Silicon Carbide and Glasses The SEM and EDS...
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Silicon Carbide and Glasses Silicon dioxide network While firing glass, some of the elements in some glasses react with silicon carbide, due to the high temperatures at which the process occurs (1100°-1450° C). The purpose of this project is to see whether the elements that react with SiC tend to move towards the substrate, thus segregating around the glass- SiC interface. To determine the veracity of this hypothesis, scanning electron microscopy and energy dispersive spectroscopy were employed. Corning 1723 2.51 11.1 11.4 4.60 9.43 60.8 Molar % BaO MgO CaO B 2 O 3 Al 2 O 3 SiO 2 Constituent Advantex 0 to 2 0.1 to 2 1 to 4 12 to 15 20 to 24 59 to 62 Wt. % K 2 O Na 2 O MgO Al 2 O 3 CaO SiO 2 Constituent Glass Compositions- red constituent indicates that it is reactive with SiC Corning 1723 Softening Point: 908°C Annealing Point: 710 °C Working Point: 1168 °C Advantex Softening Point: 916 °C Annealing Point: 736 °C Working Point: ~1250 °C The SEM and EDS The scanning electron microscope creates a magnified image of a specimen by using a series electromagnets, magnetic coils, and an aperture to direct a beam of electrons down an evacuated column towards the sample stage. In total, there are three electromagnets (two condenser lenses and one objective lens) which focus the beam, below which there is an aperture to ensure that the beam is a straight line. A magnetic coil is used to raster the beam across an area of the sample. As the beam scans across the target area, secondary electrons are released from the specimen, which are collected by a detector and converted into an image on a computer screen. In order for this to work, the specimen either has to be electrically conductive, or coated with a conductive substance such as gold. The diagram to the left illustrates what happens when the incident electron beam strikes an atom of the sample. A secondary electron from the sample is knocked out of its orbit and will eventually be picked up by the sensor. To replace the secondary electron, a higher level electron drops down one or more orbitals, thus releasing energy in the form of x-rays. By measuring the energy of the x-rays coming from the sample, it is possible to determine the elements that are present and give a standardless estimate as to how much is in the sample at a particular point. This is the basic principle behind energy dispersive spectroscopy, or EDS. Experimental Procedure The two glasses, 1723 and Advantex, were first powdered with mortar and pestle, then with milling beads. They were sieved with a 170 mesh (.088 mm) sieve. The sieved powders were then placed on silicon carbide coupons and placed in a furnace. Four samples of 1723 and two samples of Advantex were heated, each one heated at a rate of 5°C/min to the glass’s annealing point then at 4°C/min to a temperature close to or above the glass’s working point (the temperature was made different for each sample, thus introducing an independent variable). They were then cooled at a rate of 4°C/min to the annealing point and at 5°C/min to room temperature. At each annealing point and working point, the temperature was held constant for 30 minutes. The samples were then placed in resin overnight and cross- sectioned with a diamond saw. After polishing and gold sputter-coating the samples, they were placed in the SEM for imaging and EDS analysis. SEM image of Advantex-SiC interface SEM image of 1723-SiC interface Silicon carbide crystal Silicon carbide is an advanced ceramic that has quickly grown in importance over the past few years. Because of its combination of high thermal conductivity and poor electrical conductivity, it has the potential to replace current packaging materials in integrated chips. However, because it is difficult to create complex shapes of SiC, it is necessary to join multiple pieces of simple geometries by using glass as a sort of adhesive. This is achieved by mounting glass between two or more pieces of SiC and heating the pieces to the glass’s working point. Upon cooling, the pieces are “glued” together. Glasses are composed mainly of SiO 2 , which forms a non- crystalline network by sharing oxygen atoms. The two glasses used in this project, Advantex and Corning 1723, also contain a variety of other atoms which are dispersed throughout the network, taking the place of silicon atoms to form various oxides. Results and Discussion 0 200 400 600 800 1000 1200 Distance 1.54 4.63 7.71 10.8 13.88 16.97 20.05 23.14 26.22 29.31 32.39 35.48 38.56 41.64 44.73 47.81 50.9 53.98 57.07 60.15 63.24 66.32 69.41 72.49 MgK AlK SiK CaK BaL EDS analysis of glass 1723 showing relative compositions versus distance from interface (interface is at approximately 45 microns, where lines drop sharply) Visual analysis of the samples showed that the Corning 1723 samples had much fewer bubbles than the Advantex samples, indicating the composition of the Advantex glass is much more reactive with SiC (due to the presence of Na). Because boron, the only element in glass 1723 that is reactive, cannot be detected by EDS (the x-ray energy is too small to be detected), no evidence of elemental segregation could be found in this glass. Sample 8 Na:Si Ratio 0 0.005 0.01 0.015 0.02 0.025 0.03 0 20 40 60 80 100 120 Distance from Interface (microns) Na:Si Ratio Sample 9 Na:Si Ratio 0 0.005 0.01 0.015 0.02 0.025 0 50 100 150 200 250 300 350 400 Distance from Interface (microns) Na:Si Ratio The point analyses from the Advantex samples were not very conclusive. There was a slight decrease in the concentration of Na as one moved away from the interface initially. However, the concentration rose again as one moved even further from the interface. As can be seen in the graphs above, the Na:Si ratios in the two Advantex samples display a similar pattern as one moves farther from the interface. This could possibly be a sign of Na segregation near the interface, affecting only the Na ions up to a certain distance. However, since there are only two samples from which data was taken, more research would be necessary before any conclusions could be drawn. Part of the difficulty in collecting EDS data from this glass is the migration of Na ions away from the surface when subjected to an electron beam (Lineweaver, 1963). Possible solutions to this include the use of Auger electron spectroscopy in conjunction with ion milling instead of EDS (Pantano, 1975). Sources: http://www.sigmaaldrich.com/img/assets/9280/silicon_dioxide.jpg http://www.wag.caltech.edu/gallery/SiC.gif http://elec.chandra.ac.th/courses/5513101/termwork/sem/pong/1.5.jpg http://www.lko.uni-erlangen.de/Media/Web-Bilder/bild-equipment/Jeol.jpg http://geoinfo.nmt.edu/labs/microprobe/images/xrayr.gif J.L. Lineweaver, J. Appl. Phys. 34 (1963) 1786. C.G. Pantano Jr., D.B. Dove and G.Y. Onoda Jr., J. Non-Cryst. Solids 19 (1975) 41. This work was supported through the National Science Foundation: Division of Materials Research REU site program under grant number 0139125. This model of SEM was used in the project Diagram of SEM components Effect of incident beam on sample atom Na:Si ratio from both Advantex samples. Both show similar patterns of Na concentration with respect to distance from interface.
Transcript of Silicon Carbide and Glasses The SEM and EDS...
Silicon Carbide and Glasses
Silicon dioxide network
While firing glass, some of the elements in some glasses react with silicon carbide, due to the high temperatures at which the process occurs (1100°-1450° C). The purpose of this project is to see whether the elements that react with SiC tend to move towards the substrate, thus segregating around the glass-SiC interface. To determine the veracity of this hypothesis, scanning electron microscopy and energy dispersive spectroscopy were employed.
Corning 1723
2.5111.111.44.609.4360.8Molar %
BaOMgOCaOB2O3Al2O3SiO2Constituent
Advantex
0 to 20.1 to 21 to 412 to 1520 to 2459 to 62Wt. %
K2ONa2OMgOAl2O3CaOSiO2Constituent
Glass Compositions- red constituent indicates that it is reactive with SiC
Corning 1723Softening Point: 908°CAnnealing Point: 710 °CWorking Point: 1168 °C
AdvantexSoftening Point: 916 °CAnnealing Point: 736 °CWorking Point: ~1250 °C
The SEM and EDS
The scanning electron microscope creates a magnified image of a specimen by using a series electromagnets, magnetic coils, and an aperture to direct a beam of electrons down an evacuated column towards the sample stage. In total, there are three electromagnets (two condenser lenses and one objective lens) which focus the beam, below which there is an aperture to ensure that the beam is a straight line. A magnetic coil is used to raster the beam across an area of the sample. As the beam scans across the target area, secondary electrons are released from the specimen, which are collected by a detector and converted into an image on a computer screen. In order for this to work, the specimen either has to be electrically conductive, or coated with a conductive substance such as gold.
The diagram to the left illustrates what happens when the incident electron beam strikes an atom of the sample. A secondary electron from the sample is knocked out of its orbit and will eventually be picked up by the sensor. To replace the secondary electron, a higher level electron drops down one or more orbitals, thus releasing energy in the form of x-rays. By measuring the energy of the x-rays coming from the sample, it is possible to determine the elements that are present and give a standardless estimate as to how much is in the sample at a particular point. This is the basic principle behind energy dispersive spectroscopy, or EDS.
Experimental Procedure
The two glasses, 1723 and Advantex, were first powdered with mortar and pestle, then with milling beads. They were sieved with a 170 mesh (.088 mm) sieve. The sieved powders were then placed on silicon carbide coupons and placed in a furnace. Four samples of 1723 and two samples of Advantex were heated, each one heated at a rate of 5°C/min to the glass’s annealing point then at 4°C/min to a temperature close to or above the glass’s working point (the temperature was made different for each sample, thus introducing an independent variable). They were then cooled at a rate of 4°C/min to the annealing point and at 5°C/min to room temperature. At each annealing point and working point, the temperature was held constant for 30 minutes. The samples were then placed in resin overnight and cross-sectioned with a diamond saw. After polishing and gold sputter-coating the samples, they were placed in the SEM for imaging and EDS analysis.
SEM image of Advantex-SiC interface
SEM image of 1723-SiC interface
Silicon carbide crystal
Silicon carbide is an advanced ceramic that has quickly grown in importance over the past few years. Because of its combination of high thermal conductivity and poor electrical conductivity, it has the potential to replace current packaging materials in integrated chips. However, because it is difficult to create complex shapes of SiC, it is necessary to join multiple pieces of simple geometries by using glass as a sort of adhesive. This is achieved by mounting glass between two or more pieces of SiC and heating the pieces to the glass’s working point. Upon cooling, the pieces are “glued” together.
Glasses are composed mainly of SiO2, which forms a non-crystalline network by sharing oxygen atoms. The two glasses used in this project, Advantex and Corning 1723, also contain a variety of other atoms which are dispersed throughout the network, taking the place of silicon atoms to form various oxides.
Results and Discussion
0
200
400
600
800
1000
1200
Dis
tanc
e
1.54
4.63
7.71
10.8
13.8
8
16.9
7
20.0
5
23.1
4
26.2
2
29.3
1
32.3
9
35.4
8
38.5
6
41.6
4
44.7
3
47.8
1
50.9
53.9
8
57.0
7
60.1
5
63.2
4
66.3
2
69.4
1
72.4
9
MgK AlK SiK CaK BaL
EDS analysis of glass 1723 showing relative compositions versus distance from interface (interface is at approximately 45 microns, where lines drop sharply)
Visual analysis of the samples showed that the Corning 1723 samples had much fewer bubbles than the Advantex samples, indicating the composition of the Advantex glass is much more reactive with SiC (due to the presence of Na). Because boron, the only element in glass 1723 that is reactive, cannot be detected by EDS (the x-ray energy is too small to be detected), no evidence of elemental segregation could be found in this glass.
Sample 8 Na:Si Ratio
0
0.005
0.01
0.015
0.02
0.025
0.03
0 20 40 60 80 100 120
Distance from Interface (microns)
Na:
Si R
atio
Sample 9 Na:Si Ratio
0
0.005
0.01
0.015
0.02
0.025
0 50 100 150 200 250 300 350 400
Distance from Interface (microns)
Na:
Si R
atio
The point analyses from the Advantex samples were not very conclusive. There was a slight decrease in the concentration of Na as one moved away from the interface initially. However, the concentration rose again as one moved even further from the interface. As can be seen in the graphs above, the Na:Si ratios in the two Advantex samples display a similar pattern as one moves farther from the interface. This could possibly be a sign of Na segregation near the interface, affecting only the Na ions up to a certain distance. However, since there are only two samples from which data was taken, more research would be necessary before any conclusions could be drawn. Part of the difficulty in collecting EDS data from this glass is the migration of Na ions away from the surface when subjected to an electron beam (Lineweaver, 1963). Possible solutions to this include the use of Auger electron spectroscopy in conjunction with ion milling instead of EDS (Pantano, 1975).
Sources:http://www.sigmaaldrich.com/img/assets/9280/silicon_dioxide.jpg
http://www.wag.caltech.edu/gallery/SiC.gifhttp://elec.chandra.ac.th/courses/5513101/termwork/sem/pong/1.5.jpg
http://www.lko.uni-erlangen.de/Media/Web-Bilder/bild-equipment/Jeol.jpghttp://geoinfo.nmt.edu/labs/microprobe/images/xrayr.gif
J.L. Lineweaver, J. Appl. Phys. 34 (1963) 1786.C.G. Pantano Jr., D.B. Dove and G.Y. Onoda Jr., J. Non-Cryst. Solids 19 (1975) 41.
This work was supported through the National Science Foundation: Division of Materials Research REU site
program under grant number 0139125.
This model of SEM was used in the project
Diagram of SEM components
Effect of incident beam on sample atom
Na:Si ratio from both Advantex samples. Both show similar patterns of Na concentration with respect to distance from interface.
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