A. Morphological damages

Figure 1 shows a series of SEM micrographs of the surface morphologies of a SiN_x film. The film was deposited at 448 K and was 1100-nm thick. The images were taken from a few samples, which had been heat treated at different temperatures in air for 1.8 ks prior to the observation. Each image shown is from a different sample. Fig. 1(a) shows an as-deposited film. The film surface was smooth without any sign of defects to the resolution of the SEM (∼5 nm). Figure 1(b) shows a sample heated at 873 K. The sample was found to have developed a population of nearly perfect circular craters. The sizes of the craters ranged from 30 to 70 μm. Figure 1(c) shows a sample heated to 1148 K. It is seen that the population of the circular craters had increased and the sizes of the craters, appearing in between 10 and 30 μm, had decreased. These craters were found to be formed by penny-shaped disks of the film fractured off the surface, as evident in the micrograph in the circled area. These circular craters are referred to as “penny cracks.” Figure 1(d) shows a detailed view of the surface of the film in a similar area indicated by “A” in Fig. 1(c). It is seen that the film developed uniformly dispersed dark dots, which are identified to be small pinholes. The inset in Fig. 1(d) shows a pinhole at a higher magnification from an area indicated by the white rectangle. The pinholes are <100 nm in size. Figure 1(e) shows a sample heated to 1273 K. This sample exhibited fewer penny cracks compared with the sample heated to 1148 K but more roughened and damaged surface. Figure 1(f) had been heated to 1373 K. This sample exhibited massive uniformly distributed random cracks, referred to as “dry mud” cracks. In addition, very few small pockets of crystals were observed, as indicated in the circled area.

FIG. 1.

Morphology of plasma-enhanced chemical vapor deposition (PECVD) SiN_x films deposited at 448 K and subsequently heat treated to different temperatures up to 1373 K for 1.8 ks.

Figure 2 shows a selection of SEM images of the surface morphologies of a film deposited at 573 K. The as-deposited film showed similar “featureless” morphology to that shown in Fig. 1(a). Figure 2(a) shown in Fig. 2 is for a sample heated to 773 K. The sample was found to exhibit some long straight-line cracks, mostly parallel in a few self-defined directions. Figure 2(b) shows a sample heated to 973 K. This sample shows more extensive line cracks and some spalling of large pieces from the substrate. Occasionally, some penny cracks were also seen. Sample (c) was heated to 1148 K. This sample developed more penny cracks but no thin-line cracks. The sizes of the penny cracks were similar to those observed in the film deposited at 448 K, but the population was much lower, as evident in the comparison with Fig. 1(c). Sample (d) was heated to 1373 K. This sample exhibits a high population of small “pits,” ∼3 μm in size, in coexistence with some penny cracks, which were just under 10 μm in size. Close examination indicates that each pit is actually a nest of small crystals of ∼100–300 nm in size [as shown in Fig. 5(a)]. The film also contains pinholes and random cracks.

FIG. 2.

Morphology of PECVD SiN_x films deposited at 573 K and subsequently heat treated to different temperatures up to 1373 K for 1.8 ks.

These observations are unique in the literature. Some other but similar forms of physical damages have been reported in the literature for other thin film materials. Cordill et al. analyzed “telephone cord” shaped delamination buckling of compressively stressed tungsten thin films.Reference Cordill, Bahr, Moody and Gerberich¹³ Coupeau et al. conducted similar analysis on circular buckling blister of Au thin film on Si substrate.Reference Coupeau, George, Regache, Colin and Grilhe¹⁴ The most relevant observations are reported by Hughey and Cook.Reference Hughey and Cook¹⁵ In their work, they observed circular blistering of severely compressively stressed PECVD SiN_x thin films caused by thermal cycling. The formation of these blisters has been attributed to the release of bonded hydrogen and the compressive stresses existing in the films.

Figure 3 shows a schematic expression summarizing the types of the thermally induced morphological damages and the temperature regimes of the damages observed in the two films. It is generally found that the film deposited at 573 K is more resistant to heating than the film deposited at 448 K. The low-temperature film exhibited more penny cracks and dry mud cracks, whereas the high-temperature film exhibited much more extensive crystallization.

FIG. 3.

Overview of the thermal damages of PECVD SiN_x deposited at 448 K and 573 K.

B. Chemical damages

Figure 4 shows the FTIR measurements of the two SiN_x films. The spectra shown in (a) are for the film deposited at 448 K, and those shown in (b) are for the film deposited at 573 K. Sample (i) in each figure was in the as-deposited state. The as-deposited films contained high populations of Si–N bonds and N–H bonds and trace amounts of Si–O bonds. Samples (ii), (iii), and (iv) have been heat treated in air for 30 min at 673, 1148, and 1373 K, respectively. It is seen that heating in air has led to progressive loss of N–H bonds, weakening of the intensity of Si–N bonds, and the appearance and strengthening of the intensity of Si–O bonds. These measurements demonstrate that the PECVD SiN_x films were actually SiN_xH_y films and that heating in air to elevated temperatures causes oxidation of the films at the expense of their N and H contents. A detailed analysis of the chemical changes has been reported elsewhere.Reference Jehanathan, Liu, Walmsley, Dell and Saunders^12, Reference Jehanathan, Walmsley, Liu and Dell¹⁶ It has been found that the original SiN_xH_y exists in a structure conforming to a “random bonding” model and that oxidation leads to the decomposition of the matrix to a “random mixing” structure of amorphous SiN_x and crystalline SiO₂.Reference Aydinli and Ay^17–Reference Ray, Das, Maiti, Lahiri and Chakraborti¹⁹

FIG. 4.

FTIR spectra for SiN_x films deposited at (a) 448 K and (b) 573 K; subsequently heat treated at temperatures up to 1373 K for 1.8 ks.

It has also been generally observed that the film deposited at 448 K is less oxidation-resistant and contains a higher concentration of hydrogen than the film deposited at 573 K. This is consistent with previous studies.Reference Santana, Fandino, Ortiz and Alonso³

C. Crystallization

Figure 5 shows SEM images of the crystals formed. Figure 5(a) shows a detailed view of a crystal pit from the same sample shown in Fig. 2(d). The white particles appearing at the bottom of the pit are small, equiaxial, and multiple faceted crystals of 100–300 nm in size. The dark spots appearing with the white crystals are empty pockets left behind by some crystals that had dropped out. The pit appears to be significantly larger in volume than the crystals, implying a significant missing volume.

FIG. 5.

SEM images of the crystals formed in the matrix of SiN_x.

Figure 5(b) shows an SEM image of the cross section of a SiN_x film deposited at 448 K and subsequently heat treated to 1148 K. The cross-section sample was prepared by FIB. The Pt and Au layers on the top are the working layers coated after the heat treatment for FIB processing and are not part of the original sample. The dashed horizontal line marks the boundary between the SiN_x film and the Si substrate. The micrograph reveals two hidden crystals formed inside the body of the film. It is evident that large cavities are formed around these crystals. Energy-filtered transmission electron microscopic analysis indicates that the crystals contain only Si and N, whereas the surrounding matrix contains Si, Ni, and low level of O.Reference Jehanathan, Saunders, Liu and Dell²⁰ Electron diffraction analysis indicates that the crystals have a hexagonal structure similar to β-Si₃N₄.Reference Jehanathan, Saunders, Liu and Dell²⁰

D. Damage mechanisms

It is seen that the amorphous PECVD SiN_x films experience physical, chemical, and structural damages during heating to elevated temperatures in air. The chemical damage is mainly in the form of oxidation, at the expense of N and H contents of the films. The structural damages include crystallization of the amorphous SiN_x matrix to β-Si₃N₄. The physical damages are in multiple forms, including the penny cracks, pinholes, random dry mud cracks, line cracks, and spalling. From the evidences collected in this study, the line cracking and spalling are considered specific cases related to the particular conditions of the samples tested. The pinholes are possibly related to the loss of the N and H contents at high temperatures in air; but the current study does not provide more information to enable a convincing identification of its possible mechanisms.

The penny cracks have been found to form by circular buckling or circular blistering. Such failure mode implies compressive in-plane stresses. The circular shape implies that the compressive stresses in-plane isotropy. The random cracks are typical of the morphology of cracking under the influence of in-plane tensile stresses. It is commonly considered that internal stresses, mostly thermal stresses, cause film fracture and delamination.Reference Hughey and Cook^15, Reference Audoly²¹ In comparison, little attention is paid to the effect of possible changes in chemical structure of the films. It is evident that heating to elevated temperatures causes oxidation of the amorphous SiN_x into crystalline SiO₂ at relatively lower temperatures and the crystallization of the amorphous SiN_x into Si₃Ni₄ at the higher temperatures, i.e.,

(1)

(2)

The PECVD SiN_x films used in this study, with an approximate atomic ratio of SiN_1.3,Reference Jehanathan, Walmsley, Liu and Dell¹⁶ have a density 2.4 g/cm³.Reference Walmsley, Liu, Hu, Bush, Winchester, Martyniuk, Dell and Faraone²² The volume changes of the above two reactions are estimated as summarized in Table I.

TABLE I.

Volume changes of SiN_x due to oxidation and crystallization.

It is seen that the oxidation of SiN_x to SiO₂ is associated with a volume expansion of 19.7%, whereas the crystallization of amorphous SiN_x to Si₃Ni₄ is associated with a volume contraction of 24.4%. It needs to be pointed out that these volume changes are estimated for the complete conversion of the reactions. In reality, the reactions are highly incomplete. Such volume changes are consistent with the forms of cracking observed in the respective temperature ranges. Figure 6 shows a schematic illustration of the mechanisms of cracking damages of the films.

FIG. 6.

Cracking mechanisms of PECVD SiN_x films.

In addition, it is also expected that the oxidation process is time-dependent, whereas in contrast the crystallization is more a temperature-dependent process. This condition is critical for the cracking of the films heated to above 1273 K, in which dry mud cracks are observed. In this case, the fast heating the samples through the intermediate temperature range does not provide enough diffusion time for oxidation, thus the build-up of excessive compressive stresses to cause the formation of the penny cracks. This expectation is also consistent with the observation that the high-temperature deposited PECVD SiN_x film is more chemically stable and oxidation resistant, thus lower extent and higher temperatures of penny cracking, than the low-temperature deposited film.