Electron channeling pattern acquisition from small crystalline areas

A method for crystal analysis includes identifying a crystalline region on a device where an electronic channeling pattern is needed to be determined, acquiring a whole image for each of a plurality of different positions for the crystalline region using a scanning electron microscope (SEM) as the crystalline region is moved to different positions. Relevant regions are extracted from the whole images. The images of the relevant regions are stitched together to form a composite map of a full electron channeling pattern representative of the crystalline region wherein the electronic channeling pattern is provided due to an increase in effective angular range between a SEM beam and a surface of the crystal region.

BACKGROUND

Technical Field

The present invention generally relates to crystal analysis, and more particularly to systems and methods for acquiring images for identifying an electron channeling pattern in a crystalline material.

Description of the Related Art

Continued performance improvements in devices require heterogeneous integration of different materials, nanoscale fin geometry, as well as synthesis of metastable phases. Such techniques can be prone to crystalline defect generation which can have deleterious effects on device performance. Crystalline defects are most often not visible to conventional non-destructive techniques such as conventional scanning electron microscopy (SEM) and optical tools.

Methods for improving transistor performance include the introduction of permanent strain in a channel region using either external stressor or strained epitaxial channel layers. Along with the introduction of strain in complementary metal oxide semiconductor (CMOS) technology, comes greater susceptibility of forming dislocations during device processing. Detection of such dislocations and other crystallographic defects is conventionally achieved by transmission electron microscopy (TEM) which allows only limited statistics of defects, and their distribution due to the limited imaging area (typically <10×10 μm2). However, for manufacturing control, the statistics of the distribution of defects from a much larger area is required because defects are known to affect performance and reliability adversely. Thus, the defect density needs to be lower than is typically feasible to statistically investigate by TEM. For example, for materials with high defect density a small area is needed to get adequate defect statistics, but for low defect densities (which are needed for manufacturing ‘good’ devices) larger areas are needed to acquire significant statistics on defect distribution and densities.

SUMMARY

In accordance with an embodiment of the present invention, a method for crystal analysis includes identifying a crystalline region on a device where an electronic channeling pattern is needed to determine the exact crystal orientation of the crystalline area with respect to the electron beam. A partial image of the whole electron channeling pattern (ECP) is acquired for each of a number of different positions of the crystalline region with respect to the electron beam using a scanning electron microscope (SEM). After a partial image of the ECP is acquired, the position of the crystalline region is translated using the SEM stage to physically move the crystalline region to an at least one new location in the SEM microscope and an at least second image is taken with the SEM. The partial images of the ECP are stitched together to form a composite map of a larger portion of the ECP. This larger area of the ECP provides an increase in effective angular range between a SEM beam and a surface of the crystalline region allowing for more accurate identification of crystal structure and orientation.

Another method for crystal analysis includes identifying a crystalline region on a device where an electronic channeling pattern is needed to be determined, and acquiring a whole image for each of a plurality of different positions for the crystalline region using a scanning electron microscope (SEM) as the crystalline region is moved to different positions. Relevant regions are extracted from the whole images. The images of the relevant regions are stitched together to form a composite map of a full electron channeling pattern representative of the crystalline region wherein the electronic channeling pattern is provided due to an increase in effective angular range between a SEM beam and a surface of the crystal region.

Yet another method for crystal analysis includes identifying a crystalline region on a device where electronic channeling pattern is needed to be determined; stepping through a plurality of positions for the crystalline region by adjusting a location where a sample is located while a scanning electron microscope (SEM) image is to be taken; acquiring a whole image for each of the plurality of different positions of the crystalline region using the SEM; processing the whole images to stitch together a composite map of the crystalline region at different locations; and displaying a full electronic channeling pattern on a display.

A system for crystal analysis includes a scanning electron microscope (SEM) configured to scan a crystalline region of a device and a stage configured to mount the device and to step the device through a plurality of imaging positions on the crystalline region. A memory device includes imaging software configured to store whole images taken at each of the plurality of imaging positions on the crystalline region; extract portions of the whole image; and stitch the portions of the whole images together to form a composite map of the crystal region wherein a full electronic channeling pattern is provided in the composite map due to an increase in effective angular range between a SEM beam and a surface of the crystal region.

DETAILED DESCRIPTION

In accordance with embodiments of the present invention, systems and methods are provided for determining electron channel patterns on samples with only small areas of crystalline material. In particularly useful embodiments, electronic channel patterns are employed to determine exact alignment of the crystalline material and the electron beam in an SEM to acquire electron channeling contrast images (ECCI) which help to map and detect defects in crystal/crystalline materials. To determine crystal structure and orientation, a plurality of images of partial ECP images are stitched together to create a complete map or full ECP for a substrate, chip or device.

In accordance with one embodiment, ECCI is employed as a technique for rapid and high resolution characterization of individual crystalline defects in a scanning electron microscope (SEM). However, in accordance with the present embodiments, the ECCI application is no longer limited to bare semiconductor material samples in plan-view geometry. Instead, new modalities of ECCI techniques are provided herein with relevance to semiconductor manufacturing and failure analysis.

ECCI can also be employed to reveal misfit dislocation defects along a cleaved cross-section of a compositionally graded buffer (e.g., SiGe grown on Si). Plan-view imaging can be performed on patterned materials where only a small region or small regions of the crystalline material are present at the surface of the semiconductor device (e.g., SiGe/Si fins). The present embodiments permit the use of ECCI in inspecting crystallographic defects non-destructively and in a high throughput mode which is highly desirable for substrate quality control in manufacturing of products based on crystalline materials.

ECCI is highly effective in imaging defects with a spatial resolution similar to that of TEM. ECCI uses the variation in back-scattered electron (BSE) yield related to changes in the channeling of the electrons caused by lattice distortions around crystalline defects, which is greatest when a sample is aligned to a Bragg condition. Precise sample alignment is determined using the electron channeling pattern that results when the electron beam scans across the sample impinging at a range of angles. In backscattering geometry with a pole-piece mounted BSE detector, large areas of exposed single crystal sample area are traditionally necessary to obtain a wide range of beam-sample angles and hence a useful electron channeling map.

Thus far, ECCI imaging in semiconductors has been restricted to plan-view images on blanket films to view both misfit and threading dislocations. In accordance with the present embodiments, ECP images can be taken to enable ECCI to be employed in a plan-view mode on patterned devices with only small areas of the crystalline material or a cross-sectional mode to view misfit and threading dislocations in crystalline materials including a compositionally graded buffer (e.g., Si/Si0.8Ge0.2) over areas much larger than is possible with TEM with no sample preparation beyond a simple cleave. Thin layers of SiO2that normally reduce channeling contrast can be present and still permit for dislocation imaging without loss of resolution.

ECCI is a non-destructive advanced SEM-based technique that can be employed to characterize crystalline defects, offering advantages over destructive techniques such as TEM. The ECP is useful to determine the correct tilt for channeling contrast. Single crystal material requires areas greater than 100s of square microns to be exposed to see the channeling pattern. There are no convenient solutions to obtain channeling patterns using a regular SEM that does not involve beam rocking.

Referring now to the drawings in which like numerals represent the same or similar elements and initially toFIG. 1, a cross-sectional view8and a plan view20of a semiconductor device or sample10are illustratively shown. The device10includes a substrate or wafer12. The substrate12can include any suitable substrate structure, e.g., a bulk semiconductor, a semiconductor-on-insulator (SOI) substrate, etc. In one example, the substrate12can include a silicon-containing material. Illustrative examples of Si-containing materials suitable for the substrate12can include, but are not limited to, Si, SiGe, SiGeC, SiC and multi-layers thereof. Although silicon is the predominantly used semiconductor material in wafer fabrication, alternative semiconductor materials can be employed as additional layers, such as, but not limited to, germanium, gallium arsenide, gallium nitride, silicon germanium, cadmium telluride, zinc selenide, etc.

The substrate12includes a crystalline structure (e.g., monocrystalline). The substrate12can be etched to form a crystalline region or portion14, or the crystalline portion14can be grown through an amorphous or non-crystalline layer16. The layer16can include an amorphous form of the substrate material, a dielectric material, e.g., an oxide, any other non-crystalline material or a crystalline material not of interest. The portion14and the substrate12below the layer16are the target for electronic channel patterning (ECP) as these components include a crystallographic structure. In a particularly useful embodiment, the region14is a small size. The small size refers to being smaller than needed to obtain a full ECP pattern (which can be an area of 100's of square microns), and more particularly less than about 10×10 μm2).

A scanning electron microscope will be employed from over the plan view20to scan the device10. The crystalline region14is demarcated on the wafer whose ECP is desired. Scanning the device10includes a raster of a stage motor (not shown) to acquire a whole image for each raster step. The device10is mounted on a stage or platform that can be subjected to y direction and x direction (and z-direction, if needed) scanning or stage movement18,22, respectively. The SEM imaging conditions are adjusted to the settings needed to acquire an ECP. The complete image can include exposed or covered crystalline material (e.g., of the exposed substrate12(region14) or the covered (layer16) substrate.

Referring toFIG. 2with continued reference toFIG. 1, the portion of the ECP is only evident where the crystalline material of interest14is exposed. The complete image is cropped and only the portion24of the ECP is extracted with the location of the stage position indicated for reference. The sample is now translated at least once in x and/or y to a new stage position and an at least one new image is acquired. The crystalline region in the at least second image24now depicts a portion of the ECP related to the new position and the angular orientation change of the scanning beam associated with this new position. A relevant portion24of the ECP pattern is extracted from the complete image at each new stage position. The portions24of the ECP are arranged with respect to the stage position for each portion as inFIG. 2. As the stage moves, portion images are collected for each desired location. The full ECP28is generated by stitching each extracted portion image24together forming a composite image.

An electron channeling pattern (ECP) can be obtained when imaging along the portion14. Due to the absence of a large enough area for the full channeling pattern, a truncated channeling pattern is exhibited in the portion14. To obtain the full channeling map (full ECP), several images24of the partial electron channeling pattern are acquired by translating the sample10parallel to the substrate or wafer12thickness using movement of the stage where the sample or device10is mounted. The plurality of images at the different locations increases the effective angular range between the SEM beam and the surface of the sample10and reveals aspects of the ECP.

A complete map (full ECP)28is generated by compositing the plurality of portion images24together. This image28is similar to what could be obtained with a substrate that had a much larger continuous area of crystalline material of interest. Each image24is collected at a different stage location with the crystalline region14acting as a window to the full ECP channeling map28. The window provides a clearer image of the crystal structure and can be employed to better stitch together the other images. The system can automatically stitch together the full ECP28from the images24using the visible demarcated portion14of the ECP.

The full ECP28is obtained by compositing the images24using an image stitch program or algorithm26. Once generated, channeling bands are indicated in the channeling pattern28. The presence of higher order bands indicates the high quality of the surface.

In another embodiment, the e-beam of the SEM may be rastered only in the demarcated area (14). The stage is concurrently moved while acquiring images only of the demarcated area (region14). Then, the full ECP28can be stitched together using only the portion14of the ECP visible in the demarcated area.

Referring toFIG. 3, in another embodiment, if multiple disconnected crystalline areas34are visible, all areas or a subset of areas can be demarcated. Then, the stage motor can be stepped across the visible fields in a manner so as to avoid repeatedly collecting portions of ECP images. The final full ECP can be stitched from the images collected across the areas34in the same manner as described.

The accuracy of the stepper motor of the stage determines the accuracy of the ECP. The more images taken the greater the accuracy of the full ECP. Low quality ECP images are acceptable to obtain the tilt angle. The ECP can be employed to learn about the crystal structure, discover defects or identify other issues related to the crystal structure and orientation of the samples or device. The present embodiments employ non-destructive SEM imaging to provide highly accurate ECP maps comparable to those obtained with more destructive techniques (e.g., TEM).

Referring again toFIG. 1, another embodiment includes imaging a cross section (view8) of a sample10where a full ECP cannot be imaged because the cross section of the material (14) is smaller than the ECP. Therefore, the sample can be translated and multiple images stitched to create a full ECP as described above for patterned samples in other embodiments. Such samples can be cleaved to expose the cross-section for destructive testing.

Testing for such an embodiment is described in accordance with illustrative examples. In one example, SiGe samples were synthesized using reduced pressure chemical vapor deposition (RPCVD) at a temperature of about 800° C. with SiH2Cl2and GeH4as precursors. To demonstrate ECCI in cross-sectional geometry, a 4.5 micron compositionally graded buffer was grown on Si, with a surface composition of Si0.8Ge0.2. The sample was scribed and cleaved by hand in a laboratory ambient, exposing the (110) plane. Subsequent chemical cleaning procedures were not used.

Referring toFIGS. 4A and 4B, a 20 kV electron channeling map52is illustratively shown that was collected on the cleaved (110) cross-section of the silicon wafer. The wafer thickness is ˜700 micrometers (FIG. 4A). A composite image54(FIG. 4B) created by combining partial electron channeling pattern images collected by translating the sample as described above but in the vertical direction is also shown. The 220 and 004 channeling lines are indicated.

Image52(FIG. 4A) shows an electron channeling pattern (ECP) obtained when imaging along the cleaved (110) cross-section of the silicon substrate. Due to the absence of a large enough area for the full channeling pattern, the 700 μm substrate thickness exhibits a truncated channeling pattern. It can be challenging to tilt the sample to the desired Bragg condition using only a partial view of the channeling map. To circumvent this, several images of the partial electron channeling pattern were acquired by translating the sample parallel to the substrate thickness. This increased the effective angular range between the beam and the sample surface and a complete map was generated by compositing the images together.

Image54(FIG. 4B) shows the channeling map obtained by compositing six images. The 220 and 004 channeling bands are indicated. The presence of higher order 440 and 660 bands indicate the high quality of the cleaved surface.

Referring toFIG. 5, a sample is shown that was tilted to a Bragg condition located at the intersection of the 220 and 004 Kikuchi bands shown in cross-section at two different scales, 5 microns (image62) and 2 microns (image64). The range of conditions accessible was limited by the single tilt axis of the stage. The backscatter yield of Ge is greater than that of Si and hence the BSE signal increased with layer height, an effect not encountered in plan-view samples. The magnitude of ECCI contrast was weak compared to the compositional contrast but the slowly varying nature of the compositional contrast permitted background subtraction. A high-pass filter was used to remove only features with a length greater than 100 nm. The image contrast and brightness were then adjusted. A 20 μm wide view of the SiGe layer revealed regularly spaced misfit dislocations. The stage was translated across millimeters of the sample cross-section while staying at the Bragg condition. Inspection of large areas in cross-section, similar to plan-view ECCI, was demonstrated with misfit dislocations clearly visible in the images, with misfit dislocations perpendicular to the plane of the image also being visible with contrast similar to threading dislocations. Misfit dislocations lying below the Si/SiGe interface highlighted the advantages of cross-sectional ECCI as an inspection tool.

The use of electron channeling contrast in an SEM allows for rapid characterization of crystalline defects over areas much larger than those possible with TEM samples. The use of the technique can be extended to cleaved cross-sectional samples with only small areas of exposed crystalline material. To enable ECCI in these conditions, the ECP was acquired using multiple images and translating the stage.

The present embodiments can be employed as supplementary to traditional focused ion beam (FIB) and TEM-based defect analysis and can even inform the location of TEM sample preparation.

A speaker132can be operatively coupled to system bus102by the sound adapter130. A transceiver142is operatively coupled to system bus102by network adapter140. A display device162is operatively coupled to system bus102by display adapter160.

A scanning electron microscope (SEM)170is coupled to the system100, e.g., to the I/O adapter120. System100controls the SEM170, one or more stepper motors176and any other devices and functions on the SEM170. The system100coordinates movement of a stage174in one, two or three dimensions by employing the one or more stepper motors176to appropriately position the stage. A sample or device172is placed on the stage174for failure mode analysis, characterization of channel patterns, or any other ECCI related imaging. The sample172includes crystal structures to be evaluated or characterized. The motor176is synchronized with image collection of the SEM170to provide a plurality of images over one or more regions of the sample or device172. The images are stored in memory (e.g., first storage device122).

Imaging software180is employed to extract relevant portions of the images and stitch together the images by aligning pixel patterns, position data or a combination thereof. The imaging software180is employed to create a full ECP map from a number of smaller images as described herein. The imaging software180can include contrast, color/black and white controls, filters (e.g., high pass filters for removing uniform regions to focus on defects, etc.) and other visualization functions and image processing algorithms to highlight different aspects of the images (e.g., ECP definition, defect identification, etc.).

Referring toFIG. 7, methods for crystal analysis including electron channeling pattern acquisition from crystalline areas are illustratively shown. The methods for crystal analysis include identifying a crystal or crystalline region on a device where an electronic channeling pattern is needed to be determined. The system ofFIG. 6may be employed to carry out these methods. A whole image is acquired for each of a number of different positions for the crystal region using a scanning electron microscope (SEM). The crystalline region is moved to each new position with a stepper motor or other movement apparatus. Relevant portions of the whole images are extracted to remove any area that does not display a portion of the electron channeling pattern. The images portions are stitched together to form a composite map of the full electron channeling pattern, the electronic channeling pattern is provided due to an increase in effective area and angular range between a SEM beam and a surface of the crystalline region. The crystalline region can be buried below a thickness of non-crystalline material and the whole image can be acquired through the non-crystalline material.

In a particularly useful embodiment, in block302, one or more crystal or crystalline regions are identified on a device where an electronic channeling pattern is needed to be determined.

In block304, a whole image is acquired for each of a plurality of different positions for the crystalline region using a scanning electron microscope (SEM) as the crystalline region is moved to different positions. The whole image includes a full resolution image for each position in question. This can be adjusted in accordance with the capabilities of the SEM or in accordance with the acquisition plan (e.g., number of positions where the images are acquired, etc.). The whole image can be acquired by stepping through different positions using a stepper motor to change positions in at least two dimensions.

Relevant portions of the whole images are extracted to remove any area that does not display a portion of the electron channeling pattern. This can also include removing redundant portions or selecting a best version or versions of redundant portions (and removing the others). The whole image can be acquired with a plurality of different exposed crystalline regions on a same substrate or wafer, or a crystalline region of cross-sectioned device (e.g., cleaved for cross sectional analysis) can be employed.

In block306, the ECP portions from the crystalline regions of the whole images are processed and/or stitched together to form a composite map of the full electron channeling pattern representative of the crystal structure and orientation of the exposed crystalline region. This gives an increase in effective angular range between a SEM beam and a surface of the crystal region. The use of many images permits scanning electrons at different angles to obtain more features in the images. In useful embodiments, the crystalline region or regions are smaller than an area needed to view a full electronic channeling pattern for the device. The area needed to view the full electronic channeling pattern can be hundreds of square microns. The present embodiments can expand a small area (e.g., as low as 10 nm on a side) into a full electronic channeling pattern.

In block308, the composite map can be displayed on a display device to show a full electronic channeling pattern for the device. The full electronic channeling pattern permits the alignment of the electron beam into a known channeling condition. The present embodiments can be used in combination with electron channeling contrast imaging after the proper alignment permits identification of defects in at least the crystal region of the device.

Having described preferred embodiments for electron channeling pattern acquisition from small crystalline areas (which are intended to be illustrative and not limiting), it is noted that modifications and variations can be made by persons skilled in the art in light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiments disclosed which are within the scope of the invention as outlined by the appended claims. Having thus described aspects of the invention, with the details and particularity required by the patent laws, what is claimed and desired protected by Letters Patent is set forth in the appended claims.