Wolfgang Lehnert

In situ spectroscopic ellipsometry for advanced process control in vertical furnaces

Abstract For the first time, a spectroscopic ellipsometer (SE) has been integrated as an in situ layer thickness sensor into a vertical batch furnace for industrial LPCVD layer deposition. A SOPRA MOSS-OMA SE was selected because of its high accuracy and versatility. In the vertical furnace, the SE can be used for in situ sensing of the layer growth as well as for post-process control of the batch. The adaptation of the SE to the furnace was performed with only minor modifications to the furnace geometry. The light beam of the SE is guided through the base plate into the furnace tube and directed onto the wafer by quartz glass prisms operating in total internal reflection mode. This arrangement introduces a well-defined additional phase shift in the polarization state of the light, which can be calculated and subtracted from the measured phase shift. The system was used in the first step to determine the optical reference data for crystalline silicon, silicon oxide and silicon nitride as a function of temperature. These data were implemented in the refractive index library of the in situ SE for endpoint monitoring and control of layer composition in the second step. The arrangement of the in situ SE also enables post-process measurements on selected wafers of the batch during the unloading sequence. In situ as well as post-process data were used by the furnace for immediate and automated correction of parameter settings. Rapid process optimization and real-time control of integrated multilayer processing have been demonstrated to be the major benefits of the novel real-time SE technique.

In situ layer characterization by spectroscopic ellipsometry at high temperatures

The demand for increased cost-effectiveness in semiconductor manufacturing is the driving force for the development of in situ and in-line measurement tools. Some of the most critical manufacturing steps are high-temperature processes such as thermal oxidation and chemical layer deposition. Solutions for accessing batch furnace processes by high-temperature single wavelength and spectroscopic ellipsometry for layer thickness and composition control have been proposed and studied intensively in the past. These techniques require comprehensive knowledge of the optical parameters at high temperatures. Therefore, a systematical study has been started to determine the optical high-temperature data (refractive index, extinction coefficient) of relevant semiconductor materials. Moreover, optical data of amorphous and polycrystalline silicon at high temperature are under investigation. All measurements were performed with a spectroscopic ellipsometer integrated in a vertical LPCVD-batch furnace. Optical access is...

In situ measurement of the crystallization of amorphous silicon in a vertical furnace using spectroscopic ellipsometry

Abstract The in situ measurement of the crystallization of amorphous silicon at 600°C was carried out during the annealing of amorphous silicon-on-oxide samples inside a vertical furnace using spectroscopic ellipsometry. The ellipsometer arrangement was adapted to the furnace geometry using a special beam-guiding system to have a minimum impact on the furnace process performance. Modifications in the furnace geometry were restricted as far as possible, to show that a fast integration in an existing industrial equipment with minor costs can be done. The dielectric function of the changing structure was calculated using the Bruggeman-effective medium approximation (B-EMA). Two optical models were compared. In the first model, the crystallinity was described by the ‘mixture’ of single-crystalline silicon and amorphous silicon. A better fit was obtained using the second model, in which fine-grained polycrystalline silicon and amorphous silicon are used. The high-temperature reference data were obtained by a measurement on the annealed samples at the beginning and at the end of the annealing process. Using this method the crystallization can be monitored by the change of the ratio of amorphous and fine-grained polycrystalline silicon in the best-fit model.

In situ spectroscopic ellipsometry for the characterization of polysilicon formation inside a vertical furnace

Abstract A spectroscopic ellipsometer was integrated in a vertical furnace for measurement and control during chemical vapor deposition and thermal oxidation processes. The major goal of this activity was to adapt the ellipsometer arrangement to the furnace geometry with a minimum impact on the furnace process performance. Modifications in the furnace geometry were restricted as far as possible, to show that a fast integration in existing industrial equipment with minor costs can be done. This aim led to a novel beam-guiding system. This setup has been used for the in situ characterization of polysilicon formation, inside the vertical furnace during high temperature processes. The polysilicon-on-oxide structures were modeled using the Bruggeman effective-medium approximation (B-EMA). The crystallization process of amorphous silicon layers during annealing at 600°C has been monitored. Layer thickness and the degree of crystallinity can be obtained simultaneously. The crystallinity can be described in the optical model using a ‘mixture’ of amorphous silicon and single-crystalline silicon in the B-EMA.

Advanced process control system for vertical furnaces

For the first time, a layer thickness sensor has been integrated into a vertical furnace for in situ sensing of the layer growth as well as for post process control of the batch. Because of its high accuracy and versatility, an in situ spectroscopic ellipsometer (SE) was selected. The adaption of the SE to the vertical furnace was performed with only minor modifications to the furnace geometry. The ellipsometer light beam is guided through the base plate into the furnace tube and directed onto the wafer by quartz glass prisms operated in total internal reflection (TIR) mode. This arrangement introduces an additional phase shift in the polarization state of the light which can be determined and subtracted from the measured phase shift. The ellipsometer setup is mechanically fixed to the boat loader. SE measurements therefore can be performed with the wafer boat out of the furnace tube as well as during the process run with the wafer boat inserted. The SE thus can be used for in situ end-point detection during layer growth and also for post process measurements on selected wafers of the batch during the unloading sequence. A realtime controller and a run-by-run controller integrated into the furnace controller utilize the layer thickness data measured in the in situ and in the post process mode, respectively, for immediate and automated correction of the parameter settings during the actual process run and for the following process run. The advantages of this novel furnace control system such as a reduction in time needed for process optimization, the avoidance of monitor wafers and a better control in integrated multilayer processing can be beneficial for future thermal batch/minibatch processing.

Equipment and wafer modeling of batch furnaces by neural networks

In semiconductor manufacturing there is a great demand for innovations towards higher cost-effectiveness. The increasing employment of advanced control systems for process and equipment control is one means to improve manufacturing processes effectively and, hence, to lower costs. A precondition for an accurate and fast control is the availability of process models. In this paper neural networks are applied to non-linear system identification as an alternative or addition to physical models. Neural empirical models are developed with the help of measured input and output data of a system or process. After a brief summary of the theory of neural networks their application to system identification is described in detail. The capabilities of the neural network models are demonstrated by several examples. The temperature dynamics of a vertical furnace for the oxidation of 300 mm wafers as well as the zone temperatures of a 150 mm LPCVD furnace are simulated and the results are verified by measurements. Moreover, in order to control wafer temperatures in batch furnaces, an appropriate model was developed and implemented in a model- based controller.