Patent Publication Number: US-2023139375-A1

Title: In-situ etch rate or deposition rate measurement system

Description:
BACKGROUND AND SUMMARY 
     The invention relates to an in-situ etch rate or deposition rate measurement system and a method thereof. 
     Ion beam etching is a common tool for polishing and shaping of samples of various materials, e.g. optical components or thin films. In particular, an ion beam is a type of charged particle beam consisting of ions. A variety of ion beam sources exists, where the most common ion beams are of singly-charged ions. 
     The ion beam is directed onto a sample&#39;s surface where the impact of the ions erodes the sample&#39;s surface, abrading away (etching) a certain amount of material. The amount of material that is etched away by the ion beam depends, for example, on the ion beam parameters, the material composition of the sample and the time of exposure. The etch rate is defined as the amount of material removed from the sample&#39;s surface over time. 
     The ion beam is typically focused onto the sample&#39;s surface and has a Gaussian profile with a peak (maximum of the Gaussian ion beam profile) and a full width at half maximum (FWHM). The etch profile of the sample&#39;s surface can thus also be characterized by a peak etch rate and a FWHM. 
     In prior art systems, in order to determine the etch rate of the ion beam for a specific sample, a sample probe (reference sample) is pre-characterized using an interferometric measurement device based on surface form interferometric measurements, e.g. using a Fizeau-interferometer. Subsequently the sample probe is inserted into a vacuum chamber. The sample probe is ideally of the same material as the actual sample to be exposed to the ion beam afterwards. The sample probe is exposed to the ion beam for a predetermined amount of time using predetermined beam parameters, so-called footprint etching. The sample probe is taken out of the vacuum chamber and the etched profile is subsequently measured ex-situ by the interferometric device based on surface interferometric measurements. Thus, the peak etch rate, the FWHM and the etched volume can be determined. Thereafter, the actual sample is inserted into the vacuum chamber and processed in accordance with the determined etching parameters of the ion beam. 
     However, the prior art systems require a time consuming process to determine the etch rate parameters of the ion beam as the sample probe has to be inserted into and taken out of the vacuum chamber in addition to performing the aforementioned interferometric measurements before and after ion beam etching. 
     In addition, the sample probes have a limited lifetime, because the fringes of the surface form interferometric measurements can only be resolved for a certain depth of the etching profiles. That is, after a certain amount of etching on the sample probe, the depth of the etched profile cannot be resolved anymore by the prior art interferometric measurement devices. Such sample probes have to be exchanged regularly or newly polished, which is cost intensive. 
     Moreover, in order to determine the full ion beam profile, prior art systems have to use sample probes that are three to four times the size of the ion beam diameter, which necessitates large reference objects for the interferometric measurements and restricts the metrology and is cost intensive at the same time. 
     Similar problems arise in conventional deposition methods, where the deposition rate is measured ex-situ, i.e. before inserting the sample into the vacuum chamber for material deposition and after extracting the sample from the vacuum chamber. 
     It is desirable to overcome the above mentioned deficiencies of the prior art systems. 
     According to one aspect of the present invention a system for in-situ ion beam etch rate or deposition rate measurement is provided, the system comprising: a vacuum chamber; an ion beam source configured to direct an ion beam onto a first surface of a sample located within the vacuum chamber and to etch the first surface of the sample with an etch rate; or a material source configured to deposit material onto a first surface of a sample located within the vacuum chamber with a deposition rate; and an interferometric measurement device located at least partially within the vacuum chamber and configured to direct light onto a second surface of the sample and to determine the etch rate of the ion beam or the deposition rate of the deposited material in-situ based on light reflected from the sample. 
     That is, the system may be used to either determine the ion beam etch rate or the deposition rate of a material deposition technique. The deposition technique may be any one of chemical vapor deposition, CVD, physical vapor deposition, PVD, low-pressure chemical vapor deposition, LPCVD, plasma-enhanced chemical vapor deposition, PECVD, or plasma-assisted CVD. 
     In the deposition rate determination, the material coming from a material source is deposited onto the sample&#39;s surface and the deposition rate may be determined as the thickness variation (increase) over time. In contrast, in the ion etch rate determination, the material is removed from the sample&#39;s surface and the etch rate may be determined as the thickness variation (decrease) over time. 
     As mentioned above, the interferometric measurement device may at least partially be located within the vacuum chamber, e.g. a sensor and an optical fiber may be located within the vacuum chamber, while other elements such as power supply, controller and other components of the interferometric measurement device may be located outside the vacuum chamber. 
     The interferometric measurement device may be configured to determine the etch rate while etching the first surface of the sample, preferably after a predetermined stabilization time, preferably more than 2 minutes, more preferably more than 3 minutes, even more preferably more than 4 minutes, and most preferably 4-5 minutes or longer. That is, the etch rate measurement may be performed at the same time as the etching is being performed. 
     The interferometric measurement device may be configured to determine the deposition rate while depositing the material onto the first surface of the sample. That is, the deposition rate measurement may be performed at the same time as the deposition is being performed. 
     The etch rate may be defined as the amount of material removed from the sample&#39;s first surface over time. 
     The deposition rate may be defined as the amount of material deposited onto the sample&#39;s first surface over time. 
     The interferometric measurement device may be a high precision optical low-coherence interferometer, HP-OLCI. 
     The HP-OLCI as analyzing unit may be an interlaced interferometer and may be based on optical coherence tomography, OCT, technology. It may comprise two internal and two external arms. One internal arm may be equipped with a voice coil, which provides through its throw the working field. The second internal arm may be movable to provide scan range (thickness preferably 0-135 mm). 
     One external atm may be constituted by the sensor itself within the vacuum chamber of the ion beam figuring, IBF. The second arm may be an adjustable reference. This reference may be formed by an air gap. Both external arms may provide a signal; thickness is measured by the determination of the distance between both signals. For example, a DFB laser with 1550 nm may serve as scale. 
     The reference arm may be used to set an air gap with the optical thickness of the sample. The air gap may be generated from a glass plate (e.g. fused silica), which is movable (e.g. 135 mm) and a mirror. The HP-OLCI may consider only the interference burst of the interface and may compare it with the interference burst of the interface of the air gap (transition from fused silica to air). If both signals lie on top of each other, the thickness difference is zero. If the surface of the sample is etched off by the ion beam, the interference burst moves away from the reference burst. The distance between the two is determined with high precision by the maxima of the two bursts and the DFB laser interference signal (scale). The change over time may be output in nm/s and can be measured more precisely than 0.1 nm/s. 
     In other words, HP-OLCI is a highly accurate comparative thickness measurement device. The reference signal is normally generated by a sample whose thickness is known exactly and the difference to this reference sample is to be determined. The absolute thickness is not important for the present invention. Rather, the change over time is determined. Thus, an air gap may be used, which has the advantage of being variable. 
     The second surface of the sample may be opposite the first surface of the sample. That is, one side of the sample may be etched using the ion beam and the opposite side may be used to measure the etch rate. Similarly, material may be deposited onto one side of the sample and the opposite side may be used to measure the deposition rate. 
     The interferometric measurement device may be configured to determine the thickness of the sample over time and to determine the etch rate or the deposition rate based on the thickness variation of the sample over time. 
     The interferometric measurement device may comprises a sensor configured to direct the light onto the second surface of the sample and to collect light reflected back from the sample. 
     The sensor may comprise optical components to direct the light onto the second surface of the sample and to collect light reflected back from the sample. 
     The sensor may be located within the vacuum chamber. 
     The sensor may comprise the sample. That is, according to a preferred embodiment, the sample may be located inside the sensor. 
     The sensor may comprise a housing and the sample may be located inside the housing, preferably with the optical components also located inside the housing. 
     The housing may comprise an opening and the sample may be located at the opening such that the ion beam or the material to be deposited can reach the sample&#39;s first surface through the opening. In other words, the sample&#39;s first surface may be located at the opening of the housing. 
     The housing may comprise a graphite outer surface. Thus, leaving only the sample in exposition to the ion beam or the material to be deposited. 
     The interferometric measurement device may comprise a cooling device, e.g. a copper cooling device, in contact with the sample and may be configured to cool the sample. 
     The interferometric measurement device may comprise a positioning device configured to position the light at at least one predetermined location onto the second surface of the sample. The positioning device may be a x,y,z-stage on which the optical components may be mounted so that the optical components may be moved in three dimensions. In some embodiments also a two-dimensional x,y-stage may be sufficient to move the optical components in two-dimensions, preferably in parallel to the sample&#39;s second surface. 
     The controller may be configured to control the positioning device to position the light at the second surface of the sample at a location corresponding to the center of the ion beam to determine a peak etch rate of the ion beam. 
     The peak etch rate may be defined as the etch rate at the maximum intensity of the ion beam, wherein the ion beam has a substantially Gaussian form. 
     The controller may further be configured to control the positioning device to position the light at the second surface of the sample at a plurality of locations corresponding to locations within the ion beam profile to determine an etch rate of the ion beam profile. 
     For example, by obtaining several measurements at different locations corresponding to locations within the ion beam profile, a Gaussian fit can be used to determine the volume etch rate of the ion beam profile. 
     The controller may further be configured to control the positioning device to position the light at the second surface of the sample at a plurality of locations to determine a deposition rate at the plurality of locations. 
     The system may further comprise an alignment device configured to align the ion beam with respect to the first surface of the sample. 
     The alignment device may comprise a Faraday cup and a position camera or a characterization device configured to characterize the ion beam by a point-scan and to calculate the offset to an initial start point. 
     According to a further aspect of the invention a method for in-situ ion beam etch rate or deposition rate measurement is provided. The method comprises: directing an ion beam onto a first surface of a sample located within a vacuum chamber to etch the first surface of the sample with an etch rate; or depositing material onto a first surface of a sample located within the vacuum chamber with a deposition rate; directing light of an interferometric measurement device located at least partially within the vacuum chamber onto a second surface of the sample; and determining the etch rate of the ion beam or the deposition rate in-situ based on light reflected from the sample. 
     Normally, the sample (also called sample probe) as described herein may relate to the actual sample on which the ion beam processing or the deposition is finally to be executed or to a test sample to determine the ion beam etch rate or the deposition rate. This test sample should ideally be of the same material as the actual sample on which the ion beam processing or the deposition is finally to be executed. 
     In particular, if the sample is thin enough, the present invention may be used to measure the sample while etching the sample or during material deposition onto the sample without the need for using a test sample. For example, this “live” measurement on the actual sample while etching or depositing may be used to determine the etch rate or deposition rate by determining the center thickness of an optical lense. If the temperatures are expected to be high, a thin sample, e.g. having an optical thickness of up to 135 mm, may be advantageous since heat dissipation in thin samples is easier compared to thick samples. In other words, heat on the sample may be removed more effectively from thin samples due to the fact that optical materials may tend to be bad heat conductors. 
     However, in case of a constant temperature inside the sample, there is no restriction in the thickness of the sample. In any case, it is preferable to have a constant temperature inside the sample. 
     As apparent from the above description of the present invention, the present invention provides a system and method that reduces processing time in that the interferometric measurements of the sample can be executed in-situ. Thus, time consuming insertion and extraction of the sample into and out of the vacuum chamber can be avoided. In addition, the sample does not have to be measured before and after ion beam etching or material deposition, which saves additional time. 
     In addition, by the present invention, the sample can be used (reused) more often without the necessity to exchanged or newly polish the sample due to use of a thickness based interferometric measurement and thus costs may be saved. 
     Moreover, the present invention does not require the sample to be three to four times the size of the ion beam diameter for determining the etching profile of the ion beam due to use of a thickness based interferometric measurement, which again saves costs. 
     Further, the present invention allows for etch rate measurements under a non-orthogonal impact angle. This is particularly advantageous for highly curved samples, where the respective positioning system may not be able to allow for perfect orthogonal impact angles over the whole extend of the sample. 
     More particular, the etch rate is highly influenced by the impact angle and is different between different materials. The possibility for etch rate determination for such non-orthogonal impact angles is one of the advantageous of the present invention. 
     These and other features, advantages and aspects of the present invention will become apparent from the following description of the figures outlining exemplary embodiments of the present invention. The following description of exemplary embodiments of the present invention is not to be construed limiting for the scope of the present invention. Rather, the exemplary embodiments are intended to provide some non-exhaustive examples of how to implement the present invention. It will be apparent to the skilled person that the present invention is not limited by the combination of features of the following exemplary embodiments. Rather, features of different exemplary embodiments may be combined with features of other exemplary embodiments if not explicitly stated otherwise. That is, the scope of the present invention is solely defined by the features of the independent claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Some preferred embodiments are now described with reference to the drawings, where: 
         FIG.  1    illustrates one preferred embodiment of the system of the present invention, 
         FIG.  2    illustrates another embodiment of the system of the present invention, 
         FIG.  3    illustrates a flow chart of a preferred embodiment of the method of the present invention. 
         FIG.  4    illustrates a sample probe with differently etched footprints using different ion beam parameters, 
         FIG.  5    illustrates in-situ raw etch rate measurements on a sample using different ion beam parameters, 
         FIG.  6    illustrates in-situ raw etch rate measurements on a sample, 
         FIG.  7 ( a )-( e )  illustrates a 48 hour measurement of the source stability measured each hour; 
         FIG.  8    is a graph showing the sample thickness during the geometry determination; 
         FIG.  9    illustrates the sample thickness during geometry determination, and 
         FIG.  10    illustrates another preferred embodiment of the system of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG.  1    one preferred embodiment of the system of the present invention is described.  FIG.  1    shows a system  100  for in-situ ion beam etch rate measurement. The system comprises a vacuum chamber  110 , an ion beam source  111  and an interferometric measurement device  114 . 
     The ion beam source  111  is configured to direct an ion beam  112  onto a first surface  113   a  of a sample  113  located within the vacuum chamber  110  and to etch the first surface  113   a  of the sample  113  with a certain etch rate. 
     The interferometric measurement device  114  is located at least partially within the vacuum chamber  110  and configured to direct light  115  onto a second surface  113   b  of the sample  113  and to determine the etch rate of the ion beam  112  in-situ based on light reflected from the sample  113 . 
     The interferometric measurement device  114  may be a low coherence interferometer, e.g. build up of two entangled Michelson-interferometers having a modulator arm with a scanning range of approximately 400 micrometer, a measurement range arm (adjustable range between 0-135 mm thickness), the actual measurement arm, which is adjusted on the reference sample and an adjustable reference arm, which provides the reference signal for the thickness measurement. The light sources may be a combination of super luminescence diodes with a center wavelength of 1280 nm and 1310 nm. As a measure, a distributed feedback, DFB, laser with a wavelength of 1550 nm is coupled in for the interferometric measurement of the paths. This is just one example of an interferometric measurement device that can be used with the present invention. However, the present invention is not limited thereto. Any interferometric measurement device  114  which is able to determine the thickness of the sample  113  can be used together with the present invention. The aforementioned set-up of the interferometric measurement device  114  may, however, be advantageous over other such interferometric measurement device in terms of precision. 
       FIG.  2    illustrates another embodiment of the system  100  of the present invention. In particular,  FIG.  2    illustrates the features already described with reference to  FIG.  1    and further optional features of the system  100 . 
     In particular,  FIG.  2    shows the vacuum chamber  110 , the ion beam  112 , the sample  113 , the light  115 , optical components  116 , a sensor  117 , an optical fiber  118 , a high precision optical low-coherence interferometer, HP-OLCI, rack  119 , a display and input device  120 , an ion beam figuring, IBF, control computer  121 , a network device  122 , and an adjustable thickness reference module  123 . 
     The ion beam  112 , the sample  113 , the light  115 , the optical components  116 , the sensor  117 , and partly the optical fiber  118  are located within the vacuum chamber  110 . The sample  113  and the optical components  116  are located within the sensor  117 . 
     The ion beam  112  is directed onto the sample&#39;s first surface  113   a , e.g. through an opening (not shown) in the sensor&#39;s housing. The light  115 , e.g. infrared light, is directed (focused) onto the opposite surface (second surface  113   b ) of the sample  113  by the optical components  116 , wherein the light is provided to the optical components  116  through an optical fiber  118 . 
     The sample  113  is located on an inner surface of the sensor  117 , i.e. the sensor housing. The opening (not shown) in the sensor&#39;s housing is provided such that the sample&#39;s first surface  113   a  is partially exposed to the ion beam  112 . 
     The optical components  116  are configured to focus the light  115  onto the sample&#39;s second surface  113   b  and to collect light reflected from the sample&#39;s surfaces  113   a ,  113   b . The light reflected back from the sample&#39;s surfaces  113   a ,  113   b  gives rise to a unique interference pattern. By comparing this interference pattern with an interference pattern from a comparison sample in the adjustable thickness reference module  123 , the thickness of the sample  113  can be determined. 
     For example, the interference patterns may comprise interference bursts, where the envelope of the interference bursts may be used to determine the thickness of the sample  113 . In order to increase the accuracy of the thickness determination, the distance between the interference bursts may be used to determine the thickness of the sample  113 . 
     It is even more preferable to evaluate the entire burst itself to further increase the accuracy. 
     The HP-OLCI rack  119  is connected to the optical components  116  via an optical fiber  118  and with the adjustable thickness reference module  123  via another optical fiber. The HP-OLCI rack  119  comprises the power supply, the light source(s) and respective control means for the HP-OLCI. The control means may be used to control the functionality of the interferometric measurement device  114  and may comprise a processing device, e.g. a processor, which may be configured to determine the thickness of the sample  113  based on the interferometric measurements and subsequently to determine the etch rate, i.e. the thickness variation over time. 
     The system  100  may also provide a display and input device  120  for displaying the results of said determination of the etch rate and to receive user input for controlling the different functions of the interferometric measurement device, e.g. the amplification and intensity of the light  115  onto the sample&#39;s second surface  113   b  and/or perform adjustments on the adjustable thickness reference module  123 . 
     Controlling the amplification and intensity of the light  115  is advantageous because the light reflected from the sample&#39;s surface and therefore the intensity depends on the material of the sample. In addition, the intensity can decrease with increasing lifetime of the sample. Thus, to provide for sufficient intensity of the reflected light, the amplification and intensity of the light  115  directed to the sample&#39;s second surface  133   b  can be regulated. 
     The ion beam  112 , i.e. the parameters of the ion beam  112  and the location of the ion beam  112  with respect to the sample, is controlled using the IBF control computer  121 . Although not shown in detail in  FIG.  2   , the IBF machine provides the ion beam  112  and comprises the vacuum chamber  110 . 
     Both the IBF control computer  121  and the HP-OLCI rack  119  are connected to a network  122  to provide centralized control over the ion beam  112  and the interferometric measurement device  114 . 
       FIG.  3    illustrates a flow chart of a preferred embodiment of the method of the present invention. In particular,  FIG.  3    shows a first step S 10  of directing an ion beam  112  onto a first surface  113   a  of a sample  113  located within a vacuum chamber  110  to etch the first surface  133   a  of the sample  113  with an etch rate. 
     At a second step S 20  light  115  of an interferometric measurement device  114  located at least partially within the vacuum chamber  110  is directed onto a second surface  113   b  of the sample  113 . 
     At step S 30  the etch rate of the ion beam  112  is determined in-situ based on light reflected from the sample  113 . 
       FIG.  4    illustrates a sample probe with differently etched footprints using different ion beam parameters. In particular, the sample probe&#39;s surface is characterized, i.e. interferometric surface measurements are performed on the sample probe, before inserting the sample into the vacuum chamber. Subsequently, the footprints are etched into the sample probe&#39;s surface using the ion beam. The ion beam parameters are varied as follows: a) 1000 V and 50 W (upper left footprint in  FIG.  4   ), b) 1000 V and 70 W (upper right footprint in  FIG.  4   ), c) 1000 V and 100 W (lower left footprint in  FIG.  4   ), and d) 800 V and 50 W (lower right footprint in  FIG.  4   ). In a further step the sample probe is taken out of the vacuum chamber and the interferometric surface measurements are performed again to determine the etch rate of the different footprints. This corresponds to a prior art procedure. 
     For comparison, a sample of the same material, in this case fused silica, is measured using the present invention, i.e. in-situ, while the same beam parameters are used. The raw values obtained by the in-situ measurement, i.e. the thickness over time, is illustrated in  FIG.  5    (together with the rolling mean values). Each measurement was performed for 20 minutes and a new measurement was started after 5 minutes after the previous measurement. 
     As can be seen from  FIG.  5   , the thickness initially, i.e. in the first 4-5 minutes, increases, which is due to effects of thermal expansion and change of optical index with increasing temperature. By using active cooling of the sample, it is possible to achieve stable conditions after 4-5 minutes, depending on neutralizer and beam power. 
     The beam geometry can be gained by running a point scan, i.e. measurements at different locations on the sample&#39;s surface. An initial heat up time (4-5 minutes) is needed for the first measuring-point, all other points only need 1-2 minutes heat up time to stabilize the measurement. The measurement points are preferably scanned in a spiral manner beginning in the center and advancing more outward. Thus, energy input can be kept almost constant and the required time for each measuring point can be reduced. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Peak etchrate in nm/s 
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 Interferometric 
                   
                 Error 
               
               
                 Sample 
                 ISERM 
                 method 
                 Factor 
                 in nm/s 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 1 
                 1.061378412 
                 1.1578 
                 0.916719996 
                 0.09642159 
               
               
                 2 
                 1.271374238 
                 1.2821 
                 0.991634224 
                 0.01072576 
               
               
                 3 
                 1.324530057 
                 1.3943 
                 0.949960594 
                 0.06976994 
               
               
                 4 
                 1.121599131 
                 1.129 
                 0.993444758 
                 0.00740087 
               
               
                 5 
                 1.079770321 
                 1.0722 
                 1.00706055 
                 0.00757032 
               
               
                 6 
                 1.187326918 
                 1.1944 
                 0.99407813 
                 0.00707308 
               
               
                 7 
                 1.189589273 
                 1.1962 
                 0.994473561 
                 0.00661073 
               
               
                 8 
                 1.178635264 
                 1.2135 
                 0.971269274 
                 0.03486474 
               
               
                 9 
                 1.162560841 
                 1.1449 
                 1.015425662 
                 0.01766084 
               
               
                   
               
            
           
         
       
     
     Table 1 shows nine comparison measurements between a conventional interferometric measurement on a 3 inch probe fused silica and a 1 inch 1 mm sample of the measurement device according to the present invention (in-situ etch rate measurement-ISERM). Depending on the manufacturing process of the material, the etch rate may vary significantly. Therefore, the reference sample of the measurement device should be made of the same material as the sample to be processed. Otherwise, a correction factor may be determined beforehand to account for the respective variation. 
       FIG.  6    is a beam scan graph showing the signal as well as the average values of the etch rate over time using the present invention. After an initial heat up of the sample, the etch rate decreases continuously. 
       FIG.  7 ( a )-( e )  depict a 48 hour measurement of the source stability. In particular, measurement values were taken every hour to obtain a stability of the peak etch rate (see  FIG.  7 ( d ) ), full width half max (FWHM) over time ( FIG.  7 ( b ) ) and the respective deviations thereof ( FIGS.  7 ( c ) and  7 ( e ) ).  FIG.  7 ( a )  shows one example measurement of the 48 hour measurement. As can be seen from the figures, a stable peak etch rate and FWHM was achieved during the long time measurement using the present invention. 
       FIG.  8    is a graph showing the sample thickness during the geometry determination. Again, after an initial heat up of the sample, where the sample thickness increases, the relative sample thickness decreases over time. 
       FIG.  9    illustrates etch rate measurements of different locations on the sample&#39;s surface and respective Gaussian fitting to obtain the etch rates of the ion beam profile, i.e. the volume etch rate. In  FIG.  9   , the y-axis shows the normalized etch rate values and the x-axis shows the displacement of the respective scan from the center. 
     A Gaussian fit is used to obtain the volume etch rate, which corresponds to the Gaussian profile of the ion beam. As can be seen, the present invention can be used to determine the volume etc rates in-situ. 
     The point scan functionality of the present invention permits centering of the light directed to the sample&#39;s surface with respect to the ion beam by calculating the offset from the configured position. In addition, the ion beam etch rate geometry can be visualized, similar to the electric geometry visualization with Faraday scan. By using 3D Gaussian fit, volume etch rate can be calculated. The point scan is subject to the same restrictions as the peak etch rate measurement and thus exposure time for each point has to be chosen in a process requirement matching deviation grade. 
     As apparent from the above description, in-situ measurements can be used to determine the etch rate in-situ in a short time with process matching accuracy. No cost-intensive reference probe is necessary and the locking procedure dispenses. The sample can be reused many times before it has to be replaced and the sample material can be exchanged easily. 
     With reference to  FIG.  10    another preferred embodiment of the system of the present invention is described.  FIG.  10    shows a system  100  for in-situ deposition rate measurement. The system comprises a vacuum chamber  110 , a material source  111   a  and an interferometric measurement device  114 . 
     The material source  111   a  is configured to direct material  112   a  onto a first surface  113   a  of a sample  113  located within the vacuum chamber  110  and to deposit the material  112   a  onto the first surface  113   a  of the sample  113  with a certain deposition rate. 
     The interferometric measurement device  114  is located at least partially within the vacuum chamber  110  and configured to direct light  115  onto a second surface  113   b  of the sample  113  and to determine the deposition rate of the deposited material  112   a  in-situ based on light reflected from the sample  113 . 
     The interferometric measurement device  114  may be the same one as described with reference to the etch rate determination. Thus, a further description at this point is omitted to avoid unnecessary repetitions. 
     As the present invention may be embodied in several forms without departing from the scope or essential characteristics thereof, it should be understood that the above-described embodiments are not limited by any of the details of the foregoing descriptions, unless otherwise specified, but rather should be construed broadly within the scope as defined in the appended claims, and therefore all changes and modifications that fall within the present invention are therefore intended to be embraced by the appended claims. 
     Furthermore, in the claims the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single unit may fulfil the functions of several features recited in the claims. The terms “essentially”, “about”, “approximately” and the like in connection with an attribute or a value particularly also define exactly the attribute or exactly the value, respectively. Any reference signs in the claims should not be construed as limiting the scope.