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Copyright copy 2010 American Scientific PublishersAll rights reservedPrinted in the United States of America

Journal ofNanoscience and Nanotechnology

Vol 10 7808ndash7811 2010

Subsurface Damage of Single Crystalline SiliconCarbide in Nanoindentation Tests

Jiwang Yan1lowast Xiaohui Gai2 and Hirofumi Harada31Department of Nanomechanics Tohoku University Aramaki Aoba 6-6-01 Aoba-ku Sendai 980-8579 Japan

2Department of Metallurgy Material Science and Material Processing Tohoku UniversityAramaki Aoba 6-6-02 Aoba-ku Sendai 980-8579 Japan

3Siltronic Japan Corporation 3434 Shimata Hikari Yamaguchi 743-0063 Japan

The response of single crystalline silicon carbide (SiC) to a Berkovich nanoindenter was investigatedby examining the indents using a transmission electron microscope and the selected area electrondiffraction technique It was found that the depth of indentation-induced subsurface damage wasfar larger than the indentation depth and the damaging mechanism of SiC was distinctly differentfrom that of single crystalline silicon For silicon a broad amorphous region is formed underneaththe indenter after unloading for SiC however no amorphous phase was detected Instead a poly-crystalline structure with a grain size of ten nanometer level was identified directly under the indentertip Micro cracks basal plane dislocations and possible cross slips were also found around theindent These finding provide useful information for ultraprecision manufacturing of SiC wafers

Keywords Nanoindentation Silicon Carbide SiC Subsurface Damage Phase TransitionSemiconductor Manufacturing

1 INTRODUCTION

Single crystalline silicon carbide (SiC) is a next-generationsemiconductor material for advanced power devices andmicronano electromechanical systems (MNEMS) Com-pared with silicon SiC has much higher thermal conduc-tivity higher current density higher breakdown electricfield strength and broader band gap thus it is an excellentmaterial for high-temperature high-frequency and high-power applications1 The manufacturing of damage-freeprecision SiC wafers is an essential technical step fordevice manufacturing However due to its high hardnessand high brittleness SiC is very difficult to machineThus the difficulty in wafer manufacturing is one of themain problems limiting the development of SiC devicetechnologyTo improve wafer integrity subsurface damage gener-

ated by micronano machining is an emerging researchfocus from multidiscipline researchers2 Since the mechan-ical contact in an indentation test is geometricallyakin to that in a machining process understandingthe indentation-induced material deformation microfrac-ture and microstructural change can provide use-ful information for wafer manufacturing to eliminatemachining damages34 However to date literature on

lowastAuthor to whom correspondence should be addressed

indentation-induced damage of single crystalline SiC is stillvery limited5ndash7 A few fundamental aspects such as thesubsurface damage mechanism the relationship betweenthe damage depth and the indentation depth have not beenclarified Whether crystalline SiC undergoes amorphiza-tion in nanoindentation tests or not is still a controversialissueIn the present paper we investigated the responses of

single crystalline SiC to a sharp Berkovich nanoindenter atvarious loads The indentation-induced damage was inves-tigated by examining the load-displacement curves and theresidual indents using transmission electron microscope(TEM) and the selected area electron diffraction (SAED)techniques

2 EXPERIMENTAL DETAILS

A nanoindentation tester ENT-1100a produced by ElionixCo Ltd was used for the experiments Tests were per-formed using a Berkovich indenter made of single crys-talline diamond This kind of indenter is geometricallyakin to an extremely sharpened single point diamondtool in an ultraprecision machining process The maxi-mum load was varied from 10 mN to 100 mN by anincrement of 10 mN and then changed from 100 mNto 1000 mN by an increment of 100 mN The time for

7808 J Nanosci Nanotechnol 2010 Vol 10 No 11 1533-48802010107808004 doi101166jnn20102895

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Yan et al Subsurface Damage of Single Crystalline Silicon Carbide in Nanoindentation Tests

loading and unloading was the same and fixed to 5 sthus the loadingunloading rate changed in the range of02sim40 mNs The holding time was 1 s Ten indenta-tion tests were made for each maximum load and thepitch between every two neighboring indents was set to25 mA nitrogen-doped N -type single-crystal 6HndashSiC wafer

with a surface orientation of (0001) silicon face was usedas the specimen It has a hexagonal structure (zinc-blendestructure) and the lattice parameters are a = 3081 Aringand c = 15117 Aring The wafer was 508 mm in diameter0234 mm in thickness and obtained with a chemomechan-ical polished finish The SiC wafer has a Mohs hardnessof sim9 The circumferential orientation of the SiC waferto the indenter was adjusted to make the orientation flat[1010] parallel to one face of the indenter Thin-foil spec-imens for cross-sectional TEM (XTEM) observation wereprepared using a focused ion beam (FIB) apparatus in sucha way that the specimen contains the loading direction andthe center of the indent These specimens were then exam-ined using a TEM Hitachi H-9000NAR at an accelerat-ing voltage of 300 kV Selected area electron diffraction(SAED) was performed using the same TEM at a cam-era length of 10 m and a field area having a diameterof 100 nm

3 RESULTS AND DISCUSSION

31 XTEM Observation

Figure 1 is a bright field XTEM micrograph of an indentmade at a maximum load of 100 mN The long stripesseen cross the entire field are interference fringes causedby the bending of the TEM sample due to FIB process-ing It is seen that just below the indent surface thereis a region of intensive material deformation as indicatedby the dotted line the depth of which is approximately1 m The depth of material deformation is 25 times thatof the maximum indentation depth (04 m) and 5 timesthat of the residual depth of the indent (02 m) identifiedfrom the corresponding load-displacement curves Belowthe deformation region a long microcrack (median crack)extends straightly downwards to a depth of approximately4 m This result indicates that the critical condition forcrack initiation has been exceeded at this load althoughthere were no cracks observed on the wafer surface underthis condition That is to say subsurface cracks takeplace at a smaller load than surface cracks The relation-ship between damage depth and indentation depth in thisstudy is similar to that of 4HndashSiC in previous works56

However for 4HndashSiC the first cracks to form were lat-eral and radial microcracks5 rather than median cracksThis difference in cracking behavior might be a resultof difference of crystalline structure between 4Hndash and6HndashSiC

Microcrack

Deformation region

[1010]

[0001]

[1210]1 microm

Fig 1 XTEM micrograph of an indent generated at a maximum load of100 mN showing a region of subsurface deformation and a microcrackextending into the bulk

Figures 2(a) and (b) are magnified views of the left sideand the right side of the indent shown in Figure 1 Theregions indicated by circles A and B in Figure 2(a) are forSAED analysis Different from that of silicon indentation8

where beneath the indent a low-contrast grey region wasgenerated due to the amorphization of silicon in Figure 2however the deformation region shows a non-uniform con-trast indicating that the strain is significant and strain dis-tribution is not uniform In silicon indentation8 distinctboundaries between the amorphous region and the sur-rounding region could be identified in Figure 2 howeverthe boundaries between the deformation region and thesurrounding region are not very clear In Figure 2(b) dis-locations on the basal plane (0001) can be clearly iden-tified on the right side of the indent A dislocated region(the dark area indicated by the arrow) is expanding down-ward from the basal plane indicating that cross slippingwith other planes such as the pyramidal plane 1013 andthe prismatic plane 1010 might have occurred althoughthe Burgers vector of these dislocations cannot be clearlyindentified under the present conditionsIt is known that the slip systems of single crys-

talline SiC are (0001)1210 (0001)0110 10131210and 10101210 Under a load in [0001] directionthe Schmid factors for these slip systems are 03780282 0131 and 0019 respectively9 Dislocations of SiCare most likely to take place on the basal plane slipalong (0001)1210 or (0001)0110 cross slips along

J Nanosci Nanotechnol 10 7808ndash7811 2010 7809

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Subsurface Damage of Single Crystalline Silicon Carbide in Nanoindentation Tests Yan et al

AB

300 nm

(a)

(b)

Cross slipDislocationson basal plane

300 nm

Fig 2 Magnified views of (a) left side and (b) right side of the sameindent as that in Figure 1 Circles A and B in (a) indicate regions forSAED analysis in (b) basal plane dislocations and cross slip can beidentified

10131210 and 10101210 may also take place ifspecific conditions are satisfied

32 SAED Analysis

Figures 3(a) and (b) are SAED patterns of the regionsA and B indicated in Figure 2(a) In both figures onlydiffraction spots of [1010] incidence can be seen andno halo rings can be identified Therefore we can saythat SiC did not undergo amorphization under the presentindentation conditions In Figure 3(a) diffraction spots arestrongly twisted indicating that the crystalline periodic-ity has been severely disordered The diffraction spots areoriented along three different directions indicating that atleast three crystal grains exist within the selected area

(a)

(b)

Fig 3 SAED patterns of the selected regions (a) A and (b) B indicatedin Figure 2(a) showing a poly crystalline structure and a strained singlecrystalline structure respectively

In contrast the diffraction spots in Figure 3(b) are relativelyregular with less deformation

4 DISCUSSION

It has been reported that SiC undergoes phase transforma-tion from the zinc-blende structure to the rocksalt structurein high-pressure (sim105 GPa) diamond-anvil cell tests10ndash14

Amorphization of SiC has also been demonstrated inmicro cutting15 and molecular dynamics (MD) simula-tions of nanoindentation16 In the present study we did

7810 J Nanosci Nanotechnol 10 7808ndash7811 2010

RESEARCH

ARTIC

LE

Yan et al Subsurface Damage of Single Crystalline Silicon Carbide in Nanoindentation Tests

not find amorphization of SiC but detected its microstruc-tural change to a poly crystalline phase This structuralchange might be caused by two reasons one is disloca-tion movement of under a high shear stress and the otheris phase transition under a high pressure As the dislo-cation mobility of the zinc-blende SiC is very poor atroom temperature between the aforementioned two rea-sons the latter should be predominant That is to say inthe region directly beneath the indenter tip (region A inFig 2(a)) where an extremely high pressure (sim105 GPa)exists SiC transforms from the zinc-blende structure to therocksalt structure during loading Dislocations are easierto move in the rocksalt structure thus plastic deformationoccurs Then in unloading a reverse transition from therocksalt structure to the zinc-blende structure takes placeleaving a poly crystalline structure with intensive disloca-tions It is presumed that this phase transformation onlytakes place around the indenter tip where the pressure isextremely high in the surrounding region (region B inFig 2(a)) where the pressure is lower phase transforma-tion does not occur and elastic strain is dominant To fur-ther clarify this issue FEM analysis of the stress field inthe nanoindentation tests is necessary

5 CONCLUSIONS

Nanoindentation tests were performed on single crystallinesilicon carbide (SiC) using a sharp Berkovich nanoinden-ter The resulting indents were investigated by using ahigh resolution transmission electron microscope and theselected area electron diffraction technique We found thatthe depth of indentation-induced subsurface damage isfar larger than the indentation depth Directly under theindenter tip SiC changed into a poly-crystalline structurewith a grain size of ten nanometer level Micro cracks

and basal plane dislocations together with possible crossslips were also found around the indent while no amor-phous phase was confirmed The results from this studystrongly demonstrated that the subsurface damage mech-anism of SiC is distinctly different from that of singlecrystalline silicon These findings are important for ren-ovating the manufacturing processes for high-quality SiCwafers

References and Notes

1 T Kimoto Technical Report of IEICE Electron Devices 104 73(2004)

2 J Yan T Asami H Harada and T Kuriyagawa Prec Eng 33 378(2009)

3 J Yan H Takahashi J Tamaki X Gai H Harada and J PattenAppl Phys Lett 86 1 (2005)

4 J Yan H Takahashi J Tamaki X Gai and T Kuriyagawa ApplPhys Lett 87 1 (2005)

5 J Lankford and D L Davidson J Mater Sci 14 1669 (1979)6 H Kishimoto K H Park S Kondo K Ozawa and A Kohyama

J Electr Micro 53 515 (2004)7 J J Huening Walter M Liang X B Chen J Jang L Bergman

J A Patten G M Pharr and R J Nemanich Proc Mater ResSoc Symp 843 (2005)

8 J Yan H Takahashi X Gai H Harada J Tamaki andT Kuriyagawa Mater Sci Eng A 423 19 (2006)

9 K Maeda J Ceram Assoc Jap (in Japanese) 94 784 (1986)10 M Yoshida A Onodera M Ueno K Takemura and

O Shimomura Phys Rev B 48 10587 (1993)11 M S Miao and W R L Lambrecht Phys Rev B 68 092103

(2003)12 M Durandurdu J Phys Condens Matter 16 4411 (2004)13 S Eker and M Durandurdu EPL 84 26003 (2008)14 Y P Lu D W He J Zhu and X D Yang Physica B 403 3543

(2008)15 J A Patten W Gao and K Yasuto Trans ASME J Manuf Sci

Eng 127ndash3 522 (2005)16 I Szlufarska R K Kalia A Nakano and P Vashishta Appl Phys

Lett 85 378 (2004)

Received 4 September 2009 Accepted 30 October 2009

J Nanosci Nanotechnol 10 7808ndash7811 2010 7811

RESEARCH

ARTIC

LE

Subsurface Damage of Single Crystalline Silicon Carbide in Nanoindentation Tests Yan et al

AB

300 nm

(a)

(b)

Cross slipDislocationson basal plane

300 nm

Fig 2 Magnified views of (a) left side and (b) right side of the sameindent as that in Figure 1 Circles A and B in (a) indicate regions forSAED analysis in (b) basal plane dislocations and cross slip can beidentified

10131210 and 10101210 may also take place ifspecific conditions are satisfied

32 SAED Analysis

Figures 3(a) and (b) are SAED patterns of the regionsA and B indicated in Figure 2(a) In both figures onlydiffraction spots of [1010] incidence can be seen andno halo rings can be identified Therefore we can saythat SiC did not undergo amorphization under the presentindentation conditions In Figure 3(a) diffraction spots arestrongly twisted indicating that the crystalline periodic-ity has been severely disordered The diffraction spots areoriented along three different directions indicating that atleast three crystal grains exist within the selected area

(a)

(b)

Fig 3 SAED patterns of the selected regions (a) A and (b) B indicatedin Figure 2(a) showing a poly crystalline structure and a strained singlecrystalline structure respectively

In contrast the diffraction spots in Figure 3(b) are relativelyregular with less deformation

4 DISCUSSION

It has been reported that SiC undergoes phase transforma-tion from the zinc-blende structure to the rocksalt structurein high-pressure (sim105 GPa) diamond-anvil cell tests10ndash14

Amorphization of SiC has also been demonstrated inmicro cutting15 and molecular dynamics (MD) simula-tions of nanoindentation16 In the present study we did

7810 J Nanosci Nanotechnol 10 7808ndash7811 2010

RESEARCH

ARTIC

LE

Yan et al Subsurface Damage of Single Crystalline Silicon Carbide in Nanoindentation Tests

not find amorphization of SiC but detected its microstruc-tural change to a poly crystalline phase This structuralchange might be caused by two reasons one is disloca-tion movement of under a high shear stress and the otheris phase transition under a high pressure As the dislo-cation mobility of the zinc-blende SiC is very poor atroom temperature between the aforementioned two rea-sons the latter should be predominant That is to say inthe region directly beneath the indenter tip (region A inFig 2(a)) where an extremely high pressure (sim105 GPa)exists SiC transforms from the zinc-blende structure to therocksalt structure during loading Dislocations are easierto move in the rocksalt structure thus plastic deformationoccurs Then in unloading a reverse transition from therocksalt structure to the zinc-blende structure takes placeleaving a poly crystalline structure with intensive disloca-tions It is presumed that this phase transformation onlytakes place around the indenter tip where the pressure isextremely high in the surrounding region (region B inFig 2(a)) where the pressure is lower phase transforma-tion does not occur and elastic strain is dominant To fur-ther clarify this issue FEM analysis of the stress field inthe nanoindentation tests is necessary

5 CONCLUSIONS

Nanoindentation tests were performed on single crystallinesilicon carbide (SiC) using a sharp Berkovich nanoinden-ter The resulting indents were investigated by using ahigh resolution transmission electron microscope and theselected area electron diffraction technique We found thatthe depth of indentation-induced subsurface damage isfar larger than the indentation depth Directly under theindenter tip SiC changed into a poly-crystalline structurewith a grain size of ten nanometer level Micro cracks

and basal plane dislocations together with possible crossslips were also found around the indent while no amor-phous phase was confirmed The results from this studystrongly demonstrated that the subsurface damage mech-anism of SiC is distinctly different from that of singlecrystalline silicon These findings are important for ren-ovating the manufacturing processes for high-quality SiCwafers

References and Notes

1 T Kimoto Technical Report of IEICE Electron Devices 104 73(2004)

2 J Yan T Asami H Harada and T Kuriyagawa Prec Eng 33 378(2009)

3 J Yan H Takahashi J Tamaki X Gai H Harada and J PattenAppl Phys Lett 86 1 (2005)

4 J Yan H Takahashi J Tamaki X Gai and T Kuriyagawa ApplPhys Lett 87 1 (2005)

5 J Lankford and D L Davidson J Mater Sci 14 1669 (1979)6 H Kishimoto K H Park S Kondo K Ozawa and A Kohyama

J Electr Micro 53 515 (2004)7 J J Huening Walter M Liang X B Chen J Jang L Bergman

J A Patten G M Pharr and R J Nemanich Proc Mater ResSoc Symp 843 (2005)

8 J Yan H Takahashi X Gai H Harada J Tamaki andT Kuriyagawa Mater Sci Eng A 423 19 (2006)

9 K Maeda J Ceram Assoc Jap (in Japanese) 94 784 (1986)10 M Yoshida A Onodera M Ueno K Takemura and

O Shimomura Phys Rev B 48 10587 (1993)11 M S Miao and W R L Lambrecht Phys Rev B 68 092103

(2003)12 M Durandurdu J Phys Condens Matter 16 4411 (2004)13 S Eker and M Durandurdu EPL 84 26003 (2008)14 Y P Lu D W He J Zhu and X D Yang Physica B 403 3543

(2008)15 J A Patten W Gao and K Yasuto Trans ASME J Manuf Sci

Eng 127ndash3 522 (2005)16 I Szlufarska R K Kalia A Nakano and P Vashishta Appl Phys

Lett 85 378 (2004)

Received 4 September 2009 Accepted 30 October 2009

J Nanosci Nanotechnol 10 7808ndash7811 2010 7811

RESEARCH

ARTIC

LE

Yan et al Subsurface Damage of Single Crystalline Silicon Carbide in Nanoindentation Tests

not find amorphization of SiC but detected its microstruc-tural change to a poly crystalline phase This structuralchange might be caused by two reasons one is disloca-tion movement of under a high shear stress and the otheris phase transition under a high pressure As the dislo-cation mobility of the zinc-blende SiC is very poor atroom temperature between the aforementioned two rea-sons the latter should be predominant That is to say inthe region directly beneath the indenter tip (region A inFig 2(a)) where an extremely high pressure (sim105 GPa)exists SiC transforms from the zinc-blende structure to therocksalt structure during loading Dislocations are easierto move in the rocksalt structure thus plastic deformationoccurs Then in unloading a reverse transition from therocksalt structure to the zinc-blende structure takes placeleaving a poly crystalline structure with intensive disloca-tions It is presumed that this phase transformation onlytakes place around the indenter tip where the pressure isextremely high in the surrounding region (region B inFig 2(a)) where the pressure is lower phase transforma-tion does not occur and elastic strain is dominant To fur-ther clarify this issue FEM analysis of the stress field inthe nanoindentation tests is necessary

5 CONCLUSIONS

Nanoindentation tests were performed on single crystallinesilicon carbide (SiC) using a sharp Berkovich nanoinden-ter The resulting indents were investigated by using ahigh resolution transmission electron microscope and theselected area electron diffraction technique We found thatthe depth of indentation-induced subsurface damage isfar larger than the indentation depth Directly under theindenter tip SiC changed into a poly-crystalline structurewith a grain size of ten nanometer level Micro cracks

and basal plane dislocations together with possible crossslips were also found around the indent while no amor-phous phase was confirmed The results from this studystrongly demonstrated that the subsurface damage mech-anism of SiC is distinctly different from that of singlecrystalline silicon These findings are important for ren-ovating the manufacturing processes for high-quality SiCwafers

References and Notes

1 T Kimoto Technical Report of IEICE Electron Devices 104 73(2004)

2 J Yan T Asami H Harada and T Kuriyagawa Prec Eng 33 378(2009)

3 J Yan H Takahashi J Tamaki X Gai H Harada and J PattenAppl Phys Lett 86 1 (2005)

4 J Yan H Takahashi J Tamaki X Gai and T Kuriyagawa ApplPhys Lett 87 1 (2005)

5 J Lankford and D L Davidson J Mater Sci 14 1669 (1979)6 H Kishimoto K H Park S Kondo K Ozawa and A Kohyama

J Electr Micro 53 515 (2004)7 J J Huening Walter M Liang X B Chen J Jang L Bergman

J A Patten G M Pharr and R J Nemanich Proc Mater ResSoc Symp 843 (2005)

8 J Yan H Takahashi X Gai H Harada J Tamaki andT Kuriyagawa Mater Sci Eng A 423 19 (2006)

9 K Maeda J Ceram Assoc Jap (in Japanese) 94 784 (1986)10 M Yoshida A Onodera M Ueno K Takemura and

O Shimomura Phys Rev B 48 10587 (1993)11 M S Miao and W R L Lambrecht Phys Rev B 68 092103

(2003)12 M Durandurdu J Phys Condens Matter 16 4411 (2004)13 S Eker and M Durandurdu EPL 84 26003 (2008)14 Y P Lu D W He J Zhu and X D Yang Physica B 403 3543

(2008)15 J A Patten W Gao and K Yasuto Trans ASME J Manuf Sci

Eng 127ndash3 522 (2005)16 I Szlufarska R K Kalia A Nakano and P Vashishta Appl Phys

Lett 85 378 (2004)

Received 4 September 2009 Accepted 30 October 2009

J Nanosci Nanotechnol 10 7808ndash7811 2010 7811