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Improve LPCVD nitride particle with film stress cracking

Posted: 10 Oct 2014 ?? ?Print Version ?Bookmark and Share

Keywords:chamber seasoning? Low Pressure Chemical Vapor Deposition? LPCVD? nitride? vertical furnace?

These days, low pressure silicon nitride deposition process in our vertical furnace is facing high defect density challenge. It has resulted not only to product yield loss but also to long process equipment down time. Product quality and delivery commitment to customers have been jeopardized. This article introduces the new concept of chamber seasoning, and discusses experiments in different process conditions on Low Pressure Chemical Vapor Deposition (LPCVD) silicon nitride ASM vertical furnace system. Nitride film stress characteristics were studied and proven with new chamber seasoning method to lower down defects density while achieving higher equipment stability.

Silicon nitrides deposited by the LPCVD vertical furnace is often used as dielectric and chemical barrier in semiconductor manufacturing process. This deposited silicon nitride filmis high tensile stress in nature which tends to flake off during process. Huge temperature gap during loading until deposited nitride film exceeds ultimate film bonding strength and causes shedding of the film particulates generated in the process chamber.

With the high complexities and criticality of process especially for automotive integrated circuit application, defect density improvement has become a big hurdle to achieving customer satisfaction. Thus, none pattern test wafer is used to monitor particle performance in LPCVD system during the process and particles on wafers are detected by none pattern dark field inspection equipment.

The deposition of silicon nitride process from the thermal reaction of dichlorosilane (SiH2Cl2) and ammonia (NH3) creates large amount of by-product in the fore line and pump line.

This process can be clearly explained by the chemical reaction of silicon nitride deposition:

3(SiH2Cl2) + 10(NH3) Si3N4 + 6(NH4Cl) + 6H2

During process, differential pressure between process chamber and pump line incur turbulence, causing back streaming of by-product into process chamber and these particles get embedded in the nitride film. Implementation of pump down purging loops in process running sequence is able to improve on baseline particle count, but particle excursion still happened without early warning or symptom due to particles flaking from deposited nitride layer in the chamber.

Ratio of gas flow DCS: NH3 at 1:4 is a typical process gas ratio used in the wafer fabrication industry. Si3N4 film is obtained from the high deposition temperature, 700-800C and low pressure, Experimentation
The experiment was carried out on ASM A400 LPCVD silicon nitride furnace. The chamber seasoning concept was based on the relationship of nitride film cracking process effectiveness to temperature effect that helped to release intrinsic and extrinsic deposited film stress. Figure 1 included four groups of film cracking experiment. Group A and B were the temperature experiment at low pressure. Group C and D were boat speed and process time variable experiment at atmospheric pressure. Each test group was prepared with three test wafers for particle performance check. Group A and B with cracking temperature target at 500C and 400C respectively; where typical nitride deposition temperature was run at 700C. Both processes started after boat load is completed and at low pressure with N2 flow, temperature ramped down from 700C to 500C and 700C to 400C respectively according to test groups. After 10 minutes of stabilisation, temperature ramped up to 700C. On the other hand, group C process started at 400C with boat speed 300mm/min during boat load step. Process continued at temperature 700C after boat load completed. As for group D, temperature ramped down to 400C and stayed for 60 minutes with process tube open before boat load started, then process continued to ramp to 700C after boat load completed with boat speed 100mm/min.

Figure 1: Evaluate four group of experiment Group A-D for effectiveness of film cracking.

Figure 2: Summary of cracking result from experiment.

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