate at 200C has a strong linear fit for .38A/cycle.

tio2 process monitoring 2011-09

The monthly quality monitoring of TiO2 in the Savannah.

tio2 temperature dependence

Temperature dependence of deposition rate for TiO2.

tio2 xps data

XPS evaluation of the composition of titania films indicates a Ti:O ration of approximately 1:3.

tio2 afm measurement

AFM analysis of sample with 200 cycles of titania deposited at 200C.  The roughness of the film is on the order of twice that of a polished Si wafer.

HfO2 Standard Process Results

HfO2 Deposition rate of .99A/cycle for standard recipe

(Thanks to Woo-Shik Jung and  for his help with data collection.)

hafnia deposition rate

Deposition rate of Hafnia ALD at 200C (note the native oxide of the Si wafer was also tracked and plotted on this graph).

hfo2 processing monitoring 2011-09

Process monitoring for hafnia on the savannah over time.

hfo2 deposition temperature dependence

Temperature dependence of the deposition rate of hafnia.  Note that this dependence is much stronger than observed with alumina and titania.

hfo2 film roughness measurements

AFM measurements of film roughness with variation in deposition temperature.  Little dependence on temperature is observed and the film has a roughness on the order of a polished Si wafer.

hfo2 CV  hfo2 normalized CV

Representative CV and normalized CV for MOS capacitors fabricated with hafnia as the insulator and Al as the metal layer.  The films are treated with a 300C FGA anneal.  From this data we can extract the following material parameters:

Parameter Value
 Dielectric Constant 15 (at 1MHz) 
Doping Concentration  1.5 x 1016 /cm3 
Fixed Charge Density  2.9 x 1012 / cm2 (negative) 

hfo2 breakdown voltage and leakage current

Breakdown voltage and leakage current for Hafnia ALD at various temperatures (with the number of cycles varied to provide ~100A +/- 5A in each case).

ZrO2 Standard Process Results

ZrO2 Deposition rate of .89A/cycle for standard recipe

(Thanks to Ze Yuan for his help with data collection.)

zro2 deposition rate

Deposition rate for ZrO2 films.

zro2 deposition rate

Process monitoring for Zirconia films.

zro2 deposition temperature dependence

Temperature dependence of the deposition rate of zirconia.

TiN Standard Process Results

TiN thickness

The two different lines are for ellipsometry fits for either TiN or TiO2 films.  Both fit the optical constants only so-so.  By measurement on Auger spectroscopy we see there is roughly 20% (atomic) nitrogen content in the TiN film.  So it is really more TiOxNy, with x > y.  Effort is ongoing to reduce the oxygen content in the TiN films.

 

 

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