Final ReportSummary - THERMOGRIND (Thermally Controlled Rotational Grinding of Sapphire Wafers for Highly Efficient Manufacturing of Modern White LED Light Sources)
The market for discrete light emitting diodes (LEDs) is very fast growing. Basic materials for white and blue LEDs are wafers of silicon carbide (SiC) and sapphire (Al2O3). Today European enterprises, most of them small and medium-sized enterprises (SMEs0, hold less than 5 % of worldwide wafer production due to high production costs compared to the main producers Russia, United States of America (USA) and Japan.
The most time consuming and therefore expensive process steps in production of sapphire wafers are lapping and polishing. This process chain can be significantly shortened by substituting lapping by grinding as grinding allows for a much better surface quality in a shorter time. As a result, the time needed for polishing will be diminished as well. This will be a decisive step for the European wafer manufacturers to gain significant shares in a highly profitable market.
As grinding of silicon wafers, basic material for red and yellow LEDs, today is state of the art, grinding of sapphire wafers fails due to the variability and the interaction of individual effects during the grinding process. Due to this reasons, a direct measurement of the in-process parameters within the contact zone between work piece and grinding wheel is of eminent interest for developing a stable grinding process.
The THERMOGRIND concept allows the measurement of one of the key process parameters, the grinding temperature. For this, the optical transparency of sapphire for infrared radiation initiated in the contact zone is exploited. Following this principle, an innovative infrared transparent wafer clamping system (chuck) was developed. The system allows temperature measurement by capturing the infrared radiation transmitted through wafer and clamping system. In a second project step, a temperature-based loop control of the grinding process was developed in order to achieve optimum process stability at large-scale production conditions.
Project context and objectives:
In the fast growing market for discrete light emitting diodes devices, the reduction of lead time and an increase of cost efficiency are the main targets of any company involved. The main substrate material for the manufacturing of the gallium nitride based lighting devices is single crystalline sapphire (Al2O3) with c-plane orientation. Within the manufacturing chain of the disk shaped substrates - called wafers -, chemical mechanical polishing is applied as a final step in order to achieve the required microscopic surface quality regarding roughness and crystal defect density. The required polishing efforts are comparatively high and are strongly dependent on the surface quality achieved in the upstream planarisation process. Compared to state-of-the-art applied lapping processes, the rotational grinding process offers the potential of generating superior surface quality in less time.
Due to the extremely high hardness of the sapphire material, conventional grinding process development is a true challenge. For this reason, the THERMOGRIND approach focused on the development of an innovative process control system. This system is based on the in-process measurement of the grinding temperature within the contact zone. The obtained process information can be used for grinding process development in two different ways. First of all, improved process understanding enables enhanced grinding tool development. Secondly, process stability can be increased by real time process control based on thermal information. Within the THERMOGRIND project, all of the following development objectives towards the realization of the thermal control system were achieved:
- Prototype sensor integrated vacuum chuck for sapphire wafers: The system enables temperature measurement by capturing the infrared (IR) radiation from the process which is transmitted through the vacuum chuck.
- Prototype of a thermal loop control system for rotational grinding, which includes all hardware and software components for the application of loop control algorithms: In data acquisition mode, the system can be used for in-process temperature measurement and therewith facilitates process and grinding tool development.
- Proof of concept of thermally controlled wafer grinding process: The loop controlled process is established by integrating loop control algorithms and criteria into the achieved loop control system.
Project results:
The feature specification for the THERMOGRIND system was defined at the beginning of the project. For this, all project partners delivered detailed input from their area of expertise. One requirement that could not be defined from available literature or previous work was the expected temperature range for the in-process measurement. For this, a test bench was realised at Fraunhofer IPT in order to perform analogy tests on a lathe under realistic grinding conditions. A thermal imaging camera was installed to detect the temperature distribution within the contact zone. Different grinding wheels and process parameters were analyzed in order to cover a broad application range. During the full project duration, the feature specification document was effectively used as a guideline for the development and research activities performed in the different work packages (WPs).
RTD Partner DIEGM from the University of Udine performed a study on suitable IR sensors for the integration into the chuck system. Based on given product specification and manufacturer's advice, a pre-selection of systems was made. In addition, the required interfaces of the data acquisition chain were analyzed in due consideration of the available G&N machine tool and the IR sensor. Due to diverse incomparable system limitations of the pre-selected sensors, application tests under grinding conditions were performed in order to determine the best suitable system. For this, an additional rigid test setup within the G&N machine tool offering free optical access to the grinding area was realised.
The selected sensor system, consisting of three highly sensitive single colour pyrometers, provided the starting point for the downstream design process of the chuck system. Here, a major challenge was faced in the integration of an additional function into the chuck system. Next to the functions of rotating and clamping the wafer, the optical accessibility of the contact zone under grinding conditions is required. Several different design concepts regarding the optical path of the emitted IR radiation were developed. A thorough evaluation of the concepts finally lead to the selection of the 'hollow spindle' principle. Following this principle, the core element of the chuck system, a 4'' IR-transparent vacuum chuck made of single crystalline sapphire is mounted on a hollow spindle. In this way, the three pyrometers installed at the rear end of the spindle obtain direct access to any preferred position on the wafer.
A major challenge in the realization of the chuck system was faced in the manufacturing of the sapphire vacuum chuck. The required geometrical features for the vacuum function in addition the high form accuracies required by the wafer grinding application played an important role. Here, the expertise of TKF Frömgen regarding the manufacturing of complex ceramic components came in to play. Another critical element in the manufacturing of the chuck system was defined in the rotary joint for the vacuum supply. Due to the hollow spindle concept, the rotary joint is positioned on a larger diameter of the spindle. As a result, a rather complex oil based sealing solution was developed.
In the first nine months of the project, the G&N MultiNano machine tool (available at Fraunhofer IPT) was already prepared for the integration of the chuck system. Whereas the mechanical preparation was limited to the disassembly of one of the three existing chuck systems, the preparation of the machine control required extensive modifications. Since direct access to the existing control system is not possible, a more complex solution composed of several hard- and software components was developed. A THERMOGRIND PC which is able to control the spindle and chuck rotational speed as well as the feed rate of the G&N machine tool was realised.
After the chuck system components were manufactured, the spindle unit was assembled and integrated into the G&N machine tool. Functionality tests were performed in order to verify the system design and the achieved precision in manufacturing. Parallel to the mechanical developments, the data acquisition chain was prepared. Starting from the applied sensor system, primary signal processing elements were designed and a DAC software solution with integrated data analysis module was developed. After the installation of the chuck system, the pyrometers were installed below and aligned with selected radial positions within the contact zone.
In order to determine temperatures from the detected IR radiation directly, the emissivity coefficient e of the examined body must be set on the pyrometers. For the examined contact zone, this parameter is unknown. In addition, the vacuum chuck as well as the wafer act like a filter, depending on their geometrical and surface properties. In order to consider all of these effects, system calibration was performed by determining calibration curves for a variety of configurations. The calibration curves describe the relation between the measured temperature value Tpyr for the e=1 and the real grinding layer temperature detected by a thermocouple attached to it.
Achieving the objective of a thermally controlled wafer grinding process required a solid technological data basis combined with thorough process understanding. For this reason, the performance of extensive grinding tests at Fraunhofer IPT played an eminent role within the THERMOGRIND project. In a first stage of the work, grinding tools developed by Atlantic Diamond were tested regarding the dependencies of process output parameters like material removal, wheel wear, surface integrity and grinding power on process input parameters like cutting speed, feed rate and chuck speed. After the sensor integrated chuck system was installed, tests were extended to the temperature measurement in three radial positions on the wafer. The dependency of the temperature signals from the machining parameters as well as their correlation with the grinding power were analysed.
Based on the obtained data, thermal control criterions for the grinding process were derived. Within an iterative process, different controlling strategies were developed, implemented into the control software and evaluated by performing grinding tests. The obtained results show that controlling the infrared thermal emission, cutting power and specific energy when grinding sapphire wafers is possible by regulating the feed speed and the spindle speed during the process.
In order to obtain a realistic and objective view on the project results in terms of technological and economical impact, a benchmark activity was performed during full project duration. CrystalQ, the industrial partner involved in sapphire wafer manufacturing evaluated the quality of the ground wafers produced by comparing them to the standard industrial lapping process and by post-processing them with the standard industrial polishing process. CrystalQ defined the quantitative specification of state of the art lapped wafers and analysed the impact on the subsequent polishing process. Based on this reference, the evaluation of the grinding process was performed and the potentials of the technology were derived.
Potential impact:
The exploitation, dissemination and implementation of the projects outputs are essential elements of Seventh Framework Programme (FP7) projects. In order to structure, schedule and document these activities a plan for use and dissemination of foreground was developed and updated regularly in course of the project. Being a first measure for dissemination of the project, the project's homepage (see http://www.thermogind.eu online) was developed and regularly updated. In addition to the website, a project technology flyer and a number non-confidential results of the developed technology were published by the RTD performers. For the latter, following measures we taken:
- poster and system presentation at the Aachener Werkzeugmaschinen Kolloquium in Aachen, Germany (May 2011);
- paper and oral presentation at the International Conference on Advanced Manufacturing Systems and Technology in Mali Losinj, Croatia (June 2011);
- oral presentation at the GrindTec Exhibition in Augsburg, Germany (March 2012);
- paper and poster presentation at the Euspen International Conference in Stockholm, Sweden (June 2012).
The developed sapphire vacuum chuck is one of the key components of the THERMOGRIND chuck spindle system. Within the project the SME partner TKF obtained the technological know-how for the manufacturing steps of the vacuum chuck. Application tests that were performed proved the full functionality of the chuck regarding clamping the wafer and offering optical transmissivity to the IR radiation. As a result, TKF will be able to commercially supply sapphire chucks for the THERMOGRIND system directly after the end of the project.
For the chuck spindle system, the data acquisition chain and the machine tool integrated loop control system, the THERMOGRIND project delivered the proof of concept. Herewith, SME partner G+N becomes the possibility to commercially offer the system as an additional module for their grinding machines. Due to the modular character of the system, also existing machine tools installed at customers' facilities can additionally be equipped with the system. Some of the mechanical components of the system might to a certain extent require some additional engineering efforts in order to ensure or even improve the durability of the system. Due to the duration of the project, no long term tests covering these issues could be performed. Regarding the data analysis and loop control system, further software related development efforts are required to achieve a 'easy to use' and 'fool proof' qualification. In addition, the interface communication between the THERMOGRIND control system and the machine tool control must be adapted to the different machine tool control systems that are applied and supported by SME partner G+N. The mentioned development steps for achieving a market-ready system are estimated to be performed within 18 months after the end of the project.
SME partner ATL can benefit from the developed system by applying it for the further and more efficient development of grinding wheels. The additional information from the grinding process gained by means of the temperature measurement is of significant interest for the understanding of superimposed cutting and wear mechanisms. A copy of the prototype system that was realised within the project can be installed into the grinding machine tool available at ATL. In this way, the company can directly benefit from the project achievements and further build on the technological know-how gained within the THERMOGRIND project. A time frame of 12 months is estimated for the according implementation phase.
Project website:
http://www.thermogrind.eu
RTD partners
Fraunhofer Institute for Production Technology IPT (Coordinator)
Maurice Herben
Steinbachstraße 17
52074 Aachen
Germany
http://www.ipt.fraunhofer.de
University of Udine
Department of Electrical, Management and Mechanical Engineering DIEGM
Prof. Elso Kuljanic
Via delle Scienze, 208
33100 Udine
Italy
http://www.diegm.uniud.it
SME partners
G&N Genauigkeits Maschinenbau Nürnberg GmbH
Gerhard Könnemann
Wetterkreuz 35
91058 Erlangen
Germany
http://www.grinders.de
Atlantic Diamond Ltd.
Mícheál Ó Ceallaigh
Docklands Innovation Park, East Wall Road
Dublin 3
Ireland
http://www.atlanticdiamond.com
CrystalQ
Peter Rust
Electronicaweg 1
9503 GA Stadskanaal
the Netherlands
http://www.crystalq.nl
TKF Technische Keramik Frömgen GmbH
Gerhard Frömgen
Dieselstr. 3
41352 Korschenbroich-Glehn
Germany
http://www.tkf-froemgen.de
The most time consuming and therefore expensive process steps in production of sapphire wafers are lapping and polishing. This process chain can be significantly shortened by substituting lapping by grinding as grinding allows for a much better surface quality in a shorter time. As a result, the time needed for polishing will be diminished as well. This will be a decisive step for the European wafer manufacturers to gain significant shares in a highly profitable market.
As grinding of silicon wafers, basic material for red and yellow LEDs, today is state of the art, grinding of sapphire wafers fails due to the variability and the interaction of individual effects during the grinding process. Due to this reasons, a direct measurement of the in-process parameters within the contact zone between work piece and grinding wheel is of eminent interest for developing a stable grinding process.
The THERMOGRIND concept allows the measurement of one of the key process parameters, the grinding temperature. For this, the optical transparency of sapphire for infrared radiation initiated in the contact zone is exploited. Following this principle, an innovative infrared transparent wafer clamping system (chuck) was developed. The system allows temperature measurement by capturing the infrared radiation transmitted through wafer and clamping system. In a second project step, a temperature-based loop control of the grinding process was developed in order to achieve optimum process stability at large-scale production conditions.
Project context and objectives:
In the fast growing market for discrete light emitting diodes devices, the reduction of lead time and an increase of cost efficiency are the main targets of any company involved. The main substrate material for the manufacturing of the gallium nitride based lighting devices is single crystalline sapphire (Al2O3) with c-plane orientation. Within the manufacturing chain of the disk shaped substrates - called wafers -, chemical mechanical polishing is applied as a final step in order to achieve the required microscopic surface quality regarding roughness and crystal defect density. The required polishing efforts are comparatively high and are strongly dependent on the surface quality achieved in the upstream planarisation process. Compared to state-of-the-art applied lapping processes, the rotational grinding process offers the potential of generating superior surface quality in less time.
Due to the extremely high hardness of the sapphire material, conventional grinding process development is a true challenge. For this reason, the THERMOGRIND approach focused on the development of an innovative process control system. This system is based on the in-process measurement of the grinding temperature within the contact zone. The obtained process information can be used for grinding process development in two different ways. First of all, improved process understanding enables enhanced grinding tool development. Secondly, process stability can be increased by real time process control based on thermal information. Within the THERMOGRIND project, all of the following development objectives towards the realization of the thermal control system were achieved:
- Prototype sensor integrated vacuum chuck for sapphire wafers: The system enables temperature measurement by capturing the infrared (IR) radiation from the process which is transmitted through the vacuum chuck.
- Prototype of a thermal loop control system for rotational grinding, which includes all hardware and software components for the application of loop control algorithms: In data acquisition mode, the system can be used for in-process temperature measurement and therewith facilitates process and grinding tool development.
- Proof of concept of thermally controlled wafer grinding process: The loop controlled process is established by integrating loop control algorithms and criteria into the achieved loop control system.
Project results:
The feature specification for the THERMOGRIND system was defined at the beginning of the project. For this, all project partners delivered detailed input from their area of expertise. One requirement that could not be defined from available literature or previous work was the expected temperature range for the in-process measurement. For this, a test bench was realised at Fraunhofer IPT in order to perform analogy tests on a lathe under realistic grinding conditions. A thermal imaging camera was installed to detect the temperature distribution within the contact zone. Different grinding wheels and process parameters were analyzed in order to cover a broad application range. During the full project duration, the feature specification document was effectively used as a guideline for the development and research activities performed in the different work packages (WPs).
RTD Partner DIEGM from the University of Udine performed a study on suitable IR sensors for the integration into the chuck system. Based on given product specification and manufacturer's advice, a pre-selection of systems was made. In addition, the required interfaces of the data acquisition chain were analyzed in due consideration of the available G&N machine tool and the IR sensor. Due to diverse incomparable system limitations of the pre-selected sensors, application tests under grinding conditions were performed in order to determine the best suitable system. For this, an additional rigid test setup within the G&N machine tool offering free optical access to the grinding area was realised.
The selected sensor system, consisting of three highly sensitive single colour pyrometers, provided the starting point for the downstream design process of the chuck system. Here, a major challenge was faced in the integration of an additional function into the chuck system. Next to the functions of rotating and clamping the wafer, the optical accessibility of the contact zone under grinding conditions is required. Several different design concepts regarding the optical path of the emitted IR radiation were developed. A thorough evaluation of the concepts finally lead to the selection of the 'hollow spindle' principle. Following this principle, the core element of the chuck system, a 4'' IR-transparent vacuum chuck made of single crystalline sapphire is mounted on a hollow spindle. In this way, the three pyrometers installed at the rear end of the spindle obtain direct access to any preferred position on the wafer.
A major challenge in the realization of the chuck system was faced in the manufacturing of the sapphire vacuum chuck. The required geometrical features for the vacuum function in addition the high form accuracies required by the wafer grinding application played an important role. Here, the expertise of TKF Frömgen regarding the manufacturing of complex ceramic components came in to play. Another critical element in the manufacturing of the chuck system was defined in the rotary joint for the vacuum supply. Due to the hollow spindle concept, the rotary joint is positioned on a larger diameter of the spindle. As a result, a rather complex oil based sealing solution was developed.
In the first nine months of the project, the G&N MultiNano machine tool (available at Fraunhofer IPT) was already prepared for the integration of the chuck system. Whereas the mechanical preparation was limited to the disassembly of one of the three existing chuck systems, the preparation of the machine control required extensive modifications. Since direct access to the existing control system is not possible, a more complex solution composed of several hard- and software components was developed. A THERMOGRIND PC which is able to control the spindle and chuck rotational speed as well as the feed rate of the G&N machine tool was realised.
After the chuck system components were manufactured, the spindle unit was assembled and integrated into the G&N machine tool. Functionality tests were performed in order to verify the system design and the achieved precision in manufacturing. Parallel to the mechanical developments, the data acquisition chain was prepared. Starting from the applied sensor system, primary signal processing elements were designed and a DAC software solution with integrated data analysis module was developed. After the installation of the chuck system, the pyrometers were installed below and aligned with selected radial positions within the contact zone.
In order to determine temperatures from the detected IR radiation directly, the emissivity coefficient e of the examined body must be set on the pyrometers. For the examined contact zone, this parameter is unknown. In addition, the vacuum chuck as well as the wafer act like a filter, depending on their geometrical and surface properties. In order to consider all of these effects, system calibration was performed by determining calibration curves for a variety of configurations. The calibration curves describe the relation between the measured temperature value Tpyr for the e=1 and the real grinding layer temperature detected by a thermocouple attached to it.
Achieving the objective of a thermally controlled wafer grinding process required a solid technological data basis combined with thorough process understanding. For this reason, the performance of extensive grinding tests at Fraunhofer IPT played an eminent role within the THERMOGRIND project. In a first stage of the work, grinding tools developed by Atlantic Diamond were tested regarding the dependencies of process output parameters like material removal, wheel wear, surface integrity and grinding power on process input parameters like cutting speed, feed rate and chuck speed. After the sensor integrated chuck system was installed, tests were extended to the temperature measurement in three radial positions on the wafer. The dependency of the temperature signals from the machining parameters as well as their correlation with the grinding power were analysed.
Based on the obtained data, thermal control criterions for the grinding process were derived. Within an iterative process, different controlling strategies were developed, implemented into the control software and evaluated by performing grinding tests. The obtained results show that controlling the infrared thermal emission, cutting power and specific energy when grinding sapphire wafers is possible by regulating the feed speed and the spindle speed during the process.
In order to obtain a realistic and objective view on the project results in terms of technological and economical impact, a benchmark activity was performed during full project duration. CrystalQ, the industrial partner involved in sapphire wafer manufacturing evaluated the quality of the ground wafers produced by comparing them to the standard industrial lapping process and by post-processing them with the standard industrial polishing process. CrystalQ defined the quantitative specification of state of the art lapped wafers and analysed the impact on the subsequent polishing process. Based on this reference, the evaluation of the grinding process was performed and the potentials of the technology were derived.
Potential impact:
The exploitation, dissemination and implementation of the projects outputs are essential elements of Seventh Framework Programme (FP7) projects. In order to structure, schedule and document these activities a plan for use and dissemination of foreground was developed and updated regularly in course of the project. Being a first measure for dissemination of the project, the project's homepage (see http://www.thermogind.eu online) was developed and regularly updated. In addition to the website, a project technology flyer and a number non-confidential results of the developed technology were published by the RTD performers. For the latter, following measures we taken:
- poster and system presentation at the Aachener Werkzeugmaschinen Kolloquium in Aachen, Germany (May 2011);
- paper and oral presentation at the International Conference on Advanced Manufacturing Systems and Technology in Mali Losinj, Croatia (June 2011);
- oral presentation at the GrindTec Exhibition in Augsburg, Germany (March 2012);
- paper and poster presentation at the Euspen International Conference in Stockholm, Sweden (June 2012).
The developed sapphire vacuum chuck is one of the key components of the THERMOGRIND chuck spindle system. Within the project the SME partner TKF obtained the technological know-how for the manufacturing steps of the vacuum chuck. Application tests that were performed proved the full functionality of the chuck regarding clamping the wafer and offering optical transmissivity to the IR radiation. As a result, TKF will be able to commercially supply sapphire chucks for the THERMOGRIND system directly after the end of the project.
For the chuck spindle system, the data acquisition chain and the machine tool integrated loop control system, the THERMOGRIND project delivered the proof of concept. Herewith, SME partner G+N becomes the possibility to commercially offer the system as an additional module for their grinding machines. Due to the modular character of the system, also existing machine tools installed at customers' facilities can additionally be equipped with the system. Some of the mechanical components of the system might to a certain extent require some additional engineering efforts in order to ensure or even improve the durability of the system. Due to the duration of the project, no long term tests covering these issues could be performed. Regarding the data analysis and loop control system, further software related development efforts are required to achieve a 'easy to use' and 'fool proof' qualification. In addition, the interface communication between the THERMOGRIND control system and the machine tool control must be adapted to the different machine tool control systems that are applied and supported by SME partner G+N. The mentioned development steps for achieving a market-ready system are estimated to be performed within 18 months after the end of the project.
SME partner ATL can benefit from the developed system by applying it for the further and more efficient development of grinding wheels. The additional information from the grinding process gained by means of the temperature measurement is of significant interest for the understanding of superimposed cutting and wear mechanisms. A copy of the prototype system that was realised within the project can be installed into the grinding machine tool available at ATL. In this way, the company can directly benefit from the project achievements and further build on the technological know-how gained within the THERMOGRIND project. A time frame of 12 months is estimated for the according implementation phase.
Project website:
http://www.thermogrind.eu
RTD partners
Fraunhofer Institute for Production Technology IPT (Coordinator)
Maurice Herben
Steinbachstraße 17
52074 Aachen
Germany
http://www.ipt.fraunhofer.de
University of Udine
Department of Electrical, Management and Mechanical Engineering DIEGM
Prof. Elso Kuljanic
Via delle Scienze, 208
33100 Udine
Italy
http://www.diegm.uniud.it
SME partners
G&N Genauigkeits Maschinenbau Nürnberg GmbH
Gerhard Könnemann
Wetterkreuz 35
91058 Erlangen
Germany
http://www.grinders.de
Atlantic Diamond Ltd.
Mícheál Ó Ceallaigh
Docklands Innovation Park, East Wall Road
Dublin 3
Ireland
http://www.atlanticdiamond.com
CrystalQ
Peter Rust
Electronicaweg 1
9503 GA Stadskanaal
the Netherlands
http://www.crystalq.nl
TKF Technische Keramik Frömgen GmbH
Gerhard Frömgen
Dieselstr. 3
41352 Korschenbroich-Glehn
Germany
http://www.tkf-froemgen.de