Thin film deposition control system incorporating an optical cavity.

Abstract

The invention relates to a control system for the deposition of thin films by gas-phase growth in a deposition reactor (10), the gas phase being a gaseous state or a plasma state. The control system comprises: - a hollow cathode lamp (2); - an optical cavity (3) having at least two mirrors (31), and configured such that light emitted by the light source (2) is trapped in the optical cavity (3) and interacts with the gas phase, resulting in the deposition of thin films; and - a light analysis system (4) configured to analyze light emitted by the hollow cathode lamp (2) and passing through the optical cavity (3) by measuring the optical absorbance of at least one species of the gas phase through which said light passes. Figure for the abstract: Fig. 3

Description

Title of the invention: Thin film deposition control system comprising an optical cavity.

[0001] The invention relates to the deposition of thin layers. It relates more particularly to a system for controlling such deposition.

[0002] A thin film is an element whose one of the dimensions, the thickness, is nanometric or micrometric, ranging from a few atomic planes to a few thousand atomic planes. Thin films are obtained by depositing a material on a substrate.

[0003] The deposition of thin layers is used in many industrial fields (optics, chemistry, mechanics, micro-optoelectronics, renewable energies).

[0004] Thin film deposition processes use known technologies, including: molecular beam epitaxy (MBE), laser ablation type systems, cathode sputtering or electron beam evaporation, chemical vapor deposition (CVD) systems, etc.

[0005] The performance of many components / devices and their manufacturing costs depend directly on the level of control of the thin film deposition processes.

[0006] Various systems exist for controlling the growth of thin films. One of the principles consists of determining the composition of the gas phase used for the deposition of the thin film (directly linked to the composition and growth rate of the thin film) by measuring the optical absorbance of the species that compose it. In other words, it is a question of measuring the ratio of the intensity of an optical beam partially transmitted by the gas phase (denoted I in the following) to that of the incident optical beam (denoted Io in the following). The species composing the gas phase exhibit a certain number of absorption lines in specific spectral ranges, as illustrated in [Fig.l]. This figure gives the line strengths of several species of interest for the manufacture of thin films, in particular for applications in micro-optoelectronics.Line strength characterizes the propensity of a chemical species to absorb optical radiation at a given wavelength: at equal concentration in the gas phase, optical absorption increases linearly with line strength.

[0007] OFM sensors for “optical flux monitoring” are based on this principle of measuring optical absorbance and comprise a light source and an atomic absorption spectrometer. For example, document US20150284851 disclosing such an OFM sensor is known.

[0008] OFM sensors use hollow cathode lamps (HCL) as their light source. These lamps emit a set of lines characteristic of a single element. Their emission spectrum is discrete: the lines are separated from each other by spectral regions where the emitted intensity is zero.

[0009] An example of an OFM sensor according to the prior art is shown in [Fig. 2]. The light emitted by the HCL lamp is separated into two paths: a measuring beam and a reference beam. The measuring beam (denoted Meas) makes one passage through the gas to be analyzed. The reference beam passes, for example, outside the reactor. The two paths are then analyzed in a light analysis system. The intensity of the Meas path corresponds to I, that of the Ref path to Io.

[0010] [Fig.2] shows two channels. This number of channels depends on the number of elements to be measured. There are as many Meas and Ref channels as there are elements to be measured.

[0011] The spectral width of the absorption lines to be measured, ranging from a few tenths of a pm to a few pm, is difficult to resolve with conventional monochromators. The advantage of HCL lamps is that they emit lines with spectral characteristics very close to those of the absorption lines to be measured, thus removing the constraint on the necessary resolution.

[0012] On the other hand, HCL lamps emit a low flux of photons per unit area, and produce an extended and relatively divergent beam, a large part of which is lost in the low numerical aperture elements of the optical path (fibers, spectrometer, in particular). Consequently, the number of photons arriving at the detector is limited, which limits the signal-to-noise ratio of the measurement (SNR) with an impact on the accuracy and sensitivity of the sensor.

[0013] The performance of conventional OFM sensors is also intrinsically limited by the very principle of measurement: the interaction length between the light beam and the gas phase to be analyzed, limited to a few tens of centimeters, leads to relatively low absorbances, and therefore to relatively high measurement uncertainties. The best relative uncertainties on the control of the composition and growth rates reported for OFM sensors are of the order of tens of % and at best a few % (cf. [Y. Du et al. 2014]), for growth rates of the order of a few monolayers per minute. This is well below the precision required for thin film deposition processes (uncertainty on thickness and composition less than a %).

[0014] The invention aims to provide a thin film deposition control system that is more efficient and more precise in controlling the growth of thin films.

[0015] The invention proposes for this purpose a system for controlling the deposition of thin layers by gas phase growth, the gas phase corresponding to a gaseous state or a plasma state. The control system comprises: - a hollow cathode lamp; - an optical cavity comprising at least two mirrors, and configured so that light emitted by the light source is trapped in the optical cavity and interacts with the gas phase giving rise to the deposition of thin layers; and - a light analysis system configured to analyze light emitted by the hollow cathode lamp and passing through the optical cavity by measuring the optical absorbance of at least one species of the gas phase passed through by said light.

[0016] Thanks to its configuration and in particular to that of the optical cavity, the invention makes it possible to achieve considerable sensitivities, in particular for low growth speeds, and therefore very high precision in the control of growth processes.

[0017] Since the gas phase to be analyzed is located in the optical cavity, the light is reflected by the mirrors. Thus, instead of interacting only once with the gas as in existing OFM systems, the light makes a large number of round trips (up to 1000 passages). This considerably increases the light / gas interaction length, and consequently the absorbance and therefore the signal-to-noise ratio of the measurement (SNR), thus leading to a strong reduction in measurement uncertainty. A precision on the measurement of the growth rate better than % is thus easily achieved, even at low growth rates.

[0018] Particularly convenient preferred features of the thin film deposition control system according to the invention are presented below.

[0019] The optical cavity surrounds the gas phase.

[0020] The optical cavity is confocal.

[0021] The mirrors are assembled on optical accesses of the reactor. The mirrors are arranged inside or outside the reactor.

[0022] The length of the optical cavity is chosen so that the spectral width of the absorption lines measured by the light analysis system covers a minimum number of resonance modes, said minimum number preferably being equal to 10.

[0023] The light analysis system may be a spectrometer.

[0024] The spectrometer comprises a medium resolution monochromator of the order of a nanometer.

[0025] The spectrometer is equipped with a camera, preferably a CCD or CMOS camera.

[0026] The light analysis system may comprise a photodetector and a demultiplexer, the photodetector comprising a device for time or frequency multiplexing of light signals received from the hollow cathode lamp, the demul triplexer being configured to separate said light signals received by the photodetector.

[0027] Other features and advantages of the invention will become apparent in the description below with reference to the appended drawings, given by way of non-limiting example: - [Fig.l] represents transition intensities of certain elements in the UV-visible range; - [Fig.2] represents an OFM sensor of the prior art; - [Fig.3] represents a thin layer deposition control system according to a first embodiment of the invention; and - [Fig.4] represents the thin layer deposition control system according to a second embodiment of the invention.

[0028] The aim of the invention is to determine the composition of a gas phase used for the deposition of thin layers by measuring the optical absorbance of species composing the gas phase.

[0029] The term gas phase means a gaseous state or a plasma state. The gas may in particular be in a molecular regime.

[0030] The gas phase corresponds to the atmosphere containing the precursors in which a substrate is placed for growth. This can in fact be a gas or a plasma whose pressure is between 105 Torr and atmospheric pressure, for growth processes of the evaporation, sputtering, CVD or laser ablation type, but also a set of molecular beams whose pressure can drop to 10 10 Torr or less, for growth processes of the molecular beam epitaxy type.

[0031] The invention uses a thin-film deposition control system 1 for determining the composition of the gas phase. The control system 1 is used in a gas-phase deposition reactor 10.

[0032] The layer deposition control system 1 comprises at least one light source 2. The light source 2 is a hollow cathode lamp or HCL lamp.

[0033] HCL 2 lamps emit a set of lines characteristic of a single chemical element constituting the cathode. Therefore, one HCL 2 lamp is generally required per element to be measured. In the exemplary embodiments of Figures 3 and 4, the control system 1 comprises two HCL 2 lamps. Two elements are therefore measured in these non-limiting examples.

[0034] The emission spectrum of the HCL lamp 2 is discrete. In other words, the lines are separated from each other by spectral regions where the emitted intensity is zero. The spectral characteristics of these lines, and in particular their spectral width, are very close to those of the absorption lines. This makes it possible to remove the constraint on the resolution. The spectral selectivity of the control system 1 must simply allow to separate the lines of the different elements to be measured.

[0035] The light emitted by each HCL 2 lamp is separated into a measurement channel Meas and a reference channel Ref. The reference channel is used to correct the variations in intensity of the HCL 2 lamp.

[0036] According to another exemplary embodiment, the light source may be a multi-element cathode HCL lamp.

[0037] The layer deposition control system 1 further comprises an optical cavity 3 and a light analysis system 4.

[0038] The optical cavity 3 surrounds the gas phase giving rise to the deposition of thin layers. For this purpose, the optical cavity 3 is configured so that light emitted by the light source 2 is trapped in the optical cavity and interacts with the gas phase.

[0039] The optical cavity 3 comprises at least two mirrors 31.

[0040] Since the gas phase to be analyzed is located in the optical cavity, the light is reflected by the mirrors. Thus, instead of interacting only once with the gas as in existing OFM systems, the light makes a large number of round trips (up to 1000 passages). This considerably increases the light / gas interaction length, and consequently the absorbance and therefore the signal-to-noise ratio of the measurement (SNR), thus leading to a strong reduction in measurement uncertainty. A precision on the growth rate measurement better than % is thus easily achieved, even at low growth rates.

[0041] The mirrors 31 may be arranged inside the reactor 10 (as in FIGS. 3, 4) or outside the reactor 10. The mirrors 31 are assembled on optical accesses (not shown) of the reactor 10. For example, the mirrors 31 may be assembled to the reactor 10 by means of mounting flanges.

[0042] The mounting flanges are preferably protected from parasitic deposits that may form on the mirrors 31 and which would alter the optical properties of the optical cavity 3. The mounting flanges are for example placed back from the gas phase. Alternatively or in addition, differential pumping systems near the mirrors 31 may be installed to avoid parasitic deposits. Alternatively or in addition, the mounting flanges may be heated to avoid parasitic deposits.

[0043] The reflectivities R of the mirrors 31 may be different. Preferably, the mirrors 31 have the same reflectivity value R.

[0044] The optimal reflectivity of the mirrors 31 depends on the order of magnitude of the absorbance to be measured. If several lines with very different absorbances are to be measured simultaneously, a compromise must be made on the reflectivity R.

[0045] The radii of curvature Rc of the mirrors 31 may be different. Preferably, the mirrors 31 have the same value of radius of curvature Rc.

[0046] The radius of curvature Rc is chosen so that the optical cavity 3 is optically stable.

[0047] The optical cavity 3 has a length L. The length L of the optical cavity 3 is between a few tens of cm and 3 m.

[0048] The optical cavity 3 has a free spectral interval ISLcav dependent on the length L of the cavity. The free spectral interval ISLcav decreases when the length L increases.

[0049] The length L of the optical cavity 3 and therefore the free spectral interval ISLcav are configured so that the spectral width of the absorption lines measured by the light analysis system 4 covers a minimum number of resonance modes. Preferably, the free spectral interval ISLcav must be sufficiently small so that the spectral width of the absorption line to be measured contains at least 10 resonance modes of the cavity. This makes it possible to average the noise linked to the discretization of the absorption line. The spectral width of the lines to be measured is between 2 and 10 GHz.

[0050] According to an advantageous example, the optical cavity 3 is confocal. The radius of curvature Rc is equal to the length L of the optical cavity 3.

[0051] For a confocal cavity, the free spectral interval ISLcavest given by the expression c / 2L, where c is the speed of light. An optical cavity 3 of length L greater than 30 cm makes it possible to minimize the free spectral interval ISLcavest and thus to respect the aforementioned condition of at least 10 resonance modes of the cavity. An optimal spectral distribution of the resonance modes of the optical cavity 3 is notably obtained.

[0052] As seen in the figure, the signals Measl, Meas2 are sent into the reactor 10. The signals Measl, Meas2 pass through the gas phase to be analyzed. The signals Refl, Ref2 pass outside the reactor 10 without passing through the gas phase to be analyzed.

[0053] The light analysis system 4 is configured to analyze light emitted by the hollow cathode lamp 2 and passing through the optical cavity 3 by measuring the optical absorbance of at least one species of the gas phase passed through by the light.

[0054] The light analysis system 4 may be a spectrometer, as is the case in the example of [Fig. 3]. The spectrometer comprises a medium-resolution monochromator 40. The monochromator typically comprises two mirrors and a diffraction grating. The focal length of the monochromator is generally between 5 cm and 40 cm, which makes it possible to obtain resolutions between 0.5 nm and 5 nm. The wavelength range accessible by the monochromator, linked to the efficiency of the diffraction grating, is of the order of several hundred nm.

[0055] The Meas and Ref signals are spectrally separated by the monochromator 40.

[0056] The spectrometer is equipped with a 4L camera. The Meas and Ref channels are spatially separated on the camera 41 for each channel. The camera 41 reproduces the geometry according to which the Meas and Ref channels were injected into the monochromator 40. This is achieved, for example, by means of a bundle of two optical fibers whose cores are placed one above the other. Another technique consists of placing the fiber cores next to each other as described in US patent 11226288.

[0057] The camera 41 may be a CCD camera or a CMOS camera. The camera 41 preferably has a read noise of less than a few e- RMS, and a low dark current (< 0.5 e- / pix / s) to maximize the signal-to-noise ratio of the SNR measurement. The camera 41 is preferably large to be able to measure as wide a spectral range as possible.

[0058] According to another exemplary embodiment shown in [Fig.4], the light analysis system 4 comprises a photodetector 42.

[0059] The photodetector 42 comprises a device for time or frequency multiplexing of the light signals (here Ref 1, Ref 2, Meas 1 and Meas 2) received from each hollow cathode lamp 2. Modulation is carried out for example by means of optical choppers. An optical chopper is provided per Meas channel and per Ref channel.

[0060] The light analysis system 4 further comprises a demultiplexer 43. The demultiplexer 43 is configured to separate the light signals received by the photodetector 42. Thus, the channels Ref 1, Ref 2, Meas 1 and Meas 2 are separated by time or frequency multiplexing.

[0061] The control system according to the invention allows, thanks to the insertion of an optical cavity around the gas phase to be measured, the light to make numerous passages through the gas phase. The number of passages of the light in the gas phase depends in particular on the reflectivity of the mirrors. The signal-to-noise ratio of the measurement and the accuracy of the measurement are thus improved.

[0062] Bibliography

[0063] [Y. Du et al. 2014]: Du, Yingge, et al. "Self-corrected sensors based on atomic ab sorption spectroscopy for atom flux measurements in molecular beam epitaxy." Applied Physics Letters 104.16 (2014): 163110.

Claims

Claims

1. System for controlling the deposition of thin layers by gas phase growth in a deposition reactor (10), the gas phase corresponding to a gaseous state or a plasma state, the control system comprising: - a hollow cathode lamp (2); - an optical cavity (3) comprising at least two mirrors (31), and configured so that light emitted by the light source (2) is trapped in the optical cavity (3) and interacts with the gas phase giving rise to the deposition of thin layers; and - a light analysis system (4) configured to analyze light emitted by the hollow cathode lamp (2) and passing through the optical cavity (3) by measuring the optical absorbance of at least one species of the gas phase passed through by said light.

2. A control system according to claim 1, wherein the optical cavity (3) surrounds the gas phase.

3. A control system according to claim 1 or claim 2, wherein the optical cavity (3) is confocal.

4. Control system according to one of claims 1 to 3, in which the mirrors (31) are assembled on optical accesses of the reactor (10), said mirrors (31) being arranged inside or outside the reactor (10).

5. Control system according to one of claims 1 to 4, in which the length (L) of the optical cavity (3) is chosen so that the spectral width of the absorption lines measured by the light analysis system (4) covers a minimum number of resonance modes, said minimum number preferably being equal to 10.

6. Control system according to one of claims 1 to 5, wherein the light analysis system (4) is a spectrometer.

7. A control system according to claim 6, wherein the spectrometer comprises a monochromator (40) with medium resolution of the order of a nanometer.

8. A control system according to claim 6 or claim 7, in which the spectrometer is equipped with a camera (41), preferably a CCD or CMOS camera.

9. Control system according to one of claims 1 to 5, wherein the light analysis system (4) comprises a photodetector (42) and a demultiplexer (43), the photodetector (42) comprising a device for time or frequency multiplexing of light signals (Measl, Meas2, Refl, Ref2) received from the hollow cathode lamp (2), the demultiplexer (43) being configured to separate said light signals (Measl, Meas2, Refl, Ref2) received by the photodetector (42).

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