Abstract:
A thin film is etched by irradiating charged particles to a surface of the thin film. An etching time of the thin film is measured by observing a change in intensity of secondary charged particles emitted by etched portions of the thin film. A thickness of the thin film is calculated in accordance with the measured etching time.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for measuring film thickness of a thin film formed in, for example, a semiconductor manufacturing process. 
     2. Background Information 
     Conventionally, the following methods are known as methods for measuring the thickness of a thin film which is formed in a semiconductor manufacturing process: 
     (1) In the method shown in FIG.  5 (A), the film thickness of a thin film is measured using an ellipsometer. In this measuring method, incident light IO irradiates a transparent thin film  512  placed on a substrate  511 , and the intensity of the reflected light l 1  is measured. At this time, the reflected light l 1  is interference light between the light l 11  reflected on the upper surface of the transparent thin film and the light l 12  reflected on the lower surface thereof. Therefore, the intensity of the reflected light is maximized when a difference in an optical path length between the light l 11  and the light l 12  is a value an integer multiple of times larger than the irradiation light wavelength λ. Here, a difference in an optical path length L is obtained from L=2d/cos θ, wherein θ is an incident angle of the light I/O and D is a film thickness. Therefore, by obtaining an incident angle θ when λ=L is held, the film thickness D of a transparent thin film can be obtained. 
     (2) According to the method shown in FIG.  5 (B), a needle is placed on the surface of a thin film for measuring film thickness. In this measuring method, steps are formed between respective thin films  521 ,  521 , which constitute a laminated film, using etching techniques. Then, a needle  522  is placed on the surfaces of the respective thin films  521 ,  521 , and the positions of the needle  522  tip on the respective surfaces are detected for measurement of the film thicknesses of the respective thin films  521 ,  521 . 
     (3) According to the method shown in FIG.  5 (C), a film thickness is measured through observation of an etching cross section  531  formed on the thin film. In this method, an etching cross section  531  is initially formed on a laminated film by using, for example, a focusing ion beam device etc. Then, the cross section is observed by using a scanning electron microscope or a transmission electron microscope etc, so that a film thickness is measured. 
     (4) According to the method shown in FIG.  5 (D), component analysis is applied to secondary ions while etching a laminated film. In the example. shown in FIG.  5 (D), Si ions are discharged while etching a silicon thin film  541 , and Al ions are discharged while etching an aluminum thin film  542 . Therefore, etching times for the thin films  541 ,  542  can be known through component analysis applied to the secondary ions in parallel to the etching. Also, etching rates of the thin films  541 ,  542  are measured by using another method. Then, using the etching times and rates, the film thicknesses of the thin films  541 ,  542  are calculated. 
     The above described thin film measuring methods, however, have following drawbacks. 
     The method (1) suffers from the drawbacks that the film thickness of a thin film which is not photo transmissive cannot be measure, and that the film thickness of a thin film smaller than a light wavelength cannot be measured. For example, this method cannot measure the film thickness of an oxide film of about 10 nm, which as been recently used in practice for semiconductor devices. 
     The method (2) has a drawback that a larger burden is imposed on sample production due to very complicated etching processing. Therefore, it is substantially impossible to apply this method to a thin film having a complicated laminated structure. 
     The method (3) has a defect that the film thickness of a thin film smaller than the resolution of a scanning or transmission electron microscope cannot be measured. For example, as the limit of the resolution of a scanning electron microscope is 1 to 3 nm, the thickness of a thin film thinner than 1 to 3 nm cannot be measured using this method. This method has another drawback in that it requires an expensive measurement device when using a transmission electron microscope. 
     The method (4) has a drawback in that it requires an expensive measurement device as it requires component analysis. 
     The present invention has been conceived to overcome the above problems of the related art, and aims to provide a film thickness measuring method capable of measuring the film thickness of a very thin film and realized using an inexpensive measurement device. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided a film thickness measuring method using a focused ion beam device, comprising the steps of etching a thin film by irradiating charged particles to a surface of the thin film; measuring a change as time passes of strength of secondary charged particles discharged from the thin film during the step of etching; calculating an etching time of the thin film, using a point at which the strength changes quickly; and determining a film thickness of the thin film using the etching time. 
     A measurement method relative to the present invention can be realized using a very inexpensive device, compared to a case where an etching time is detected through component analysis, as an etching time is detected through observation of secondary charged particles. In addition, as a film thickness is determined using an etching time, resolution can be improved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a conceptual view showing a structure of a focusing ion beam device relative to a preferred embodiment. 
     FIG. 2 is a cross section showing a laminated structure of a sample used in this embodiment. 
     FIG. 3 is a graph showing a change as time passes of secondary electronic strength observed in a first preferred embodiment. 
     FIG. 4 is a graph showing a change as time passes of secondary ion strength observed in a second preferred embodiment. 
     FIGS.  5 (A) through (D) are conceptual views explaining a conventional film thickness measuring method. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, preferred embodiments of the present invention will be described referring to the accompanying drawings. Note that the size and shape of and positional relationships between the respective structural components are shown schematized to an extent not hindering understanding of the invention. Also, conditions regarding values described below are merely examples. 
     First Embodiment 
     A thin film measuring method relative to a first preferred embodiment of the present invention will be described referring to FIG.  1  through FIG.  3 . 
     FIG. 1 is a diagram showing a structure of a focused ion beam device  100  used in this embodiment. 
     A liquid metal ion source  110  discharges, from the tip end thereof, metal ions such as gallium ions. 
     A capacitor lens  121  of an ion optical system  120  causes an electric field to deflect an ion beam  150  to form parallel beams. 
     A beam blanker  122  causes an electric field to deflect an ion beam  150  when irradiation of an ion beam  150  to a sample  160  is not desired. 
     An aligner  123  causes an electric field to adjust an axis of an ion beam  150 . 
     A movable diaphragm  124  has a plurality of piercing holes with different diameters, and the diameter of the ion beam  150  is adjusted using any of the piercing holes. A piercing hole to be used is selected by using a driving mechanism (not shown). 
     A stigmeter  125  causes an electric field to adjust a beam shape such that an irradiation surface of the ion beam  150  becomes circular. 
     An object lens  126  adjusts a focal length through electric field strength such that the ion beam  150  is focused on the surface of the sample  160 . 
     A deflector  127  scans an irradiation position of the ion beam  150 , using an electric field. 
     A secondary electron detector  130  detects secondary electrons caused when the surface of the sample  160  is irradiated by the ion beam  150 . 
     A gas gun  140  sprays stack or etching gas, supplied from a gas supplier mechanism (not shown) to the surface of the sample  160 . 
     FIG. 2 is a cross section showing an example of a sample used in a thin film measuring method according to this embodiment. 
     As shown in FIG. 2, the sample  160  comprises a thin film formed by sequentially forming an SiO 2  thin film layer  202 , an Al thin film layer  203 , and an SiO 2  thin film layer  204  on a substrate  201 . Hereinafter, each of thin films  202 - 204  will be referred to as “thin film”. 
     A method for measuring the film thickness of a sample  160  using a focused ion beam device  100  will next be described. 
     {circle around (1)} A part to be etched of a sample  160  is determined, and the sample  160  is positioned such that the determined part is etched. Preferably, a part free from an IC, etc. is selected as a part to be etched. For example, a part close to an alignment mark of the sample  160  may be selected as a part to be etched. 
     {circle around (2)} Then, discharging of Ga+ ions from a liquid metal ion source  110  is started, and spraying of etching gas from a gas gun  140  is also started whereby etching processing is started. With this etching processing, an etching hole  205  is formed on the sample  160  at a predetermined etching rate (see FIG.  2 ). 
     Here, this is little restriction placed on the kind of metal ion that can be used, though ions having little influence on the sample  160  are preferred. For example, Ga+ ions can be used. 
     The current of the ion beam.  150  is set such that its dose amount in a unit time is constant, though it may be continuous or intermittent. 
     An electric current value and an acceleration voltage of the ion beam  150  are set such that etching processing is carried out at a predetermined etching rate. An etching rate employed in measurement of this embodiment is not particularly restricted. With a large etching rate, film thick measurement can be completed in a short time even when thin films  202  to  204  are thick. On the other hand, with a small etching rate, film thickness measurement can be achieved with improved accuracy as a change, as time passes, of an amount of secondary electron discharge can be observed in detail even when the thin films  202  to  204  are very thin. 
     An incident angle of the ion beam  150  with respect to the sample  160  is not particularly restricted, and may generally be vertical. 
     A diameter of the ion beam  150  is not particularly restricted. However, with too large a beam diameter, an IC formation area, etc. of the sample  160  may possibly be destroyed. Therefore, the beam diameter is desired to be sufficiently small with respect to a pattern size of an IC, etc. 
     The diameter of an etching hole  205  is also not particularly restricted in this embodiment. However, with a larger beam diameter, measurement accuracy can be improved as secondary electrons, generated in the etching hole  205 , can more easily be discharged to the outside of the hole. Therefore, the diameter of the etching hole  205  may be adjusted by adjusting the ion beam  150 . 
     {circle around (3)} In parallel with the above described etching processing (step 2), secondary electron detection is also applied. When etching using a focused ion beam device  100 , secondary electrons are generated in a part to be etched. The secondary electrons generated are partly discharged from the etching hole  205 , reaching the secondary electron detector  130 . The secondary electron detector  130  observes a change as time passes of the strength of the secondary electrons. 
     FIG. 3 is a graph showing a change as time passes of the strength of secondary electrons, with the horizontal axis indicating time and the vertical axis indicating the strength of secondary electrons (a standard value). 
     In FIG. 3, time t 1  is a time at which the etching hole  205  (see FIG. 2) reaches the surface of the Al thin film  203 . That is, at time t 1 , etching is completed with the SiO 2  thin film  204 , and started with the Al thin film  203 . As known from FIG. 3, the strength of secondary electrons is much larger while etching the Al thin film  203  than that while etching the SiO 2  thin film  204 . Therefore, an etching end time t 1  with respect to the SiO 2  thin film  204  can be known from a quick or abrupt increase of the strength of secondary electrons. 
     In FIG. 3, time t 2  is a time at which the etching hole  205  reaches the surface of the SiO 2  thin film  202 . That is, at time t 2 , etching is completed with the Al thin film  203 , and started with the SiO 2  thin film  202 . As described above, the strength of secondary electrons is smaller while etching the SiO 2  thin film  204  than that while etching the Al thin film  203 . Therefore, an etching end time t 2  with respect to the Al thin film  203  can be known from a quick or abrupt decrease of the strength of secondary electrons. 
     Times t 1 , t 2  can be detected through value execution processing using output data from the secondary electron detector  130 . That is, by detecting a point at which the strength of secondary electrons is changed, through this execution processing, times t 1 , t 2  can be detected. 
     Also, times t 1 , t 2  can be detected by using a high pass filter. That is, an output signal from the secondary electron detector  130  is input to a high pass filter, and times at which a peak waveform is output from the filter may be determined as times t 1 , t 2 . 
     {circle around (4)} Next, a time spent for etching the SiO 2  thin film  204  and that for etching the Al thin film  203  are calculated using times t 1 , t 2 . As known from FIG. 3, an etching time for the SiO 2  nthin film  204  is a difference between an etching start time and time t 1 , i.e., t 1 . Also, an etching time for the Al thin film  203  is a difference between time t 1  and time t 2 , i.e., t 2 −t 1 . 
     Then, the film thicknesses of the thin films  204 ,  203  are calculated through multiplication of these etching times and rates. 
     Here, an etching rate can be obtained, for example, through measurement of an enticing time of a sample with a known thickness in a manner similar to the above described processes {circle around (1)} through {circle around (3)}. 
     Note that whether the thin films  203 ,  204  are good/no good may be determined using an etching time, rather than calculating a film thickness value thereof. That is, etching times for good (i.e., a semiconductor device guaranteed for normal operation) thin films  203 ,  204  are measured in advance to be used as comparison values, and etching times, obtained through the above step (4), are compared with the comparison values, whereby whether or not the film thickness is within a tolerance range can be determined. 
     {circle around (5)} Thereafter, the etching hole  205  may be refilled, if necessary, to complete film thickness measurement. 
     As described above, according to this embodiment, film thickness can be measured using a very inexpensive device as an etching time is detected through observation of secondary electrons, compared to a case where an etching time is detected through component analysis (see related art (4)). In addition, as a conventional focusing ion beam device can be used intact for film thickness measurement, a device which is inexpensive also in this view can be used. 
     Also, a film thickness measurement method according to this embodiment can measure the film thickness of a film which is not photo transmissive, as well as the film thickness of a film smaller than a light wavelength or resolution of a scanning or transmission electron microscope. 
     Further, the film thickness of a thin film can be measured in a short time as formation of steps in a sample is unnecessary. 
     Second Embodiment 
     Next, a thin film measuring method in a second preferred embodiment of the present invention will be described referring to FIG.  4 . 
     A structure of a focused ion beam device used in this embodiment is substantially identical to that of the device  100  used in the first preferred embodiment (see FIG.  1 ), which, however, is different in that the device in the second embodiment uses a secondary ion detector (not shown) in the place of a secondary electron detector  130 . 
     A sample similar to the sample  160  (see FIG. 2) used in the first preferred embodiment is used as a sample for film thickness measurement. 
     In the following, a film thickness measurement method according to this embodiment will be described. 
     {circle around (1)} Similar to the first preferred embodiment, a part to be etched of a sample  160  is determined, and the sample  160  is positioned such that the part is etched. 
     {circle around (2)} Further, similar to the first preferred embodiment, Ga+ ions discharging and etching gas spraying are started to thereby start formation of an etching hole  205  (see FIG.  2 ). 
     Note that the type of usable metal ions, a current value, an acceleration voltage, an incident angle, and a beam diameter of an ion beam  150  can be determined similar to the first preferred embodiment. 
     {circle around (3)} In parallel to the above described etching processing (step 2), secondary ion detection is carried out. When etching using a focused ion beam device  100 , secondary ions are generated in the part to be etched. The secondary ions generated are partially discharged from the etching hole  205 , and reach a secondary ion detector. The secondary ion detector observes a change as time passes of the strength of secondary ions. 
     FIG. 4 is a graph showing a change as time passes of the strength of secondary ions, wherein the horizontal axis shows time and the vertical axis shows secondary ion strength (a regular value). 
     In FIG. 4, time t 1  is a time at which the etching hole  205  reaches the surface of the Al thin film  203 . That is, at time T 1 , etching is completed with the SiO 2  thin film  204 , and started with the Al thin film  203 . Also, in FIG. 4, at time t 2 , the etching hole  205  reaches the surface of the SiO 2  thin film  202 . That is, at time t 2 , etching is completed for the Al thin film  203 , and started for the SiO 2  thin film  202 . 
     As known from FIG. 4, the strength of secondary ions increases when etching a part close to the boundary between the SiO 2  thin film  204  and the Al thin film  203  and a part close to the boundary between the Al thin film  203  and the SiO 2  thin film  202 . Therefore, times t 1 , t 2  can be known by detecting an etching temporal increase of the strength of secondary ions. 
     Similar to the first preferred embodiment, times t 1 , t 2  can be detected, for example, by giving value execution processing to an output signal from the secondary ion detector, or filtering the output signal using a high pass filter. 
     {circle around (4)} Next, similar to the first preferred embodiment, an etching time for the SiO 2  thin film  204  and that for the Al thin film  203  are calculated using times t 1 , t 2 , and the film thicknesses of the thin films  204 ,  203  are calculated based on these etching times and rates. 
     An etching rate can be obtained in a method similar to that in the first preferred embodiment. This embodiment is similar to the first preferred embodiment also in that thin films  203 ,  204  can be determined to be good/no good, rather than calculating a value of film thickness. 
     {circle around (5)} Thereafter, the etching hole  205  is refilled, if necessary, to complete film thickness measurement. 
     As described above, according to this embodiment, an etching time can be detected through observation of secondary ions. 
     With the above, similar to the first preferred embodiment, film thickness measurement can be achieved using a very inexpensive device, compared to a case where an etching time is detected through component analysis. 
     In addition, this embodiment is similar to the first preferred embodiment in that a conventional focusing ion beam device can be used intact, that the thickness of a film which is not photo transmissive can be measured, as well as the thickness. of a thin film smaller than a light waveform or the resolution of a scanning or transmission electron microscope, and that etching processing is very easy. 
     INDUSTRIAL APPLICABILITY 
     As described in detail in the above, according to the present invention, there can be provided a film thickness measuring method capable of measuring a film thickness of a very thin film, and realized using an inexpensive measuring device.