Monocrystalline silicon, as a key material in the integrated circuit industry, has been a popular research topic for micro-cracking failure and brittle damage during processing at different temperatures [1-3]. The study of cracking damage mechanisms and processing damage thresholds at high temperatures has been a difficult problem [4-6]. High-temperature indentation testing has been
widely used to study the crack failure mode of hard and brittle materials such as monocrystalline silicon at different temperatures [7-9]. Pharr et al. found that for sharper indenter, well-developed radial cracks can be clearly observed, and the crack length decreases with the increase of angle [10, 11]. Jaya B N et al found that the resistance of monocrystalline silicon to crack extension was enhanced at temperatures above 300 ℃ [12]. Suzuki et al. concluded that the high-temperature deformation of monocrystalline silicon (above 500 ℃) occurs through plastic deformation, i.e., nucleation and slip motion of dislocations [13]. The insensitivity of the hardness to temperature below 500 ℃ is due to the Si-I to Si-II transition of the diamond structure. Nakao Shigeki et al. showed that as the temperature increased from RT to 150°C. The fracture toughness of monocrystalline silicon was reduced by the temperature change from RT to 150 ℃ [14]. The fracture toughness of monocrystalline silicon was also increased by the temperature change from RT to 150 ℃. The fracture toughness of monocrystalline silicon increases rapidly, and the enhanced dislocation activity leads to a change in the direction of crack tip extension. The fracture mode changed to a mixed mode of opening mode and tearing mode. However, the existing research has not yet elucidated the mechanism of the cracking damage mode of monocrystalline silicon under high temperature and complex stress. The transformation of the fracture mechanism for monocrystalline silicon at high temperatures is still unclear.
In this paper, the crack initiation and extension process and the brittle-plastic transition process of monocrystalline silicon from RT to 500 ℃ were investigated by in-situ indentation test. The monocrystalline silicon material used in the test was obtained from Hefei Kejing Material Technology Co., Ltd. In order to easier investigate the surface crack extension of monocrystalline silicon during the indentation process, a cubic angle indenter from Synton-MDP., Ltd. was used for the experiments. The maximum indentation load was 500 mN. The self-developed indentation device was compatible with a TESCAN VEGA4 scan electron microscope, as shown in Fig. 1 (a). The heating module independently heats the indenter and the specimen, eliminating the effect of temperature drift during contact indentation. The indentation curves of monocrystalline silicon at different temperatures from RT to 500 ℃ were obtained. Hardness and modulus of elasticity of monocrystalline silicon were calculated from the indentation curves, as shown in Fig. 1 (b) (c) (d). It can be seen that hardness of monocrystalline silicon decreases gradually with the increase of temperature. However, compared to RT, hardness of monocrystalline silicon at 200 ℃, 300 ℃, 400 ℃, and 500 ℃ is about 96.1%, 87.2%, 81.4%, and 56.8%, respectively. Hardness decreases dramatically from 400 ℃ to 500 ℃. The elastic modulus of monocrystalline silicon also decreases gradually with increasing temperature as can be seen in Fig. 1 (d). The interaction force and bonding strength between monocrystalline silicon atoms at high temperatures weaken with increasing temperature. This leads to a decrease in the elastic modulus and elastic recovery capacity.
Fig. 1 (a) In-situ high-temperature indentation test device in the SEM (b) Load-displacement curve of indentation of monocrystalline silicon at RT-500 ℃ (c) Indentation hardness of monocrystalline silicon at RT-500 ℃ (d) Modulus of elasticity of monocrystalline silicon at RT-500 ℃
Figures 2 a b c show the in-situ observed monocrystalline silicon indentation test processes at RT, 100 ℃, and 200 ℃, respectively. During the indentation process at RT, extrusion occurs at the indenter prism position due to high hydrostatic pressure, as shown in Fig. 2a I II. This is caused by the transformation of the material from the Si-I phase to the ductile metallic phase Si-II during the loading process [15, 16]. The same phenomenon was also observed in the in-situ indentation tests at 100 ℃ and 200 ℃. After the contact of the indenter with the material surface, radial cracks were first produced along the direction of the indenter edges, as shown in Fig. 2a II. Secondary radial cracks were initiated as the load increased at RT. Secondary radial cracks nucleated at the contact position of the indenter edges and extended slowly during the loading process, as shown in Fig. 2 a III IV. Shallow surface damage with small area appeared at the location of the indenter prismatic surface. It can be seen from Fig. 2 a V that both radial cracks and secondary radial cracks extended during the unloading process. However, the secondary radial cracks extended faster. In addition, at the other prismatic surface of the indenter, a shallow lateral crack was initiated during the unloading process. Shallow lateral cracks initiate at the edges of the contact imprints and extend almost parallel to the surface [17]. The driving force for shallow lateral cracks is typically large during the unloading process, causing the shallow lateral cracks to extend rapidly toward the surface.
As the temperature increased to 100 ℃, only radial crack extension and secondary radial crack initiation were observed during the loading process as well. During unloading, secondary radial cracks initially extended perpendicular to the specimen surface. Later, it changes to approximately parallel to the surface and turns to curling. Two secondary radial cracks envelop inward to form a shallow scallop-shell lateral peeling, as shown in Fig. 2 b IV V. Comparing the phenomena at RT, it can be concluded that secondary radial cracks and shallow lateral cracks are unified crack systems. In Fig. 2 b VI, it can be seen that the depth of secondary radial cracks becomes progressively shallower from the place of initiation to the end. This indicates that the driving force of radial crack extension decreases rapidly with increasing depth, while the driving force of lateral cracks is larger during unloading [18]. The radial crack stops extending downward during unloading, and its further extension can be realized by turning to the lateral path. With the combined effect of the two types of cracks, shallow scallop shell peeling is formed on the surface of the material. This is precisely the reason for chipping and surface material removal during monocrystalline silicon machining. Extrusion of metal-like phases, shallow scallop-shell peeling, and secondary radial crack systems that did not closure occurred in the residual indentation imprint after unloading, respectively, as shown in Fig. 2 c.
Fig. 2 SEM images of in-situ indentation of monocrystalline silicon at different temperatures
a RT b 100 ℃ c 200 ℃
The surface crack morphology of the monocrystalline silicon changed when the temperature was increased to 300 ℃. Only radial cracks were observed on the surface when loaded to maximum load. Secondary radial cracks and shallow lateral cracks did not occur. Similarly no chipping and peeling occurred as seen at lower temperatures, as shown in Fig. 3 a I II. Radial cracks grew further during unloading. Turning of the cracks and peeling of the material surface during unloading didn't appear due to the increase in temperature which suppressed the secondary radial cracks and shallow lateral cracks to some extent. At the end of unloading, the extrusion of the material below the prismatic surface of the indenter was still observed in Fig. 3 a III, but it was not significant compared to the previous two temperatures. In previous molecular dynamics simulation studies, it was shown that high temperatures reduced the energy barrier for structural changes. This leads to a lower contact pressure of the material, and a lower Si-II content below the indenter at 300 ℃ [19].
In contrast, radial cracks were not all observed below the indenter edges during loading at 400 ℃. The material under the prismatic surface of the indenter showed side crack growth and further extension during unloading, as shown in Fig. 3 b I-V. Extrusion was not observed throughout the indentation process. This phenomenon represents that the transformation of monocrystalline silicon from Si-I phase to Si-II phase did not occur during the indentation process. The disappearance of shallow lateral cracks and extrusion, and the change in the crack initiation location indicate deformation mode changes in monocrystalline silicon. The deformation mode of monocrystalline silicon changes from phase transition to dislocation slip. And 400 ℃ is in the transition temperature of brittle-plastic transition of monocrystalline silicon. Below the prismatic surface of the indenter is the position of highest stress during the whole indentation process. The activation of dislocation slip is not sufficient to release all the stresses. As a result, atypical side cracking occurs at this location. The above inference can be confirmed in Fig. 3 b VI. Slip lines appear near the residual indentation imprint. It represents the thermal activation of dislocation slip in monocrystalline silicon by high temperature.
When the temperature was further increased to 500 ℃, no cracking was observed in monocrystalline silicon throughout the indentation process, as shown in Fig. 3 c I-II. The plastic behavior exhibited by monocrystalline silicon at high temperatures relieves the strain/stress concentration around the crack tip. Nucleation and propagation of dislocations from the breaking of covalent brittle interatomic bonds. The increased dislocation motion shields the crack and initiates plastic deformation. It makes the cracking of the material difficult. A bulge (pile-up) in the material below the prismatic surface of the indenter was observed through Figure 3 c III. It can be seen with the gradual increase in temperature, the material in contact with the prismatic surface of the indenter deformation mode from shallow scallop-shell lateral peeling to the extrusion only, 400 ℃ and then into the coupling of dislocation slippage and linear cracking. 500 ℃ into a good plasticity of the pile-up.
Fig. 3 SEM images of in-situ indentation of monocrystalline silicon at different temperatures
a 300 ℃ b 400 ℃ c 500 ℃
In order to compare visually the improvement of monocrystalline silicon resistance to cracking with increasing temperature. Five repetitive experiments were performed for each temperature point, and the cracking thresholds of monocrystalline silicon indentation tests at different temperatures were recorded, as shown in Fig. 4. At temperature lower than 400 ℃, the cracking threshold was not sensitive to temperature changes. Lower temperatures do not activate dislocation slip in monocrystalline silicon. The stresses at the indenter edges are released through radial cracks. When the temperature increases above 400 ℃, the cracking threshold increases significantly with temperature.
Fig. 4 Indentation cracking threshold load at different
temperatures for monocrystalline silicon
Figure 5 represents the deformation and crack damage modes of monocrystalline silicon after indentation tests at different temperatures. RT to 200 ℃, monocrystalline silicon mainly exhibits brittle fracture damage. Extrusion, radial cracks and shallow scallop-shell lateral peeling appeared on the surface, as shown in Fig. 5 (a). The initiation of secondary radial cracks is inhibited when the temperature reaches 300 ℃. Lateral peeling is no longer easily generated, as shown in Fig. 5 (b). Extrusion of the material also becomes weaker compared to RT. This is due to the decrease in hardness of the material at high temperatures, which increases the deformation during the pressing process. The hydrostatic pressure decreases and the transition zone from Si-I to Si-II becomes smaller. At 400 ℃, plastic deformation replaces phase transformation. Significant slip lines and pile-ups appeared. Dislocation generation is the root cause of plastic deformation and strain concentration at the crack tip [20]. Dislocation motion coexists but competes with microcrack nucleation-expansion. High temperatures inhibit crack nucleation and extension and activate the slip motion of dislocations. The cell of monocrystalline silicon consists of two dislocated centered lattices displaced 1/4 diagonal length. High temperature activated dislocation slip is accommodated by two asymmetric slip planes [21]. Multiple dissociation planes are involved in the fracture behavior of monocrystalline silicon initiating side cracking at high hydrostatic pressure, as shown in Fig 5 (c). The brittle-plastic transition is the main reason for the increase in the cracking threshold of monocrystalline silicon in Fig. 4. When the temperature reaches 500 ℃, dislocation slip in monocrystalline silicon is fully activated. Fig. 5 (d), at 500 mN, cracks no longer appear on the surface. Pile-up phenomenon is more obvious.
Fig. 5 Schematic diagram of the effect by indentation surface of monocrystalline silicon at different temperatures (a) RT,100 ℃, 200 ℃ (b) 300 ℃ (c) 400 ℃ (d) 500 ℃
In conclusion, in-situ observation of crack initiation and extension during indentation of monocrystalline silicon from room temperature to 500 ℃ was carried out inside SEM. It is found that radial cracks and secondary radial cracks are successively generated in the loading process, and the material at the prismatic surface of the indenter also generates extrusion and shallow lateral cracks due to high pressure. Secondary radial cracks are turned and enveloped during the unloading process due to the driving force of the lateral cracks. Formation of shallow scallop-shell lateral peeling that leads to surface material removal during monocrystalline silicon processing. The reduction of hardness and the reduction of the phase transition zone at 300 ℃ lead to the inhibition of secondary radial cracking. Extrusion becomes weak and disappears at 400 ℃. The maximum hydrostatic pressure does not reach the transformation threshold of Si-II. Side cracks with slip lines symbolize the brittle-plastic transition of the material. After further temperature increase dislocation slip replaces the phase transition and the material shows good plasticity. The hardness and cracking threshold also increase dramatically. This study reveals the microzone damage mechanism of monocrystalline silicon at brittle-plastic transition temperatures. It indicates the feasibility of high-temperature plastic molding processing of monocrystalline silicon.