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Automatic Control of an ASIC Fabrication Sequence as Demonstrated in the Plasma Etch Area

Objective

The objective of ACAFS was to develop a methodology and to acquire know-how for a wafer fabrication sequence control system. Because of its critical nature, the plasma etch area was the one chosen in which to implement the methodology. The work involved s everal domains, such as optical sensor integration for "smart" information collection, plasma etching process modelling for control and supervision purposes, communication between equipment manipulators and computers, as well as hardware and software for real-time processing control. Recent powerful techniques in model identification, multivariable process control and artificial intelligence were used to efficiently address the various control, supervision, and decision-making problems related to the proposed hierarchical control structure.
In the field of thin film industries, the in situ control of layer thickness has become of great importance in monitoring, in real time, the process parameters during processing or for stopping the process when layers have reached the desired thickness. The technique developed is based on the measurement of the spectral reflectivity of a multilayer sample irradiated with a white light beam. The spectrum of the reflected light is sent to a computer which computes the optical parameters of the thin film stack and especially the thickness. The incident and reflected light are driven through optical fibres.
This technique presents several advantages such as resolution, sensitivity, ease of use and cost over the other current methods.

The objective was to develop a methodology for a wafer fabrication sequence control system. Because of its critical nature, the plasma etch area was the one chosen in which to implement the methodology. Recent powerful techniques in model identification, multivariable process control and artificial intelligence were used to address the various control, supervision, and decision making problems related to the proposed hierarchical control structure. Software was developed to control the plasma etch process parameters online, in a closed loop. The purpose of the controller software was to improve process reliability and reactor yield. Work on extracting the parameters affecting the yield and on developing models, using statistical process control (SPC) methods, to predict etch process results from process parameters was completed. Effort was devoted to transparent layer plasma etching processes. Effort was devoted to transparent layer plasma etching processes. An in situ layer thickness probe based on white light interferometry and a single point interferometer for end point detection were designed. The highly sensitive metallization etching was chosen as the process to be used in the final demonstrator of the whole automatic control concept. Optical emission spectroscopy was studied for in situ etch rate and uniformity monitoring purposes, and it was demonstrated that it can be successfully used for this purpose. Added emphasis was put on enhancing the fault detection and machine maintenance scheduling capabilities of the system. Appropriate supervision software was developed. The closed loop control concept was validated. An evaluation of various automated product tracking and transfer approaches was made. An overview of the different techniques available for in situ wafer surfacestemperature measurement during plasma etching will be provided.
Software was developed to control the plasma etch process parameters online, in a closed loop. The software runs on a PC connected to the etcher via a SECS II link. The PC receives information such as etch-rate or uniformity from an in situ sensor and compares this with a theoretical model in order to adjust the process parameters in real time. The purpose of the controller software was to improve process reliability and reactor yield.
Work on extracting the parameters affecting the yield and on developing models, using statistical process control (SPC) methods, to predict etch process results from process parameters was completed. The necessary studies were realised on appropriate teststructures.
During the first two years, effort was devoted to transparent (SiO2 and polysilicon) layer plasma etching processes. An in situ layer thickness probe based on white light interferometry and a single-point interferometer for end-point detection were designed.
Based on the experience of the first two-year period, the highly sensitive metallisation etching was chosen as the process to be used in the final demonstrator of the whole automatic control concept. Optical emission spectroscopy was studied for in situ etch-rate and uniformity monitoring purposes, and it was demonstrated that it can be successfully used for this purpose.
Over the last 16-month period added emphasis was put on enhancing the fault detection and machine maintenance scheduling capabilities of the system. Appropriate supervision software was developed. The closed loop control concept was validated at Plasma Technology on an RIE reactor. The whole automated control system and its impact on yield was planned to be demonstrated at the ES2 production line on a new ECR etcher under development by Plasma Technology. However, due to technical problems in the etcher development, the demonstration was not possible, and the project was terminated four months before its planned completion-date.
An evaluation of various automated product tracking and transfer approaches was made in the first two years. Furthermore, an overview of the different techniques available for in situ wafer surface temperature measurement during plasma etching will be provided.
Exploitation
The layer thickness and end-point detection probes developed will be commercialised by Bertin & Cie and Leybold Heraeus, respectively. Plasma Technology (now called Oxford Plasma Technology) is already using the PC controller developed on all its new lineetchers. The user companies, Mietec and ES2, developed methodologies and tools for plasma etch process modelling and general statistical process control, which they are using routinely in new process development.
Bertin & Cie is planning to market the two software packages on equipment maintenance and fault detection it developed, and is also actively looking for partners to implement the closed loop control system in a production environment.

Coordinator

Bertin & Cie
Address
15 Rue Fredéric Joliot
13762 Les Milles
France

Participants (4)

European Silicon Structures SA
France
Address
Zone Industrielle
13106 Rousset
LEYBOLD HERAEUS GMBH
Germany
Address
Siemensstrasse 100
8755 Alzenau
MIETEC NV
Belgium
Address
Westerring
9700 Oudenaarde
Plasma Technology Ltd
United Kingdom
Address
North End Yatton
BS19 4AP Bristol