In the process of the focused ion beam specimen processing, hereinafter also referred to as FIB, it is necessary to achieve as planar surface of the milled area as possible. One of the applications where this planar surface is desirable is represented for example by TSV structure cuts, where TSV stands for through-silicon vias and comprises silicone plates with e.g. embedded copper structures. Due to various material and milling ratios, the FIB processing leads to the so called curtaining effect, when grooves appear in the line of the beam incidence. It means that a structure with grooves appears on the milled surface, the grooves being formed in the direction of the beam incidence, which is highly undesirable for applications which require plane surface and/or constant thickness for further examination.
One method which is successfully used to reduce the above mentioned effect, is based on polishing the specimen surface by repeated re-scanning by the FIB beam, the axes of which form an angle with the original ones in the plane of the specimen surface, usually an angle in the interval of 5-25°. Grooves originated from the first scanning are practically erased with the second re-scanning using a different angle which results in a smooth, nearly plane surface. Re-scanning by FIB using two different angles in the plane of the milled side is usually achieved by turning the specimen around the axis perpendicular to the milled side by these two angles while milling by FIB is done in each of the positions. This process of re-polishing by tilting the specimen and milling in tilted positions is described, for example, in the article called Characterization and Failure Analysis of 3D Integrated Systems Using a Novel Plasma-FIB System written by Laurens Kwakman, German Franz, Maaike Margrete Visser Taklo, Armin Klumpp, and Peter Ramm, which was published in the AIP Conf. Proc. 1395 Collection FRONTIERS OF CHARACTERIZATION AND METROLOGY FOR NANOELECTRONICS, pages 269-273.
In most applications, it is necessary to monitor the milled surface during the milling process (end-point detection). For example, with TSV structures, the milling process must be stopped in the middle of TSV structure which is to be further examined, in other cases, milling the specimen to exactly defined thickness is necessary, etc.
All the known applications pose a disadvantage in the sense that they do not enable this simultaneous monitoring by observing the milled surface directly during milling or at least by individual stages of milling without further manipulation with the specimen. The problem is, that the position of the milled surface used for milling by FIB does not enable observing this milled surface using the FIB beam. During the milling process, the FIB beam is tangential or nearly tangential to the surface which is to be imaged.
One of the options is to tilt, after a period of milling, the specimen by several tens of degrees into the FIB observation position and backwards, but this causes a delay, brings errors due to the repeatability of the manipulator and it does not allow for the observation of the milling process in real time. Therefore, it is impossible to control the milling process adequately and timely correction of the milling parameters or its timely halt is not possible either, which may even lead to the destruction of the structure that was to be originally examined.
Using the FIB beam for milling and imaging is described for example in the US patent application 2012/0091360 A1. This application also deals with the problems in imaging when using ion beam with plasma source. A beam capable of milling with sufficient efficiency would mill the specimen too intensively during the imaging too, which is absolutely undesirable. That is why the application US 2012/0091360 A1 proposes to make radical changes in the ion beam parameters, such as different excitation of the ion optics elements in the beam path, various diaphragms limiting the beam, etc. as well as changes related to ion sources, between milling and imaging. Xe ions are proposed for milling, while H ions are preferred for imaging, there are also variations in gas pressures and RF sources frequencies and performance. These changes of parameters are, of course, complicated from the point of view of technology and they are expensive. The apparatus described in the application US 2012/0091360 A1 does not enable milling and observing the specimen at the same time.
There are solutions which enable monitoring the process of milling or its depth; however, these solutions cannot be combined with the specimen surface polishing. These solutions are realized in apparatuses with two particle beams. In these apparatuses usually one beam, the FIB beam, serves for the purposes of milling the specimen surface and the other beam, usually the electron one from scanning electron microscope, hereinafter referred to as SEM, is used to image the milled surface. Substantial disadvantage of these solutions is the fact that especially in the case when the specimen must be tilted in order to get polished with FIB, observing and checking with the second beam is impossible, at least in one of the tilted positions. Common configuration does not allow for the second beam to “see” the milled surface, i.e. to form a reasonably big angle with this surface in both tilted positions.
In the so far known solutions, the specimen is placed in the specimen holder, which is attached to the manipulator, and at the same time the manipulator enables the specimen to move and/or to turn. Manipulators commonly allow for a movement along three axes X, Y, and Z which are perpendicular to one another with a possibility of supplementary rotations or tilts. Rotation is the most frequent, usually with no angle limits, i.e. of up to 360°, around perpendicular axis, supplemented possibly by a tilt, limited to an acute angle, e.g. max. 45° around one of the horizontal axes X or Y. In the case of using the tilt of the specimen, the originally vertical rotation axis usually tilts by the same angle.
FIB and SEM axes are usually intersecting and both beams point to the area where the milled specimen is. Apart from this, the mutual position of the FIB and SEM is also limited by physical dimensions of these apparatuses.
Tilt of the specimen using the manipulator serves for the specimen to be set in a position suitable for the required application. For the observing in SEM itself, it is the non-tilted position with the surface of the specimen holder perpendicular to the SEM axis. For processing by FIB, the specimen is usually tilted so that the FIB beam in the middle position of scanning impinges on the processed surface either approximately perpendicularly or approximately tangentially. The middle position of scanning is a position where scanning coils or electrodes do not deflect the beam from its original direction, which is usually also the middle position between the highest deviations to both sides in the directions the scanning is performed in. For the applications of the TSV structure studies type, and also for numerous other ones, approximate tangential milling is advantageous.
In one of the known embodiments, the FIB and SEM axes form an angle of about 50° and the surface of the specimen holder is perpendicular to the SEM axis in its initial position. In this particular case, in order to mill the specimen side perpendicular to the specimen holder approximately tangentially by the FIB beam, the specimen holder must be tilted of about 50° compared to its initial position. In this position, the SEM beam forms in its middle position of scanning an angle of about 50° with the processed surface, which means that the specimen remains well observable in SEM even during approximately tangential milling by FIB. Unfortunately, during one scanning of the milled surface by the FIB beam there appears the above mentioned curtaining effect on the specimen surface which must be removed by polishing. Heretofore-known methods and commonly used manipulators, however, are not able to ensure the polishing so that the surface can be observed by the SEM beam in both positions, i.e. also in the tilted position, which is necessary for checking the polishing process permanently or at least in selected intervals, without any further manipulation with the specimen. This represents a major disadvantage incompatible with numerous applications, because at least in one position the milling is performed out of the field of view of the SEM, i.e. blindly, which may result even in unwanted milling and destruction of the structures which should have been observed. To tilt the specimen, it might be also necessary to change the specimen holder. In this case, the angle of the specimen tilt is defined by the angle of the tilt of the holder, i.e. it is fixed and it is impossible to change it dynamically, which poses another disadvantage.
Patent application WO 2013039891 for removing curtaining effect recommends that planarization of the examined structure surface is executed. This is achieved through milling in either tangential or nearly tangential direction to the specimen surface before the cut itself is performed. If the curtaining effect appears due to non-homogeneity of the cut itself, which is true in most cases, this solution is functional only to a limited extent.
Patent applications WO 2013082496 and WO 2012103534 specifically focus on preparation of specimens for transmission electron microscopy, hereinafter referred to as TEM. Apart from other things, they also deal with curtaining effect reduction during the specimen preparations and propose a method called backside milling. During this process TEM lamellas get thinner from the silicone substrate side, which is homogeneous and therefore curtaining effect does not originate in it. Curtaining effect appears in the structures underneath the silicone and it is insignificant if the examined layer lies directly below the silicone plate. A method to remove surface defects in the silicone substrate has been also designed.
The method described in these applications is specifically focused on the preparation of ultra-thin TEM lamellas, it concerns shallow cuts only and materials containing homogeneous silicone layer with the curtaining effect reduction occurring only closely underneath the aforementioned layer.