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In situ monitoring of plasma etch processes with a quantum cascade laser arrangement in semiconductor industrial environment

Concentrations of the etch product SiF4 were measured online and in situ in technological etch plasmas with an especially designed quantum cascade laser arrangement for application in semiconductor industrial environment, the Q-MACS Etch. The combination of quantum cascade lasers and infra red absorption spectroscopy (QCLAS) opens up new attractive possibilities for plasma process monitoring and control. With the realization of a specific interface the Q-MACS Etch system is synchronized to the etch process and allows therefore automated measurements, which is important in a high volume production environment.


Introduction
Plasma technological procedures, like etching or deposition techniques play a key role for the industrial production of semiconductor devices. The challenge doing this can be found in the need of higher reproducibility when realizing structures on nanometer scales. In particular the multitude of discrete processing steps starting from the blank silicon wafer up to the final chip requires more and more methods of process monitoring to realize stable process control. Furthermore, the usage of molecular plasmas makes special demands for methods of in-situ process control.
Up to now, typically optical emission spectroscopy (OES), self excited electron resonance spectroscopy (SEERS) and mass spectrometry is applied for the online monitoring of plasma etch processes [1]. The drawback of all these methods is the lack of direct and quantitative access to etch relevant properties, like the concentration of precursors or of molecular etch products in the plasma bulk. A powerful diagnostic approach is given by infra red absorption spectroscopy with continuous sources (FTIR) [2] as well as with narrowband led salt lasers (TDLAS) [3,4] and quantum cascade lasers (QCLAS) [5][6][7][8][9][10]. The availability of quantum cascade lasers (QCL) opens up new attractive possibilities for the application of infra red absorption spectroscopy for plasma process monitoring and control. QCL emit in the MIR spectral range at room temperature, i.e. without the need of strong cooling. Because of that it is possible to design very compact MIR sources which are characterized by 3 To whom any correspondence should be addressed. With the compact and modular measurement device Q-MACS [5] it is possible to use the potential of the QCL for plasma process control or for high sensitive trace gas analysis. It combines the advantages of absorption spectroscopy in the mid infra red spectral range with the positive properties of quantum cascade lasers. In this contribution we report actual results using the Q-MACS Etch system for the in situ process monitoring of the industrial production of DRAMs. In situ measured concentration devolutions of the etch product SiF 4 demonstrate the abilities for a process control.

The Q-MACS Etch
On the basis of Q-MACS a two channel spectroscopical system was developed, which fulfills the special requirements for an application in a semiconductor environment. A picture of this Q-MACS Etch can be seen in figure 1 showing the separate optical module and the control unit in a rack. Beside compactness and integrability a new concept for the optical coupling to an industrial dual frequency, capacitive coupled, magnetically enhanced, reactive ion etcher (MERIE) was developed. For simple operation and avoiding of open optical pathways an optical coupling with a MIR fiber to the etch chamber has been realized. The fiber can be mounted in front of KBr windows either for side access or   for top access to the chamber, see figure 2 left. In case of the side access mode the infra red beam is reflected via a mirror in the liner door and coupled back into the fiber. Because of this only one optical window is necessary. This arrangement is used for the concentration measurement of etch relevant species. In case of the top access mode, the surface of the wafer itself serves as reflective element. Because of the partial transparence of silicon and silicon dioxide in the mid infra red spectral range, this arrangement can be used for an interferometric etch rate determination. The optical setup together with the calculated beam paths via ray tracing methods is shown in figure 2 on the right. The main components consist of the laser source, the detectors, opto-mechanical parts and an MIR fiber as well. With the help of a central beam splitter both the QCL emission is separated into a measurement and a reference channel and the input MIR radiation is decoupled from the reflected signal from the chamber. In the reference channel absorption lines of the reference gas C 2 H 4 are measured simultaneously in order to fix the QCL emission at a distinct spectral position with the help of a control loop (line locking). Adapted to clean room requirements and for low maintenance level thermoelectric cooled detectors are installed on board.

Spectroscopy at the etch product SiF 4
In the plasma process under study deep trench structures were etched in Si wafers using NF 3 as precursor. One of the main etch products is known to be the molecule SiF 4 . To determine the spectral absorption characteristics for the proper selection of a QCL with its narrow spectral bandwidth, reference gas cells were filled with SiF 4 and NF 3 and were measured with a FTIR [11]. Figure 3a shows the absorption band of SiF 4 at 1030cm -1 due to the ν 3 stretch of the molecule. It becomes obvious that there is a spectral overlapping with the precursor NF 3 . Taking this into account, the tuning of the QCL was performed so that there is a minimum of overlapping with the NF 3 absorption to minimize the errors in the concentration determination of SiF4. For the selected spectral range of 1028±0.3cm -1 the ratio between the absorption cross sections of both molecules is close to 90. The part of the absorption spectrum measured with the Q-MACS is shown in figure 3b. The laser line width of approximately 0.008cm -1 impedes to resolve spectrally the single rovibronic transitions. Using a numerical approach for the treatment of such complex spectra an effective absorption cross section of SiF 4 was determined to σ=7.7(±0.7)×10 -18 cm 2 molecule -1 and for NF 3 : σ=8.7(±0.8)×10 -20 cm 2 molecule -1 [11]. With the experimentally determined absorbance and the known absorption length the concentration of the absorbing species follows from the Lambert-Beer law. Because of the broad band absorption characteristic of SiF 4 in the tuning range of the QCL it was necessary to measure the . FTIR spectra of SiF 4 and NF 3 from 1005cm -1 to 1050cm -1 , the spectral resolution is 0.11cm -1 [11].

Synchronization with the etch tool
The above discussed absorption behavior of SiF 4 impedes the simultaneous detection of the background intensity of the QCL during the measurement. Having a look on the time course of the plasma etch process different states in the chamber are distinguishable and can be related to different phases of the process. This can be used for the automated determination of the concentration, as illustrated exemplarily in figure 4 for a conditioning sequence. If the liner door is open the QCL radiation is no longer reflected back from the chamber and the detected signal corresponds to an offset I off , which is important to take into account in the calculation. As soon as the liner door is closed the background intensity I 0 is measured before the process gases starts to flow in the stabilization step. During the plasma phase (ME) the signal I(t) corresponds to the actual transmission. At the end of the plasma process again it is possible to measure the background intensity I 0 2 , which can be altered because of depositions at the window or mirror. Therefore the concentration N at a time t is given by: To be able to use these different meanings of the signal a synchronization between the Q-MACS Etch and the etch chamber control tool is required. This was realized by means of a communication via a customized network interface. With the exchange of attributes it was possible to interpret at any time point of the process the signal and therefore to calculate the concentration. From the changes of the background intensities I 0 i a degradation coefficient was determined in a linear approach and the concentration corrected correspondingly. The effect of the degradation on the concentration can be seen in figure 5 exemplarily. On the left side the signal course is shown as recorded from the measurement detector from batch processed wafers with the corresponding degradation coefficient. The sequence was measured during the etching of 25 masked wafers for deep trench structures after seasoning the chamber. Every etching process was followed by a cleaning cycle consisting of a special clean process for the cathode (ICClean) and a cleaning of the chamber (DryClean). In the right graph of figure 5 both SiF 4 concentration values are shown, the uncorrected in grey and with degradation taken into account in black. During the etching processes the differences become significant whereas the effect is less pronounced during the cleans.  With this the concentrations of the etch product SiF 4 can be transferred to the tool and used for process monitoring or control purposes. The synchronization with the etch tool fulfils therefore an important requirement for the online process monitoring with Q-MACS in typically automated high volume production environments.

Monitoring of the Si etch process
In the experiment reported here 25 masked Si wafers were processed for the plasma etching of deep trench structures in a batch mode. Between the etching sequences the chamber was cleaned with an ICClean and a DryClean procedure. Before the etching procedure the chamber was conditioned twice via a process-like plasma and blank Si wafers. Taken into account the above discussed degradation the resulting concentration courses were like in figure 6 presented. The individual traces were overlaid and normalized with respect to their starting time in order to compare the devolution of the SiF 4 concentration for each wafer. For reason of clarity all 25 graphs are split into two graphs. As already recognizable in the signal traces in figure 5 (left) the development of the SiF 4 concentrations during the deep trench etch processes are affected by a seasoning which is not present in the subsequent clean cycles, see figure 7. Furthermore, for the latter the SiF 4 concentrations were detected stable and  reproducible since the beginning of the batch. There is a time jitter in the concentration values due to different spans between two successive processes caused by the tool control. In case of the deep trench processes it takes up to 8 cycles before the concentration courses becomes more reproducible. This is seen as a clear indication of chamber conditioning effects, i.e. the balancing of chemical reactions is dominated by plasma wall interactions. In dependence of the prehistory it takes several process cycles until stable conditions prevail in the etch chamber. Because of that the measured concentration courses show a stronger scattering at the beginning of the experiment. Moreover, also negative values were detected for the SiF 4 concentration, which is an artifact and does not have any physical meaning. The appearance is related to an enhanced background during the process. With increasing number of processed wafers the concentration courses become more and more reproducible as it can be seen in the right graph in figure 6. During the etch process SiF 4 is built up to 6×10 13 cm -3 which corresponds to a partial pressure of ca. 1%. Significant different concentration values were detected for the wafer no. 22 with about 30% higher SiF 4 concentrations in the first fourth of the etch process. As in case of wafer no. 6 the previous cleaning procedure failed and was stopped from the tool control. Again the amount of detectable SiF 4 during the etch process depends sensitive on the chamber conditioning.   bottom void show an analog distribution as the trench depth, as it can be seen on the right graph. Again the influence of the failed clean cycles becomes apparent.

Summary
A quantum cascade laser system Q-MACS Etch was developed for process monitoring in the semiconductor industry environment. For the first time one of the major etch products SiF 4 was monitored in situ in silicon plasma etch processes used for the production of DRAM modules. Based on the determination of an effective absorption cross section it was possible to measure online the molecular SiF 4 concentration during the process. In order to evaluate the reproducibility and the stability of the plasma processes the measurements were expanded to a batch processing of 25 silicon wafers. The required synchronization with the etch process was realized via a customized network interface. With this the Q-MACS Etch communicates with the etch tool providing the automation of data acquisition. The usage of a special MIR fiber in combination with an internal mirror is an innovative approach to ensure robustness and flexibility in an industrial production environment. The first application of a quantum cascade laser arrangement for monitoring of industrial etch processes has opened up a challenging new option for control of demanding semiconductor production. Focused on sensitive and fast concentration measurements of molecular key components while ensuring compactness, robustness and long term stability, this new class of process control equipment has the potential to become implemented also into other fields of plasma technology.