Method for making specimen

Abstract: The present invention is intended to provide a method and an apparatus for making a specimen for use in observation through a transparent electron microscope, including a step for milling part of the specimen into a thin film part, which can be observed through a transparent electron microscope, by scanning and irradiating a focused ion beam onto the specimen, a step for observing a mark for detection of position provided on the specimen as a secondary charged particle image by scanning and irradiating a charged particle beam onto the specimen without irradiating the charged particle beam onto the portion to be milled into the thin film part during the step for milling, and a step for compensating positional drift of the focused ion beam during said step for milling in accordance with a result of the observation. The present invention provides an effect to strikingly raise the efficiency of TEM observation since a specimen for use in TEM observation can be made by precisely milling a thin film part of the specimen even though positional drift of the focused ion beam is caused during milling.

Description: BACKGROUND OF THE INVENTION The present invention is related to a method of making a specimen for observing a specific portion of a semiconductor device through a transmission electron microscope according to the preamble of claim 1. Recently, a focused ion beam (hereafter referred to as "FIB") milling has been used for making specimens of specific portions of semiconductor devices for use on a transmission electron microscope (hereafter referred to as "TEM") in analyses of a gate portion of a specific memory cell. An example of the method is described below referring to the drawings. As shown in, for example, Fig. 2, a surface of a specimen 42 is polished by a polishing apparatus to form a protruded part 1 of 30 to 100 µm (typically, 50 µm) in width and 10 to 100 µm (typically, 50 µm) in height so that a portion to be observed is positioned about the center of this protruded part. As shown in Fig. 3, both sides of the protruded part 1 are removed in a depth of d = 3 to 10 µm and a width of w = 4 to 15 µm by applying the focused ion beam 2 to leave a thin film part 3 with a thickness t at the center of the protruded part. The protruded part is milled so that a portion to be observed is formed in this thin film part 3. The thickness t of this thin film part 3 need be approximately 100 nm or less to carry out TEM observation. To leave this small thickness, the protruded part is processed to leave a film having a thickness of 1 µm by using the focused ion beam of approximately 0.5 to 1 µm in the beam diameter in the initial stage and the thin film part 3 with thickness t is further gradually milled by using a thinner beam of approximately 0.1 µm or less in the beam diameter to finish a specimen for TEM observation which is provided with a final thickness of approximately 100 nm or less. A first prior art related to the above is, for example, Japanese Patent Application Disclosure Hei 5-15981 which discloses a method for milling a specimen for use in SEM observation of a cross section which is adapted to mill a mark permitting to control a position of a cross section to be finally obtained and adapted to set a finishing position by using this scanning ion microscope image (SIM image). As a second prior art, the "J. Vac. Sci. Technol. B11, (3) (May/June), pp 531 to 535, 1993" published by the U.S. Society of Vacuum discloses a method for milling a specimen for use in TEM observation by which an electron beam is irradiated during FIB processing and a thickness of a film which is formed by milling is obtained by observing secondary electrons or reflecting electrons generated therefrom. In most cases, a positional drift of the focused ion beam 2 is approximately 0.1 µm to 0.5 µm/10 minutes. Therefore, if the laser beam drifts to the center of the thin film part 3 as shown in a plan view of an important part of a milled surface of the specimen in Fig. 4, the thin film part 3 is often excessively milled and a portion to be observed is inadvertently milled off. When the thickness t of the thin film part 3 approaches 200 nm, it becomes difficult to identify the thin film part 3 due to a scanning ion image and set a milling area 5 and therefore there is a risk that the portion to be observed may be inadvertently milled and damaged. In case of a first prior art, marks 6a and 6b are provided in an area shown as an observing area 4 of the thin film part 3 as shown in Fig. 4 and a milling area 5 has been determined by observing scanning ion microscope images (SIM image) of these marks. However, there has been a risk that, when the thickness of the thin film part 3 approaches approximately 200 nm, the top part of the thin film part 3 may be milled because the scanning ion image is observed and the thin film part 3 may be excessively thinned. In case of a second prior art, high costs are required to provide an electron gun for SEM, a power supply and a controller and it is spatially difficult to simultaneously reduce sufficiently the operating distance of the focused ion beam optics and that of the electron beam optics since the electron gun is arranged nearby a workpiece. Therefore, installation of the SEM on a practical focused ion beam milling unit includes two problems, that is, high price of the apparatus and difficulty in focusing of the laser beam to be sufficiently thin. From JP-A.58167775 a method is known to carry out a positional alignment between ion beam and a material to be processed based on a detected signal by detecting Auger electron generated by irradiating ion beam to a mark part provided on the material to be processed. From the document Materials Research Society Symposium Proceedings, Vol. 254, 65-78, Boston, 5-6 Dec. (1991), the construction and performance of an updated gas source precision ion milling system is known. This known system is based on an existing focused ion beam machine which is able to image and mill selected areas of specimen that are too thick for TEM studies. The specimen image is formed using either secondary electrons or secondary ions captured by a dual detector. This system is characterized by a specimen chamber with an improved specimen holder. This new holder enables the specimen to be moved between the TEM and the instrument for further precision thinning of the specimen, without removal of the specimen from the holder. In the above prior arts, practical means for observing the thickness distribution of the thin film part and means for detecting the electron beam which passes through the thin film part are taken into consideration. For this reason, the prior arts include a problem that, if a specimen for TEM observation is made by using an ordinary focused ion beam milling machine, a failure may be often repeated, work efficiency is extremely low and a lot of time is required to obtain the data for TEM observation. In addition, there is another problem that, if only one specimen is available, it cannot be milled to be sufficiently thin due to a fear of probable breakage. SUMMARY OF THE INVENTION The object of the present invention is to solve the above-described problems of the prior arts and to provide a method for making a specimen capable of certainly finishing a TEM specimen with an appropriate thickness by a focused ion beam milling without fail. This object is achieved by the subject-matter of independent method claim 1. Features of preferred embodiments are defined with depending claims. To attain the above object, the present invention is adapted to (1) provide a mark 6 for measuring a positional drift of a focused ion beam on a protruded part 1 at a position more outer than a thin film part 3 to be milled to a predetermined thickness, observe this mark with the focused ion beam 2 at an appropriate frequency during milling so that an observing area 4 does not include the thin film part 3, and measure and compensate a positional drift of the beam, (2) prevent excessive milling by monitoring the thickness of the thin film part 3 during milling with, for example, a light more economical then an electron beam, and (3) measure a thickness distribution of the thin film part 3 by scanning the thin film part 3 with the light or the electron beam. The following describes in detail a practical means for attaining the objects of the present invention. The object of the present invention is attained by a method for milling a workpiece, in forming a specimen with a thin film by focused ion beam milling, while compensating the positional drift of the focused ion beam by observing a mark for measuring the positional drift of the focused ion beam provided on the workpiece, at a predetermined number of times as a scanning secondary charged particle microscope image, the method being a specimen making method which comprises steps for providing the mark at the position more outer than the thin film part to be milled to have the predetermined thickness, for arranging the mark so that both one side of the milling area and that of the observing area with the focused ion beam come in contact with the milled surface of the thin film or one side of the observing area does not come in contact with the milled surface and is kept away in the opposite direction, and for forming the thin film part. It is preferable to compensate the positional drift of the ion beam and monitor a thickness of a remaining film during milling with the focused ion beam. In other words, the thin film part is formed by accurately milling the workpiece to the predetermined film thickness while monitoring the thickness of the thin film part. Various film thickness measuring methods such as, for example, (1) an optical interferometry, (2) a method for detecting a transparent light intensity of a slit-shaped beam, (3) a method for detecting a transmission image of a scanning laser beam, (4) a method for detecting the transmission light intensity of an electron beam, (5) a method for detecting the transmission image of the scanning electron beam and (6) a method for detecting a distortion of the thin film caused by irradiation of a pulse laser beam are used as a film thickness monitoring method. Particularly, the film thickness measuring method by means of optical measuring means is practical and preferable. For milling a workpiece with the focused ion beam, the method can be adapted to slightly tilt a surface to be milled of the workpiece and monitor a tilting angle. Since a current density distribution of the focused ion beam has a tail and a milled end surface of the workpiece is slightly tilted due to an effect of such tail even though the beam is reduced to be thinner, monitoring of tilting as described above is required to compensate the latter tilting of the milled surface. Therefore, the tilting angle of the milled surface of the workpiece in this case should meet the characteristic (the tail of the current density distribution) of the focused ion beam to be used. For example, a method for measuring a reflection angle of light irradiated onto the milled surface can easily be the method for monitoring the tilt of this milled surface. The object of the present invention can be also attained, in milling a specimen by the above-described specimen making method, by a focused ion beam milling method which is adapted to form a surface to be milled by irradiating the focused ion beam onto the specimen, scan the surface to be milled with the electron beam at a scanning velocity largely differing from the scanning velocity of the focused ion beam, detect secondary charged particles from the specimen, eliminate noise by passing signals of the detected secondary charged particles through a filter of a high or low frequency, and display a scanning electron microscope image or a scanning ion microscope image by using the signals from which noises have been eliminated. In this case, the thickness of the milled film can be measured from the transmission electron beam which has passed the milled surface and the workpiece can be accurately milled to a predetermined target thickness while monitoring the film thickness. The thickness distribution of the thin film part can be measured by scanning the surface, which has been milled by the focused ion beam, with a light or an electron beam. The following describes the beam irradiating conditions when the specimen is milled with the focused ion beam and those when the secondary charged particle image is obtained by scanning with the beam. Though usually the beam intensity is fixed and the scanning velocity is changed, care is taken to reduce the scanning velocity to increase an irradiation dose per unit area and improve efficiency in milling and, on the contrary, raise the scanning velocity to reduce the irradiation dose and protect the specimen wherever possible in obtaining the secondary charged particle image. In case of fixing the scanning velocity, the dose is changed over by controlling an ion source but the former scanning velocity control method is practical and preferable in view of ease in determination of a configuration of the apparatus. The following embodiments related to a specimen making apparatus are not part of the present invention but may help to understand the invention. Such a specimen making apparatus comprises means for changing over the observing area and the milling area for ion beam scanning so that respective designated one sides of the observing area and the milling area match or the one side of the observing area remains at a relative position retracted from the one side of the milling area in an opposite direction to the milled surface of the specimen, in the focused ion beam milling apparatus which at least comprises 1 ○ an optics for focusing an ion beam which is provided with a high brightness ion source, a focusing optics for focusing the ion beam to an extremely small spot, and a deflector for deflection-scanning the ion beam on the specimen, 2 ○ a secondary charged particle observing system which is provided with a secondary charged particle detector for detecting secondary charged particles from the specimen to obtain a scanning secondary charged particle microscope image and an imaging device for a scanning secondary charged particle microscope image, 3 ○ a stage for supporting and moving the specimen arranged in a sample vacuum chamber, and 4 ○ a vacuum pumping system. In the above specimen making apparatus, it is preferable to provide a means for monitoring a thickness of a thin film specimen (workpiece) which is mounted on a stage arranged in the specimen vacuum chamber 3 ○ and is being milled. As the means for monitoring the thickness of the thin film is used one of, for example, (1) means provided with an optical irradiation device and a light interferometer, (2) means provided with the optical irradiation device and an optical transmission detector, (3) means provided with a slit-shaped light irradiation device and the optical transmission detector, (4) means provided with a scanning laser beam irradiation device, the optical transmission detector and an imaging device of scanning electron transmission image, (5) means provided with an electron beam gun and an electron beam transmission detector, (6) means provided with a scanning electron beam irradiation device, the electron beam transmission detector and the imaging device of scanning electron transmission image, and (7) means provided with a pulse laser generator and an interferometer for detecting a distortion of the thin film due to irradiation of the pulse laser. In any one of the above-described means for monitoring the film thickness, the light or electron irradiation part and the detector can be fixed to the specimen stage. In the above specimen making apparatus, it is preferable to provide a means for monitoring a tilting angle of the milled surface of the thin film specimen (workpiece) which is mounted on a stage arranged in the specimen vacuum chamber 3 ○ and is being milled. This monitoring means can comprises, for example, a light irradiation device and a device for detecting a reflecting position of the light. The above specimen making apparatus can be provided with a focused electron beam irradiation device which is constructed to permit irradiation onto the thin film specimen during focused ion beam milling, a controller for scanning the electron beam at a scanning velocity independent of the scanning velocity of the focused ion beam, a secondary charged particle detector for detecting secondary charged particles from the specimen, a circuit for filtering the signals of detected secondary charged particles at a high or low frequency, and an imaging device for displaying a scanning electron microscope image or a scanning ion microscope image by means of the filtered signals. Thus the thickness and thickness distribution of the thin film to be milled can be known and therefore high precision milling with more uniformly controlled thickness distribution can be carried out by feeding back this thickness information to controlling of the thickness in focused ion beam milling. The mark for measuring the positional drift of the beam is observed with the focused ion beam so that the observing area does not include the thin film part and therefore the observing part can be protected from being milled. In addition, the thickness of the thin film part can be monitored and therefore the milling position of the focused ion beam can be accurately set. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional view of a specimen showing an example of a method for monitoring a thickness of a thin film part which is an embodiment of the present invention; Fig. 2 is a perspective view of a specimen; Fig. 3 is a perspective view showing an example of the conventional specimen making method; Fig. 4 is a plan view of the specimen showing a method for observing a scanning charged particle microscopic image (SIM image) on the specimen according to the prior art; Fig. 5 is a plan view of the specimen showing a method for observing the SIM image on the specimen according to the present invention; Fig. 6 is the SIM image on the specimen in an initial stage of milling according to the present invention; Fig. 7 is the SIM image on the specimen on which milling is further progressed; Fig. 8 is a sectional view of the specimen showing an example of the method for monitoring the thickness of the thin film part which is another embodiment of the present invention; Fig. 9 is a sectional view of the specimen showing an example of the method for monitoring the thickness of the thin film part which is a further another embodiment of the present invention; Fig. 10 is a sectional view of the specimen showing an example of the method for monitoring the thickness of the thin film part which is a further another embodiment of the present invention; Fig. 11 is a sectional view of the specimen showing an example of the method for monitoring the thickness of the thin film part which is a further another embodiment of the present invention; Fig. 12 is a sectional view of the specimen showing an example of the method for monitoring the thickness of the thin film part which is a further another embodiment of the present invention; Fig. 13 is a sectional view of the specimen showing an example of the method for monitoring the thickness of the thin film part which is a further another embodiment of the present invention; Fig. 14 is an extended perspective view showing a distortion of the thin film part obtained by milling the specimen; Fig. 15 is an extended perspective view showing a distortion of the thin film part similarly obtained by milling the specimen; Fig. 16 is an illustration showing a current density distribution of the focused ion beam and a sectional shape of the thin film part milled with the focused ion beam; Fig. 17 is a sectional view of the specimen showing a method for milling a tilted specimen which is another embodiment of the present invention; Fig. 18 is a sectional view of an apparatus showing a light irradiation part and a light receiving part which are mounted on the specimen stage; Fig. 19 is a sectional view showing another embodiment of the present invention for milling the specimen by normally irradiating the focused ion beam onto the specimen; Fig. 20 is a schematic sectional view of a principal part showing an embodiment of the apparatus provided with a thickness monitor; Fig. 21 is a schematic sectional view of a principal part showing an embodiment of the apparatus provided with another thickness monitor; Fig. 22 is a schematic sectional view of a principal part showing an embodiment of the apparatus provided with a further another thickness monitor; Fig. 23 is a schematic sectional view of a principal part showing an embodiment of the apparatus provided with a further another thickness monitor; Fig. 24 is a schematic sectional view of a principal part showing an embodiment of the apparatus provided with a further another thickness monitor; Fig. 25 is a schematic sectional view of a principal part showing an embodiment of the apparatus provided with a further another thickness monitor; Fig. 26 is a graph representing a variation of a secondary electron signal to time; and Fig. 27 is a schematic sectional view of a principal part showing an embodiment of the apparatus provided with a further another thickness monitor. DESCRIPTION OF THE PREFERRED EMBODIMENTS First of all, a principle of observation of position marks according to the present invention is described in detail referring to the drawings. As shown in Fig. 4, while the thickness of the thin film part 3 is yet sufficiently larger than a predetermined target thickness, the observing area 4 is set so that the thin film part 3 is observed at the center thereof as in the method according to the prior art. Then the milling area 5 is set and milling is carried out. However, if the observing area is set as shown in Fig. 4 when milling is progressed and the thin film part 3 is sufficiently thinned, the top portion of the thin film part 3 is milled off. Therefore, the present invention is adapted, as shown in Fig. 5a, to position the observing area 4 and the milling area 5 so that the left-side end of the milling area 5 is aligned with the left-side end of the observing area 4 or the position of the observing area 4 is determined by shifting the left-side end thereof in a direction opposite to the milled surface as shown in Fig. 5b, for observing the marks 6 on the scanning ion microscope image to measure the positional drift of the beam during milling, for example, the right side of the thin film part 3. In milling the left side of the thin film part, the relative positions are reversed and therefore the position of the observing area 4 is determined by aligning the right-side end of the observing area 4 with the right-side end of the milling area 5 or shifting the observing area 4 from the right-side end of the milling area 5. In this case, an important point is positioning of the observing area 4 at the side where the thin film part 3 is milled and the observing area 4 should not be protruded toward the thin film part 3 from a position where it is kept in contact with the milled surface of the thin film part 3 in the milling area 5. In Fig. 5a, the left-side ends of the milling area 5 and the observing area 4 are aligned. The scanning ion microscope image observed in the initial stage of milling of one thin film part is shown in Fig. 6. The marks 6 are provided in advance by using the focused ion beam at the positions which are included in this observing area 4. It is important to provide these marks 6 so that they do not come out of the observing area 4, and are always positioned within the observing area 4 and positioned outside the area of the thin film part 3. In general, the width of the cross of this mark 6 should preferably be approximately 0.1 µm. An example of the scanning ion microscopic image observed during a more advanced stage of milling is shown in Fig. 7. Expressions dx = x 2 - x 1 and dy = y 2 - y 1 derived by the positions x 1 and y 1 of the mark 6 from reference axes x and y shown in Fig. 6 and positions x 2 and y 2 shown in Fig. 7 represent the positional drift of the beam. The position of the beam is shifted as much as dx and dy by deflecting the beam. A preferable frequency of observation is one to five minutes, depending on the stability of the beam. The anticipated objects of the present invention can be attained with the above configuration. In other words, the thin film part can be accurately milled by avoiding irradiation of the focused ion beam onto the thin film part even with the positional drift of the focused ion beam while observing the mark provided on the specimen as the SIM image. Moreover, an appropriate infeed amount of milling can be set with an inexpensive apparatus by using a light as a means for monitoring the thickness of the thin film part during milling. In addition, since the thickness distribution of the thin film part which is milled with the light or the electron beam can be measured and therefore the TEM specimen can be finished to an appropriate thickness within a short period of time with a high percentage of success and the efficiency of TEM observation can be greatly raised. Embodiments of the present invention based on the above-described principle are described below. An example of the specimen making method by ion beam milling according to the present invention is described in detail along how to carry out milling of the workpiece, including the methods for observing the mark for measuring the positional drift of the ion beam and compensation of the positional drift of the beam, referring to Figs. 5 to 7. In Fig. 5, the protruded part 1 of the specimen 42 is roughly milled so that the thin film part 3 to be formed on the protruded part 1 of the specimen has a thickness of approximately 2 µm which is sufficiently larger than the final finish thickness of 100 to 200 nm. Before finish milling, the SIM image of the observing area 4 shown in Fig. 5 is set so that the left end of the SIM image is aligned with the target finished surface of the thin film part 3. On the SIM image shown in Fig. 6, a distance x 0 between the left end of the image (that is, the target finished surface) and the mark 6 in the x direction is measured in advance. At the same time, distances x 1 and y 1 between the center line of the image and the mark are measured in advance for each mark. A milling area for which the infeed amount is predetermined to be approximately 1 µm is set on the SIM image shown in Fig. 6 and milling is carried out. When milling is finished, the SIM image is observed again. In this case, if the position of the mark 6 is presented as x 2 and y 2 (provided, x 2 ≠ x 1 and y 2 ≠ y 1 ) as shown in Fig. 7, the positional drift of the beam appears as x 2 - x 1 and y 2 - y 1 and therefore the observing area 4 is translated in parallel as much as x 2 - x 1 and y 2 - y 1 in x and y directions respectively. Thus the left-side end of the image is aligned with the target finish-milled surface. It is checked again that the distance between the finish-milled surface and the mark is x 0 . Further, the infeed amount is set to 0.5 µm. In other words, the thickness of the remaining film is 2 µm - 1 µm - 0.5 µm = 0.5 µm. Then milling is carried out. The infeed amount is gradually reduced to reach the target finish-milled surface by repeating this operating procedure. Thus the fear of milling off of the thin film part 3 is eliminated by setting the observing area 4 so that the area 4 does not cover the thin film part 3 and high precision milling can be carried out by measuring a deviation of the mark during milling while compensating the positional drift of the beam. A method for monitoring the thickness and further the thickness distribution of the thin film part 3 is described referring to Figs. 1 and 8 to 19. Fig. 1 shows a schematic sectional view of an optics for measuring a thickness of the thin film part 3 being milled with the focused ion beam 2. An probe light 7 is irradiated onto the milled surface to measure the thickness of the thin film part 3 of the specimen 42 as shown and a light 8 reflected from the surface of the thin film part 3 and a light 9 reflected from the opposite side surface are caused to interfere and the thickness of the thin film part is measured by observing variations of interference which occur along milling by an interferometer 11. Fig. 8 also shows a schematic sectional view of the optics for measuring the thickness of the thin film part 3 being milled with the focused ion beam 2. The probe light 7 is transmitted through the thin film part 3 as shown and forced to interfere with a reference light 10 which is not transmitted through the thin film part, and the thickness is measured by detecting a phase deviation of the probe light due to the thin film part 3. In Figs. 1 and 8, the probe light 7 need to transmit through the thin film part and therefore an infrared ray is preferable when the specimen is made of silicon. Fig. 9 also shows a schematic sectional view of the optics for measuring the thickness of the thin film part being milled with the focused ion beam 2. The probe light 7 having a wavelength which is absorbed by the thin film part 3 is applied to the thin film part 3 as shown and the thickness is measured by measuring the intensity of the light which has transmitted through the thin film part with a measuring instrument 12 such as a photomultiplier. In this case, a shorter wavelength is better and the wavelength should be determined in accordance with a type of material of the specimen. The side wall of the thin film part 3 tends to be tilted as shown in Fig. 10. This tilting angle appears as a tail of a current distribution of the ion beam as described below, and it is important for high precision milling to understand this tail and the tilting angle of the side wall of the thin film part 3. A position of a reflected light 8 and an angle of the side wall of the thin film part 3 can be obtained by using a laser beam as the focused probe light and detecting the laser beam 8 reflected from the side wall of the thin film part 3 with an array detector 12. Fig. 11 shows a similar method to that shown in Fig. 10 and this method is intended to measure an intensity distribution of the probe light 7 as a slit-shaped beam by the array detector 12 and detect the thickness distribution of the thin film part 3, that is, an excessively thinned top part thereof in most cases. In Fig. 9, the focused laser beam can be used as the probe light 7 and a transmission image of the scanning electron beam can be obtained by scanning the laser beam in the area of the thin film part 3. Fig. 12 shows an example of measuring the thickness of the thin film part 3 by detecting a transmission electron beam, which is obtained by impinging the electron beam 14 from a focusing electron gun 13 onto the thin film part 3, by an electron beam detector 16 such as a Faraday cup or a scintillator. An appropriate transmission light intensity can be obtained by adjusting an energy of the electron beam 14. In a case that the thin film part 3 comprise a plurality of materials which have substantially different transmissivities to the electron beam 14, only measurement of an average transmission intensity of the electron beam through the thin film part is insufficient to check the thickness. Therefore the thickness distribution can be measured by adapting to scan the electron beam 14. There is an effect that the TEM image can be observed during milling with the focused ion beam by using the same construction as the transmission electron microscope. As shown in Figs. 13 to 15, a pulse laser beam 15 is irradiated onto the thin film part 3 to heat it and cause a displacement δ due to thermal expansion as shown in Fig. 14 or 15, and this displacement δ is a function of the thickness. The thickness can be estimated from the displacement by calculating in advance the thickness and the displacement from materials, the shape of the thin film part and an absorption coefficient of the laser beam, or confirming the values through experiment. For measuring the displacement δ, the probe light 7 is reflected at the surface of the thin film part 3 and made interfere with the reference light (not shown) reflected at the other fixed portion. As shown in Fig. 16, the current density distribution has a tail as a curve 50 shown in Fig. 16 (a) in milling by the focused ion beam and therefore the milled surface of the workpiece may be tilted by several degrees due to the effect of the tail as shown with 51 in Fig. 16 (b) even though the laser beam 2 is throttled. For example, if it is assumed that the tilting angle is 2° and the height of the thin film part 3 is 6 µm, a difference of the thickness of approximately 6 µm × tan 2° × 2 = 0.4 µm is found between the upper part and the lower part of the thin film part 3. A TEM specimen with a uniform thickness cannot be made and therefore a specimen as shown in Fig. 17 should be tilted in advance by  = 2° before milling in the above example. However, there is a problem in this case that, when the probe light 7 is horizontally impinged, the reflected light is tilted by 2 and the reflected light does not reach the interferometer for the reflected light fixed at a position away from the specimen. In a case that the transmission light is used, there is a problem that the length of the light path which passes through the thin film part 3 is extended as long as l/cos . Even though the specimen is not tilted, it is difficult to align the probe light 7 with the thin film part 3 of approximately 10 µm in thickness. As shown in Fig. 18, therefore, it is recommended to attach a lens part 52 for irradiating the ,probe light and a light receiving part 53 of the interferometer for the transmission light to the specimen stage 39 and connect an optical fiber 54 and an output cable thereto. The probe light irradiating part 52 and the light receiving part 53 can be positioned in advance at the positions where the thin film part is expected to be formed in the atmosphere. Otherwise, it can be adapted to finely adjust the position of the specimen even after the specimen has been set in the specimen vacuum chamber. In Fig. 18, there are shown the light irradiation part 52 and the light receiving part 53 which are respectively divided into the right and left-side parts. These parts can be located at the same side for the reflected light detector. In the above-described embodiments, there is shown an example of configuration in which the focused ion beam 2 is applied in substantially parallel to the surface of the thin film part 3. As shown in Fig. 19, a method for normally irradiating the focused ion beam 2 to the thin film part 3 is also available. In this case, the probe light irradiation part 52 and the reflected light receiving part 53 are installed faced up. Thus the thickness of the thin film part 3 can be monitored during milling. In Fig. 19, a reactive gas from a gas nozzle 65 is blown onto the milled surface to carry out focused ion beam assisted etching. Since an ion having an energy of approximately 30 kV is impinged onto the milled surface in milling with the ion beam, the milled surface is damaged due to ion impact. In the focused ion beam assisted etching, approximately 9/10 of the stock to be milled is removed by chemical reaction with the reactive gas and therefore an ion dose per milling volume is less than in milling only with the ion beam. In other words, the damage due to ion impact is reduced. Suitable reactive gases for specimens such as Si, SiO 2 and SiN include, for example, gases including fluorine such as XeF 2 , SF 6 and CF 4 or gas mixtures and those for aluminum alloy include gases including chlorine such as Cl 2 , SiCl 4 and BCl 3 or gas mixtures. If a specimen comprises various materials, halogen gas mixtures containing the above gases are suitable. It is obvious that focused ion beam assisted etching is equivalently effective not only in normal impinging of the ion beam to the milled surface as shown in Fig. 19 but also in slant or horizontal impinging of the ion beam as shown in Fig. 17. The following describes the specimen making apparatus according to the present invention which is made up by employing the method for observing the marks for measuring the positional drift of the ion beam, compensation of the positional drift of the beam, the thickness monitor for use in milling, and the method for monitoring the tilting angle of the milled surface in the focused ion beam milling machine. Fig. 20 is a schematic sectional view of a main part of an apparatus which is made up by attaching the interferometer for the reflected light shown in Fig. 1 to the focused ion beam milling machine. The main part of the focused ion beam milling machine is described below. In Fig. 20, reference numerals 21 denote an ion beam optics which comprises a liquid metal ion source 22, an extractor 23, a defining aperture 24, a electrostatic lens 26 which focuses an ion beam 25 passing through the defining aperture 24, a blanking electrode 28 connected with a blanking power supply, a blanking aperture 29, and a deflector 31 connected with a deflection controller 30. This ion beam optics 21 is housed in a vacuum chamber for the optics 38 whereby the focused ion beam 2 which is focused and deflection-controlled by the ion beam optics 21 is supplied to the specimen 42. The specimen 42 is set on stages 39a and 39b which are arranged to be rotatable around the X and Y axes as the centers and movable in parallel to the X and Y axes. The stage 39 is accommodated in a vacuum chamber for specimen 40 and the vacuum chambers 38 and 40 are evacuated to a sufficient level of vacuum by a vacuum pumping system. 32 is a control power supply which controls the ion source 22, the extractor 23 and the electrostatic lens 26. Reference numerals 33 is a computer serving as a controller which is connected with the deflection controller 30, the blanking power supply 27 and the control power supply 32. Reference numerals 37 is an image monitor. A scanning ion image is displayed by a micro channel plate (MCP) 35 which detects secondary electrons or secondary charged particles such as secondary ion, which are emitted from the specimen 42 upon irradiation thereto of the focused ion beam 2, an anode 36 for detecting a detection signal from the MCP, and an image monitor 37 [scanning ion microscope image (SIM image) monitor] which is connected with the anode 36 and the deflection controller 30. The following describes a construction of a thickness monitor which operates in conjunction with the interferometer for the reflected light. Reference numerals 61 is a coherent light source of a laser by which a beam having a wavelength which partly passes through the thin film part 3 of the specimen 42 is generated. A laser beam generated by this coherent light source is expanded by a beam expander 68 and focused by the focusing lens 71 onto the thin film part 3 of the specimen 42 through a beam splitter 69 and a window glass (coated with a reflection preventing film) of the vacuum chamber 40. A light reflected at the surface of the thin film part 3, and a light which passes through the thin film part 3 and is reflected at the opposite surface of the thin film part 3 are reflected by the beam splitter 69 to interfere and focused by the focusing lens 72 and this interfering light is detected by the interferometer 11 which employs photodetectors such as photodiodes. An operating mechanism of this apparatus is described below. In the main part of the focused ion beam milling machine, milling is carried out while compensating the positional drift of the beam and maintaining the specified relative positions of the milling area with the focused ion beam and the observing area according to the same operating method as described in the first embodiment by using the ion beam optics 21, the X/Y stage 39, the image monitor for observing the image 37, and controllers. On the other hand, the thickness monitor monitors the thickness of the thin film part during milling and feeds back the thickness information to the controller for focused ion beam milling and thus accurate milling is carried out until the thickness of the thin film part 3 of the specimen reaches the predetermined target thickness. The above configuration of the apparatus enables to facilitate milling of the specimen for TEM observation, for example, an extremely thin and uniform thickness of 100 to 200 nm order. Fig. 21 is a schematic sectional view of the main part of the apparatus which is made up by attaching the interferometer for the transmission light shown in Fig. 8 to the focused ion beam milling machine. This embodiment has the basically same construction of the optics for focusing the ion beam and the image monitor for observing the image as in the third embodiment shown in Fig. 20 and therefore the description of these parts is omitted from the following explanation and the thickness monitor for measuring the thickness of the thin film part 3 being milled is mainly described. As shown, an optics comprising a focusing lens 71 and others is constructed so that the beam from the coherent light source 61 is expanded by the beam expander 68 and divided into a P-polarized beam 75 and an S-polarized beam 76 by the Savart plate (double refraction prism) 73 through a convex lens, the P-polarized beam passes through the thin film part 3 of the specimen 42 and the S-polarized beam 76 does not pass through the thin film part 3. These beams are made polarized-interfered each other through a polarizing plate 74 which is provided so that the beams are combined again into one beam by the Savart plate 73 through the lens 71 and the window glass 70 and arranged at the angles of polarization of 45° in reference to the angle of polarization of both beams, and this interfered beam is detected by the interferometer 11. In Fig. 20 shown in the third embodiment, though preferable interference may not be obtained unless the reflectivity and transmissivity of the thin film part are appropriately provided, this embodiment provides an effect that such preferable interference may be easily obtained even though the reflectivity is low. Fig. 22 shows a schematic sectional view of the main part of the apparatus made up by attaching the photodetector for the transmission light shown in Fig. 9 to the focused ion beam milling machine. This embodiment also has the basically same construction of the optics for focusing the ion beam and the image monitor for observing the image as in the third embodiment shown in Fig. 20 and therefore the description of these parts is omitted from the following explanation and the thickness monitor for measuring the thickness of the thin film part 3 being milled is mainly described. As shown, this embodiment of the thickness monitoring optics is adapted to make the beam from the coherent light source 61 (a laser beam source can also be used), which generates a light transmitting through the thin film part of the specimen, pass through the thin film part 3 of the specimen 42 through the collimating optics 77, the window glass 70 and the focusing lens 71, and detect a degree of absorption of light of the thin film part by the photodetector 12. Though it is necessary to provide the beam splitter in Fig. 20, this beam splitter is not required in this embodiment and therefore it is advantageous in that the apparatus is inexpensive and adjustment of the apparatus is easy. Fig. 23 shows a schematic sectional view of the main part of the apparatus made up by attaching the photodetector for the transmission slit-shaped beam shown in Fig. 11 to the focused ion beam milling machine. This example also has the basically same construction of the optics for focusing the ion beam and the image monitor for observing the image as in the third embodiment shown in Fig. 20 and therefore the description of these parts is omitted from the following explanation and the thickness monitor for measuring the thickness of the thin film part 3 being milled is mainly described. As shown, this embodiment of the thickness monitoring optics is adapted to make the beam from the light source 61, as a slit-shaped beam, pass through the thin film part 3 of the specimen 42 through the collimating optics 77, the window glass 70 and the cylindrical lens 78, focus this slit-shaped beam onto the photodetector 12 comprising the photodiode array (for example, CCD sensor) through two cylindrical lenses 78, and measure the distribution of light absorption of the thin film part (a distribution along the longitudinal direction of the slit-shaped beam). Though it is difficult to obtain the thickness distribution of the thin film part (for example, thin at the upper part and thick at the lower part) in Figs. 20 to 22, this embodiment enables to check the thickness distribution in the vertical direction of the thin film part and therefore, if the thickness distribution is unsatisfactory, adjustment can be made easily by adjusting the irradiation angle of the ion beam. Fig. 24 shows a schematic sectional view of the main part of the apparatus made up by attaching a device for precisely measuring distortion or thermal expansion of the thin film part 3 when an intensity modulated laser beam shown in Figs. 14 and 15 is irradiated by using interference of the laser beam to the focused ion beam milling machine. This embodiment also has the basically same construction of the optics for focusing the ion beam and the image monitor for observing the image as in the third embodiment shown in Fig. 20 and therefore the description of these parts is omitted from the following explanation and the thickness monitor for measuring the thickness of the thin film part 3 being milled is mainly described. As shown, this embodiment of the thickness monitoring optics is adapted to expand the beam such as a laser beam emitted from the light source 61a through the beam expander 68 after the intensity of the beam has been modulated with a specified frequency by a chopper 79 such as an acoustic-optic modulator, focus the beam by the focusing lens 71 and irradiate it onto the thin film part 3 of the specimen 42 through a dichroic mirror 80 and the window glass 70. At the thin film part, a cyclic displacement due to thermal expansion synchronized with the above intensity modulated frequency is caused. On the other hand, the beam such as the laser beam emitted from the coherent light source 61b is expanded by the beam expander 68, and separated into two beams by the beam splitter 69 and one of these beams is reflected at the dichroic mirror 80, and focused and irradiated by the focusing lens 71 onto the same position as the focusing part of the above intensity modulated light on the thin film part 3 of the specimen 42 through the window glass 70. The reflected light and the reference light separated by the above beam splitter 69 and reflected by a reference mirror 83 are made to interfere each other and the interfered light is focused by the focusing lens 71 and detected by the interferometer 12 using the photodetector such as the photodiode. The detection signal is sent to a synchronous demodulation circuit such as a lock-in amplifier 81 and the amplitude and phase of a thermal expansion component are extracted with the intensity modulated signal as the reference signal. This embodiment provides an effect that, though the light which passes through the thin film part is not used as shown in Figs. 20 to 22, the displacement due to thermal expansion which results from absorption of the light can be obtained even when the transmissivity of light of the thin film part is low and the transmission light intensity is insufficient, and therefore the thickness of the thin film part can be known. Fig. 25 shows a schematic sectional view of the main part of the apparatus made up by attaching a device for measuring a transmission electron beam shown in Fig. 12 to the focused ion beam milling machine. As shown, the specimen chamber 40 is provided with the focused ion beam optics 21, the electron beam gun 13, a secondary electron beam detector 56, a detector 16 for detecting the electron beam which passes through the specimen, and the stage 39. The specimen 42 to be milled as shown in Fig. 3 is set on the stage 39. The focused ion beam 25 for milling the specimen 42 is deflection-scanned by applying signals from the deflection controller 30 to the deflector 31. The electron beam 14 is deflection-scanned by applying signals from a deflection controller for electron beam 57 to a deflector 58. Secondary electron obtained by irradiation of the focused ion beam 25 and secondary electrons obtained by irradiation of the electron beam 14 are detected by a secondary electron detector 56, and the secondary electron image obtained from irradiation of the focused ion beam 25 is used for observing the top view of the specimen 42 and the secondary electron image obtained from irradiation of the electron beam 14 is used for observing the cross section of the thin film part of the specimen. The electron beam which has passed the thin film part 3 of the specimen 42 is detected by the electron beam detector 16 and used for measuring the thickness or the thickness distribution of the thin film. The output of the secondary electron detector 56 and the output of the electron beam detector 16 are entered into an electron beam output selector 59. In this selector 59, a low frequency pass filter and a high frequency pass filter are provided as described later. Electron beam deflection signals and focused ion beam deflection signals are entered into a deflection signal selector 60. For observing the scanning ion microscope image (SIM image) by these selectors 59 and 60, the ion beam deflection signal and the secondary electron detection output are entered into the image monitor 37. For observing the scanning electron microscope image (SEM image), the electron beam deflection signal and the secondary electron detection output are entered into the image monitor 37. For observing the intensity distribution of the transparent electron beam, the electron beam deflection signal and the transmission electron beam detection output are entered into the-image monitor 37. The detector for obtaining the scanning ion microscope image may be a micro channel plate 35 as shown in Fig. 20. Though one set of the image monitor 37 is used in Fig. 25 to reduce the cost of the whole apparatus, the configuration need not be limited to this and the scanning ion microscope image, the scanning electron microscope image and the transmission electron beam image can be observed on respective dedicated image monitors by providing two or more sets of the image monitors. Two types of beams, that is, the focused ion beam 25 and the electron beam 14 can be supplied simultaneously or separately at delimited timings. By supplying these beams simultaneously, the scanning electron microscope images (SEM images) of the cross sectional views of the milled surface can be simultaneously observed while milling the specimen with the ion beam. In a case that two sets of image monitors 37 are provided, the SEM image and the SIM image can be simultaneously observed. However, in case of simultaneous scanning, the secondary electron signal to be detected by the secondary electron detector 56 is a secondary electron signal 66 obtained with the electron beam which is superposed with a secondary electron signal 67 obtained with the focused ion beam as shown in Fig. 26 (a) and therefore if the SEM image and the SIM image are displayed by using this secondary electron signal, the image displayed includes many noises. With respect to the scanning velocity, the focused ion beam is scanned at a high scanning velocity of, for example, 1 ms for one face since the milled surface in focused ion beam milling can be finished more smoothly at the higher scanning velocity, while the electron beam can be scanned at a low scanning velocity of, for example, approximately 1 s for one surface. As shown in Fig. 26 (b), therefore, the signal 66 obtained with the electron beam mainly comprises a low frequency component and the signal 67 obtained with the focused ion beam mainly comprises a high frequency component. For obtaining the SEM image by using a filter (not shown) provided in the selector 59, secondary electron signals which pass through the low frequency pass filter are used and, for obtaining the SIM image, secondary electron signals which pass through the high frequency pass filter are used. Thus, satisfactory SEM and SIM images can be obtained. This method is effective to check the progress of milling more precisely and obtain more satisfactory finishing performance since the focused ion beam milling can be carried out while observing the cross sectional SEM image not only in making a specimen for TEM observation but also in milling a specimen for SEM observation of the cross section. Fig. 27 shows a schematic sectional view of the main part of the apparatus made up by attaching a device for obtaining a scanning transmission light image by particularly scanning a focused probe light in a method for measuring the transmission light shown in Fig. 9 to the focused ion beam milling machine. This embodiment also has the basically same construction of the optics for focusing the ion beam and the image monitor for observing the image as in the third embodiment shown in Fig. 20 and therefore the description of these parts is omitted from the following explanation and the thickness monitor for measuring the thickness of the thin film part 3 being milled is mainly described. As shown, the specimen chamber 40 is provided with the focused ion beam optics 21, a laser beam focusing optics 34 (incorporating a laser beam source 61 and the galvanometer 63), the MCP 35 for detecting the secondary ion, the detector 12 for detecting the laser beam passing through the specimen 42, and the stage 39. The specimen 42 is placed on the stage 39. The probe light 14 of the laser beam is deflection-scanned by applying signals from the laser beam deflection controller 62 to the galvanometer 63. The laser beam which has passed through the thin film part 3 of the specimen 42 is detected by the light detector 12. The deflection signal from the laser beam deflection controller 62 and the transmission light intensity signal from the light detector 12 are entered into the image monitor 37b, whereby the scanning transmission image is displayed and the thickness distribution of the thin film part can be known. On the other hand, the SIM image is displayed on the image monitor 37a in the same method as in Fig. 25 and the surface of the specimen can be observed. The above configuration of the apparatus enables to observe the transmission light image of the milled thin film part while milling the specimen surface with the focused ion beam and therefore easily determine the milled thickness of the thin film part. According to the embodiments of the present invention, the milled thickness distribution (only in the vertical direction in Fig. 23) of the tin film part can be checked both in the vertical and horizontal directions even during milling and the irradiating position and angle of the ion beam can be finely adjusted and therefore a unique effect can be attained which high efficiency of precision milling is obtained at a high percentage of success. As described in detail, the present invention enables to implement the anticipated objects. Specifically, the thin film part can be prevented from being irradiated with the focused ion beam during observation of the marks provided on the specimen as the SIM image and therefore can be milled with high preciseness even though the positional drift of the focused ion beam occurs. In addition, an appropriate infeed amount can be set even with an inexpensive apparatus by using a light as means for monitoring the thickness of the thin film part during milling. The present invention provides an effect that, since the thickness distribution of the thin film part to be milled with the light or the electron beam can be measured, the TEM specimen can be finished to an appropriate thickness within a short period of time at a high percentage of success and the efficiency of TEM observation can be strikingly raised.

Claims: A method of making a specimen (42) for use in observation through a transmission electron microscope, comprising: a step of milling part of the specimen to a thin film part (3) of a predetermined thickness, which can be observed through a transmission electron microscope, by scanning and irradiating a focused ion beam (2) onto a milling area (5) on the specimen (42); a step of observing a mark (6) for detection of position provided on said specimen (42) as a secondary charged particle image, by scanning and irradiating a charged particle beam onto an observing area (4) on said specimen (42); and a step of compensating positional drift of said focused ion beam (2) during said step of milling in accordance with a result of said observation, characterized in that said mark (6) is provided in advance on said specimen (42) at a position included in said observing area (4) but outside the area of the thin film part (3), and said step of observing a mark (6) is performed so that said charged particle beam is irradiated onto said observing area (4) to avoid excessive milling of the thin film part (3) by positioning said observing area (4) and said milling area (5) either so that one side of said milling area (5) and one side of said observing area (4) are aligned and that both said sides come in contact with the milled surface of said thin film part (3) or so that the position of said observing area (4) is determined by shifting said one side of said observing area (4) in a direction opposite to said part to be milled so that the observing area (4) does not come in contact with the milled surface of said thin film part (3). A method according to claim 1, wherein said charged particle beam (2) is said focused ion beam to be used in said step for milling. A method according to claim 1, wherein said method further comprises a step for monitoring a thickness of a portion, which is milled into a thin film part (3) by irradiating said focused ion beam (2) during milling said specimen to enable milling said portion to be milled into said thin film part (3) to a predetermined specified thickness by monitoring said thickness. A method according to claim 3, wherein said monitoring of the thickness is carried out by detecting interference of light (8, 9; 1, 10). A method according to claim 3, wherein said monitoring of the thickness is carried out by detecting a transmission light (7) intensity. A method according to claim 3, wherein said monitoring of the thickness is carried out by detecting a transmission intensity of a slit-shaped light (7 in Fig. 11). A method according to claim 3, wherein said monitoring of the thickness is carried out by detecting a transmission image of a scanning laser beam (7 in Fig. 9). A method according to claim 3, wherein said monitoring of the thickness is carried out by detecting a transmission amount of an electron beam (14). A method according to claim 3, wherein said monitoring of the thickness is carried out by detecting a transmission image of a scanning electron beam (14). A method according to claim 3, wherein said monitoring of the thickness is carried out by detecting distortion of the thin film part due to irradiation of a pulse laser beam (15). A method according to claim 1, further comprising the step of displaying a cross sectional secondary electron image on a display obtained by scanning an electron beam (2) on a surface of the milling part of the specimen (42).

Inventors: ISHITANI TOHRU, ITOH FUMIKAZU, KAMIMURA TAKASHI, NAKATA TOSHIHIKO, SHIMASE AKIRA, YAMAGUCHI HIROSHI, ISHITANI, TOHRU, ITOH, FUMIKAZU, KAMIMURA, TAKASHI, NAKATA, TOSHIHIKO, SHIMASE, AKIRA, YAMAGUCHI, HIROSHI
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1995
Main classification: G01N1
IPC codes: H01J37, B23K15, H01L21, G01N1, G01B11, C23F4, G01Q60