DESIGN, CONSTRUCTION, AND QUALIFICATION OF ACHLORINATED OXIDE GROWTH PROCESS

Steven D. Ward5th Year Microelectronic Engineering Student

Rochester Institute of Technology

ABSTRACT

This project dealt with the design and installation ofa TCA bubbler system. Considerations for safety andtemperature control of the TCA were simplified by theuse of a J.C. Schumacher Flow Temperature Controlunit. Installation of the system is complete andchlorinated oxides have been grown.

I NTRODUCT ION

Wet and dry oxide growth techniques are widely used Insemiconductor fabrication. However, there are times when it isdesirable to reduce sodium ion contamination nd enhance theelectrical characteristics of the oxide. This is most commonlydone when the oxide Is being used as a gate oxide in tIOS devices.For this reason, oxidation of silicon In a chlorine—containingambient can be a very beneficial process step. It is possible toreduce the mobile charges in the oxide by two orders ofmagnitude, Increase the minority carrier lifetime by two ordersof gnitude and also grow an oxide with much better integritythus gre tly improving the ci ctrical properties. These benefitsare not without some drawbacks to the process. These includeincreased surface roughness of the oxide, the separation of theoxide from the silicon, the etching of the silicon at the Si/Si02Interface and the increased radiation sensitivity of devicescontaining chlorine.t1)

Sodium passivatlon is the process of Immobilizing sodiumIons in the gate oxide. Sodium passivation can be achieved withthe Incorporation of chlorine into the oxid tion process twoways. The first is a cleaning of the tube Itself. There Is avery large potential for sodium contamination in the furnacebecause at elevated temperatures, sodium can diffuse through thewalls of the tube. To decrease the contribution o this, it Isnecessary to perform a cleaning of the furnace tube, boats, andpaddle to remove sodium and other metal ion contaminants. Astandard procedure is to clean for 45 minutes at a temperatureapproximately 100 degrees celsius over standard processtemperature in a chlorine ambient. It is possible to decreasethe contamination 1 vels even more by increasing the cleaningtime. The mobile sodium ions In the oxide are also reduced whenchlorine is Incorporated Into the oxide growth process itself.High temperatures are required for passivation to occur.However, some passivatlon is seen at temperatures as low as 900

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degrees celsius. For a rticular oxidation time andtemperature, the amount of passivation Is dependent upon thelevel of chlorine contained in the oxide. Th sodiumconcentration decreases with Increasin chlorineconcentrations. E2)

The mechanism proposed for the removal of mobile ioncontamination Is as folIows:partfcles are present on the siliconsurface at the start of oxidation. As the oxide is grown, theseparticles interact causing defects and they are incorporated intothe oxide layer. These particles are usually metallic In nature.If the particles are incorporated into the oxide they becomesites for breakdown. During oxidation the chlorine combines withthe metallic particles to form metal chlorides which are volatileand are desorbed into the gas stream. The sodium is neutralizedby the chlorine through the formation of neutral species.Experiments have shown that 1% TCA is the optimum concentrationfor passivation and increased dielectric strength. Atconcentrations lower than 1%, not enough chlorine is present. Asthe concentration exceeds 1%, corrosion of the silicon surfacebegins to occur.[3)

Localized impurities in the oxide are also causes of lowintegrity oxides. These impurities are usually odium. Themechanism of n utralizatiori is not well understood. It isproposed that the mobile loris become trapped when they reach thevicinity of the chlorine, incorporated in a Si-0-Cl bond wherechlorine is substitutional for oxygen. Therefore, the trappingand neutralization of sodium ions occurs only at the Si/Sf 02interface.

EXPER I MENTAL

The gathering of the bubbler and needed hardware was maderelatively simple by J.C.Schumacher Company. Upon calling thecompany for technical assistance, they decided to donate acomplete TCA bubbler system to R.I.T. This system contained allsafety valves and mass flow controllers needed for installation.Included with the system was the microprocessor control unitwhich contains mass flow controllers for the oxygen and nitrogen,an oxygen sensor and solenoids in the nitrogen lines to shut thesystem down in case of a decrease in oxygen, and temperaturesensors to control the temperature of the TCA. Also Included inthe system was the bubbler housing which holds a 500 ml bubblerand heats the TCA.

The design considerations were based on what the tube wouldbe used for, who would be using it, and safety considerations.As a gate oxide furnace, it was decided that it should have thecapability of growing chlorinated oxides, dry oxides, and itshould also be equipped with a nitrogen purge for post-oxidationanneals. Since many students would be using the furnace, thesystem should also be fairly easy to operate. Lastly, sincechlorine Is a harmful gas and that TCA breaks down in the tube to

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form HCL, a good exhaust system must be placed at the end of thetube.

RESULTS

The TCA bubbler system is installed and running. Aschematic of the system Is given In figure 1. The system worksby bubbling nitrogen through the liquid TCA, which is regulatedat 25 degrees celsius. Oxygen Is mixed with the nitrogen/TCA atthe tube. Flow rates of the gases are regulated by the FlowTemperature Controller (FTC). This unit also contains all oxygensensors and solenoids to shut the system down in case of a lackof oxygen. A nitrogen purge can also be performed If the FTC Isput into standby. To do this, a switch was installed which putsthe unit in standby and allows the electric valve to be switchedto nitrogen. With the FTC in run mode, the electric valve canonly be placed in the oxygen position. The tube has been cleanedwith the TCA and oxides grown with the system show an improvementIn mobile Ion contamination ever those grown with dry oxygenonly.

CONCLUSION

The TCA bubbler system has been installed and is working asIt was designed to. Mobile Ions in the oxides have shownimprovement, which should make it possible to fabricate betterdevices here at R.I.T.

ACKNOWLEDGEMENTS

J.C.Schumacher - for their technical sslstance end donation ofthe TCA bubbler system.

Gary Runkel - for help with the Installation of the system.Dave Fatke - for help with the setup.

REFERENCES

1. Dr. J. Monkowski, Role of Chlorine In Silicon Oxidation,Part I, article reprinted by J.C. Schumacher Company

2. Dr. J. Monkowskl, Role of Chlorine in Silicon Oxidation,Part II, article reprinted by J.C. Schumacher Company

3. Edmond J. %Jansser~s, Gilbert J. Declerck, The Use of1,1,1-TrichiorOeth n as an Optimized Additive to Improve TheSilicon Thermal Oxidation Technology, article reprinted byJ.C. Schumacher Company

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Figure 1. Schematic of TCA bubbler system

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