Water diffusion and fracture in organosilicate glass film stacks

Transcription

1 Acta Materialia 55 (2007) Water diffuion and fracture in organoilicate gla film tack Youbo Lin a, Ting Y. Tui b, Joot J. Vlaak a, * a Diviion of Engineering and Applied Science, Harvard Univerity, 29 Oxford Street, Cambridge, MA , USA b Silicon Technology Development, Texa Intrument Inc., TI Boulevard, Dalla, TX 75243, USA Received 22 September 2006; received in revied form 17 November 2006; accepted 26 November 2006 Available online 30 January 2007 Abtract Organoilicate gla (OSG) coating with low dielectric permittivity are widely ued a dielectric in high-performance integrated circuit. OSG i very brittle and it i uceptible to tre-corroion cracking in water-containing environment. We have invetigated the adheion degradation of ilicon nitride/osg and ilicon carbonitride/osg interface caued by water diffuion. Experimental reult are in good quantitative agreement with an analytical model that combine water diffuion with ubcritical crack growth. Fracture experiment how that water diffuion in OSG film tack i remarkably fat and that it ha an activation energy of 0.27 ev. The adheion degradation i completely reverible under mild annealing condition. Interfacial plama treatment reult in a ignificant enhancement of the adheion in the abence of water, but thi enhancement i lot almot completely upon expoure of the film tack to water. A diffuion tudy uing deuterium a an iotopic tracer how that the Si/OSG interface i the main diffuion path. Ó 2007 Acta Materialia Inc. Publihed by Elevier Ltd. All right reerved. Keyword: Diffuion; Fracture; Organoilicate gla; Adheion; Thin film 1. Introduction Organoilicate gla (OSG) i a hybrid organic inorganic material that conit of a iloxane network imilar to that of amorphou ilicon dioxide where ome of the bridging oxygen atom have been replaced by hydrogen ( H) or hydroxyl group ( OH) and by organic group uch a methyl ( CH 3 ) or methylene ( CH 2 ) [1 3]. OSG ha found many application in the microelectronic [4 6], biomedical [7 12] and environmental field [13,14] a a reult of it excellent electronic, biological, optical and catalytic propertie. In the emiconductor indutry, OSG i ued a the dielectric material between copper interconnect in high-performance integrated circuit becaue of it low permittivity. But integration of OSG into microelectronic proce flow i challenging, becaue of the low fracture toughne of OSG and it vulnerability to trecorroion cracking [15 20]. It i well etablihed that the preence of water in the environment ha a detrimental * Correponding author. Tel.: ; fax: addre: vlaak@eag.harvard.edu (J.J. Vlaak). effect on the reitance of OSG to fracture and on the adheion of other film to it; even mall amount of water in the atmophere can ignificantly accelerate crack velocity a water molecule react with the trained Si O bond at the crack tip. The correlation between crack velocity, water partial preure and applied energy releae rate ha been extenively characterized for OSG film tack by mean of ubcritical crack growth experiment [16,18], in which crack velocity in a controlled environment i meaured a a function of applied energy releae rate. The uceptibility of OSG to tre-corroion cracking ha important conequence for the interpretation of adheion tet in the preence of reactive pecie uch a water, becaue of the kinetic of the reactive bond-breaking proce [21,22]. If, for intance, an interfacial crack i driven at a finite velocity, the applied energy releae rate doe not correpond to the equilibrium adheion energy. Rather, it i a kinetic value that depend not only on the material ytem under conideration, but alo on crack velocity and the availability of reactive pecie. Without performing extenive ubcritical meaurement, there i no way of knowing a priori the proximity of thi energy releae rate /$30.00 Ó 2007 Acta Materialia Inc. Publihed by Elevier Ltd. All right reerved. doi: /j.actamat

2 2456 Y. Lin et al. / Acta Materialia 55 (2007) to the equilibrium adheion energy. From a practical point of view, however, thee energy releae rate can be ued for ytem ytem comparion and they may be referred to a adheion energie [21,22] with the implicit undertanding that they correpond to the energy releae rate required to drive a crack at a given velocity in a given environment. Thi i the approach taken in thi tudy. If an OSG-containing film tack i expoed to water prior to delamination, diffuion of water into the film tack can everely degrade it adheion [23], a phenomenon alo oberved for ome other material ytem [22]. Even though the fracture proce in thi cae i nearly independent of the teting environment, thi adheion degradation i the reult of ubcritical crack growth: indeed, if water i already preent in the film tack, it will lower the adheion energy even if the meaurement i made in an inert atmophere; tranport of water molecule from the environment to the crack tip i not neceary. Evidently, adheion degradation i an iue of concern for OSG film tack ued in integrated circuit becaue integrated circuit are repeatedly expoed to water during the manufacturing proce. In thi paper, we preent the reult of an in-depth tudy of the adheion degradation of OSG film tack a a reult of expoure to water prior to fracture. We will how that thi phenomenon can be ued to meaure the diffuion coefficient of water in multi-layered tructure and we will explore the effect of interfacial plama treatment on the degradation proce. In Section 2, we briefly dicu a quantitative model that couple diffuion of a reactive pecie and ubcritical crack growth to predict how the adheion energy varie with expoure time. In the following ection, a erie of four-point flexure experiment i preented to evaluate the effect of water expoure time and temperature on adheion. Next, we addre the effect of interfacial plama treatment and we preent the reult of D 2 O diffuion experiment aimed at identifying the main diffuion path. We conclude our dicuion with a imple application in the form of adheion degradation map. 2. Water diffuion-induced adheion degradation in film tack In thi ection, we decribe a model for the degradation of the adheion energy of a film tack caued by the diffuion of an active pecie uch a water. We apply the model to the four-point flexure geometry, which wa ued to perform the adheion meaurement in thi tudy, but the model i eaily generalized to other geometrie. Fig. 1a how a chematic diagram of the four-point flexure pecimen. In the four-point flexure technique, which i commonly ued to meaure adheion energie [24 27], the film tack of interet i andwiched between two ubtrate and the entire aembly i deformed in bending until delamination occur. A more detailed decription of the technique i given in the experimental ection of thi paper. One of the main advantage of the technique Fig. 1. (a) Schematic view of a four-point flexure pecimen. (b) Croectional view of an adheion ample howing the geometry of the diffuion problem. i that the energy releae rate for film delamination can be calculated from macrocopic variable that are readily meaured. When a four-point flexure pecimen i expoed to water, water molecule diffue into the film tack from the edge of the ample and diffuion front develop a illutrated chematically in Fig. 1b. In a typical four-point flexure meaurement, the crack propagate at or near the filmubtrate interface in the direction perpendicular to the direction of diffuion, i.e. the water concentration varie along the crack front. Thu, if the crack travel at contant velocity, the energy releae rate mut alo vary along the crack front. It i well known that the energy releae rate to drive a crack at a given velocity in the reaction-controlled regime varie linearly with the chemical potential of water in the environment [15 18,28,29]. If the water vapor in the environment can be regarded a an ideal ga, then G ¼ B nnrt ln p H2 O ¼ B A ln p H2 O for G 6 G c ; ð1þ where n i the number of water molecule reacting with each broken bond, N i the areal bond denity, B i a contant that depend on crack velocity, R i the ideal ga contant, T i the abolute temperature at which the crack propagate and G i the energy releae rate for that crack velocity. A and B are readily determined uing ubcritical fracture meaurement [16,18,23]. Eq. (1) make it poible to couple the water diffuion profile in the film tack to the energy releae rate required to drive a crack at a contant velocity. Now conider an experiment in which water i allowed to diffue into a four-point flexure pecimen at temperature T D and the pecimen i teted at a temperature T A. If water diolved in OSG follow Henry law, the energy releae rate needed to fracture a four-point flexure pecimen decreae with increaing expoure time t according to the following expreion [23]:

3 GðtÞ ¼B A ln CT D p T C T A A v þ Uðc; Þ ; ð2þ where Z ( 1 4 X 1 1 Uðc; Þ ¼ ln 1 ð1 cþ inðð2k þ 1ÞpnÞ 0 p 2k þ 1 k¼0!) expð ð2k þ 1Þ 2 p 2 Þ dn ð3þ and Y. Lin et al. / Acta Materialia 55 (2007) c ¼ CT A C T D p T A i p T A v ; and ¼ DT D H 2 O b 2 t: ð4þ In thee expreion, C T D and C T A repreent the olubility of water at T D and T A, repectively; p T A v i the water vapor preure at T A ; p T A i i the water partial preure at T A in equilibrium with the initial water content of the film tack. D T D H 2 O i the diffuion coefficient of water in the film tack and b i the width of the four-point flexure pecimen. The quantitie c and are non-dimenional meaure for the initial water concentration in the film tack and the time the pecimen ha been expoed to water, repectively. The function U(c, ) determine how the energy releae rate change with expoure time and i readily evaluated numerically (Fig. 2a). Fig. 2b how the adheion energy required to delaminate a four-point flexure pecimen a a function of expoure time and for different level of initial water content. It hould be noted that Eq. (3) i valid only a long a the initial water content i large enough that the critical energy releae rate G c i not exceeded anywhere along the crack front. If the initial water concentration i too low, the linear relationhip between energy releae rate and chemical potential break down. The energy releae rate i then bounded by the critical energy releae rate and Eq. (3) take on a rather le convenient form. The geometry analyzed in thi ection i alo appropriate for the double-cantilever-beam pecimen, another tet geometry often ued for thin-film fracture meaurement. Eq. (2) (4) can be applied directly to double-cantileverbeam experiment, provided the contant A and B are obtained under pure mode I condition. Finally, it hould be noted that, with the exception of the diffuion coefficient D T D H 2 O, the olubility ratio, and the contant c, all parameter in Eq. (2) can be meaured independently uing ubcritical fracture meaurement. Accordingly, Eq. (2) can be ued to determine the water diffuion coefficient in a film tack from a erie of fracture experiment. Intead of expoing a dry film tack to water and meauring the change in adheion energy a a function of expoure time, it i alo poible to firt aturate the film tack with water and then drive out the water by annealing in a flowing inert ga. Such an out-gaing experiment ha everal advantage: uing thi approach, the diffuion experiment can be performed at temperature well above Fig. 2. (a) U(c,) a a function of the normalized time for different initial water content ratio of the film. U(c,) approache ln(c) for very mall value of. Curve are hown for c increment of (b) Change of the adheion energy a a function of normalized expoure time. The adheion energy i normalized by the adheion energy of a film tack aturated with water at 25 C. the boiling point of water. Conequently, it i poible to obtain diffuion data over a wide range of temperature and to etablih an accurate value of the diffuion activation energy. An out-gaing experiment alo provide information on the reveribility of the adheion degradation. If water diffue into the OSG and participate in a chemical reaction, a low-temperature anneal i unlikely to revere the degradation. If, on the other hand, there i no interaction between water and OSG except in the preence of a tre field, the degradation proce i expected to be reverible. In the latter cae, it i traightforward to how that the change in adheion energy with annealing time i given by GðtÞ ¼B A ln pt A v þ Uðc c f ; Þ ; ð5þ f where c f i now a non-dimenional meaure of the water partial preure p T D f in the annealing environment: c f ¼ CT A C T D p T D v : ð6þ p T D f

4 2458 Y. Lin et al. / Acta Materialia 55 (2007) Experimental method OSG film with a thickne of 420 nm were depoited onto bare (1 0 0)-oriented ilicon ubtrate at 400 C uing an indutrial plama-enhanced chemical vapor depoition (PECVD) proce with 1,3,5,7-tetramethylcyclotetrailoxane (TMCTS) a a precuror, oxygen a an oxidizer and carbon dioxide. The film had a poroity of approximately 7% a determined by X-ray reflectometry, and a relative dielectric contant of approximately 2.8. The OSG film were capped with one of two type of barrier layer, either a 200-nm-thick PECVD ilicon nitride (SiN x ) or a 375-nmthick PECVD ilicon carbonitride (SiCN) coating. The OSG film with the SiCN coating were treated with an ammonia or a helium plama immediately prior to the SiCN depoition; experimental detail are ummarized in Table 1. Thin-film delamination tet were conducted uing the four-point flexure technique under contant diplacement rate condition [24 26]. Such condition lead to a contant load during crack extenion, which i directly related to the adheion energy of the film tack [21]. The ample preparation technique and teting procedure have been decribed in detail elewhere [16 18]. Briefly, the Si ubtrate with the OSG film tack were bonded to upporting Si ubtrate uing a pin-on epoxy (EPO-TEK 353ND from Epoxy Technology) and cured for 1 h at 120 C. The bonded ubtrate were cut into 60 mm 6 mm beam uing a high-peed dicing aw. Notche were machined acro the ubtrate beam to facilitate crack initiation upon bending. The pecimen were baked at 180 C in a vacuum furnace with a bae preure of Pa for 14 h, before being ubmerged in deionized water at 25 or 95 C for pre-defined period of time. After ubmerion, the ample were dried and immediately teted uing the four-point flexure technique. Additionally, ome pecimen were ubmerged in deionized water at room temperature for 4 week to fully aturate them with water. Thee ample were ubequently baked at 150 C in flowing argon for pecified period of time and immediately teted uing the four-point flexure technique. All four-point flexure tet were performed in dry N 2 at 25 C at a crohead peed of 0.3 lm 1. The correponding crack velocity wa approximately 20 lm 1. All experimental loading curve diplayed well-developed load plateau during crack extenion allowing the determination of the adheion energie. A tated previouly, the adheion energie thu obtained correpond to the energy releae rate required to drive a crack at a given velocity in a given environment. After the adheion meaurement, the fracture urface were analyzed by X-ray photoelectron pectrocopy (XPS) to determine the precie fracture interface. Throughout the meaurement, all pecimen were fully randomized to eliminate ytematic error caued by ubtrate-to-ubtrate variation or ubtle variation in ample preparation and teting procedure. Subcritical crack growth meaurement in environment with controlled water partial preure were performed on the OSG/SiCN ample uing a load relaxation technique a decribed in Ref. [18]. Subcritical reult for the OSG/SiN x film tack have been reported previouly [18]. In order to determine whether water diffue through the bulk of the OSG film or mainly along the interface, a diffuion experiment wa carried out with deuterium a an iotopic tracer: three 8 mm 8 mm Si ubtrate with the OSG/SiN x film tack were expoed to heavy water (D 2 O, from Cambridge Iotope Laboratorie, Inc.) at room temperature for period of 1, 7 and 15 day, repectively. The SiN x coating i an effective barrier to D 2 O diffuion o that D 2 O could diffue into the film tack only from the edge of the ample. The compoition at the center of thee ample wa meaured a a function of depth uing econdary ion ma pectrocopy (SIMS) with a primary beam of 3 kev C + ion. Concentration were calculated from meaured econdary ion count uing relative enitivity factor determined from the abolute concentration of carbon and fluorine in a reference ample meaured by Rutherford backcattering pectrocopy. 4. Reult and dicuion 4.1. Adheion degradation Fig. 3 how the adheion energy of the OSG/SiN x interface a a function of the time the film tack were expoed Table 1 Film tack data Film tack Plama treatment Si/420 nm OSG/200 nm SiN x No treatment Si/420 nm OSG/375 nm SiCN 90 NH 3 Fig. 3. Adheion energy of the OSG/SiN x interface a a function of waterexpoure time. Dicrete point repreent experimental meaurement; olid Si/420 nm OSG/375 nm SiCN 90 NH 3 /10 He Si/420 nm OSG/375 nm SiCN 10 He/90 NH 3 line repreent the model prediction.

5 Y. Lin et al. / Acta Materialia 55 (2007) to water at 25 and 95 C. XPS analyi confirm that the delamination occurred at the OSG/SiN x interface. The arrow mark the adheion energy for film tack immediately after the vacuum bake at 180 C. The two curve in the figure are very imilar: the adheion energy i approximately 7 J m 2 prior to expoure to water, but it drop off quickly after water i allowed to diffue into the film tack. Eventually, the adheion energy reache a plateau value of le than 3 J m 2, indicating that the film tack are aturated. The figure clearly demontrate the detrimental effect of water on the adheion of the OSG/SiN x interface. Two obervation are noteworthy. Firt, the plateau value for the diffuion experiment at 95 C i lower than for the experiment at 25 C. The difference i mall, but i tatitically ignificant; it i probably aociated with an enhanced olubility of water in the OSG coating at elevated temperature. Second, the degradation of the adheion occur much fater at elevated temperature. Thi accelerated degradation i the reult of a nearly tenfold increae in the diffuion coefficient at thi temperature. The model preented in the previou ection can be ued to calculate quantitative value for the diffuion coefficient uing the procedure decribed in Ref. [23] along with the ubcritical meaurement publihed by Vlaak et al. [18]. The value of A, B, c and D thu obtained are lited in Table 2. The olid line in Fig. 3 repreent the adheion energy obtained from the diffuion model it i evident that the model i in cloe agreement with the experimental reult. Experiment were alo carried out in which OSG/SiN x film tack that were previouly aturated with water were annealed in flowing argon at a temperature of 150 C. Fig. 4 how the adheion energy required to delaminate thee OSG/SiN x film tack a a function of the annealing time. The arrow mark the value of the adheion energy prior to expoure to water. After 4 week in deionized water, the adheion energy decreae to le than 3 J m 2. During the annealing proce the adheion gradually recover and reache a value comparable to the pre-expoure adheion energy. The annealing data can be analyzed uing Eq. (2), (5) and (6) (ee Table 2) to obtain the diffuion coefficient; the value of c f wa etimated from the water content of the argon and the change in water olubility between 25 and 95 C. The diffuion coefficient for water into the OSG film tack at 150 C i m 2 1. Fig. 4. Recovery of the adheion energy a a reult of water out-gaing during annealing in flowing argon at 150 C. Dicrete point repreent experimental meaurement; the olid line repreent the model The effect of interfacial plama treatment Fig. 5 how the ubcritical crack growth curve for the OSG/SiCN film tack where the interface wa ubjected to a He plama for 10 followed by a NH 3 plama for 90. The ubcritical crack growth data for film tack with different plama treatment were imilar and are not reproduced here. All curve how a reaction-controlled regime, Fig. 5. Subcritical crack growth curve for OSG/SiCN film tack with a 10 He/90 NH 3 plama treatment at different level of relative humidity. Table 2 Summary of model parameter and diffuivitie Si/OSG/SiN x Si/OSG/SiCN 25 C 95 C 150 C a 90 NH 3 90 NH 3 /10 He 10 He/90 NH 3 A (J m 2 ) B (J m 2 ) c b D (m 2 1 ) a Sample for thi erie were prepared in a eparate batch from the other OSG/SiN x ample and had a lightly lower adheion value, hence the lower B value. b c f Value.

6 2460 Y. Lin et al. / Acta Materialia 55 (2007) where the crack velocity change exponentially with energy releae rate and where the crack velocity i a enitive function of the relative humidity, and a tranport-controlled regime, where the crack velocity change much more lowly with energy releae rate. Fig. 6 how the linear relationhip between the energy releae rate required to drive a crack at a given velocity and the natural logarithm of the water partial preure in the teting environment a decribed by Eq. (1). Since the crack velocity of interet i relatively large, the reaction-controlled regime of the ubcritical crack growth curve were extrapolated to the appropriate crack velocity. Thi wa neceary to avoid any tranport-control effect. Becaue water i already preent in the film tack in thi tudy, there i no need to tranport water molecule to the crack tip during delamination. The figure demontrate that the linear relationhip between the energy releae rate to drive a crack at a given velocity or adheion energy, and the logarithm of the water partial preure i valid over a wide range of water partial preure. The figure alo illutrate the rather ignificant effect water vapor can have on the adheion of a film tack. A the water partial preure decreae, the adheion energy increae until eventually the critical energy releae rate i reached (not hown), at which point the linear relationhip break down and the adheion energy become independent of the environment. It i evident from the figure that the lope of the linear relationhip change for different plama treatment and that the lope i larger than for the untreated OSG/SiN x film tack. According to Eq. (1), the lope i determined by A = nnrt, i.e. the lope cale with the bond denity N. Conequently, the data ugget that the bond denity increae when the interface i expoed to a plama treatment. Furthermore, it i evident from Fig. 6 that at a given water partial preure, the plama-treated OSG/SiCN film tack require a larger energy releae rate for delamination than untreated film tack. A cloer look further reveal that the precie equence of the plama treatment make a difference: Compared with film tack treated with a He plama only, film tack expoed to both He and NH 3 plama how improved adheion if the NH 3 treatment occur before the He treatment, and wore adheion otherwie. Thee obervation can be rationalized if one aume that the plama treatment modify the urface of the OSG to create a thin, dene SiO 2 -like layer that i depleted of organic group a uggeted by ome report in the literature [30 32]. If the OSG i placed in an He plama, interaction between the OSG and the excited He * pecie in the plama would leave a urface with a high denity of dangling bond a well a H Si O bond that hould form a trong bond with the SiCN capping layer depoited immediately after the He plama treatment [31]. If the OSG i placed in an NH 3 plama after the He treatment, however, ome of the dangling bond will react with NH 3 and related pecie in the plama to form amine group. Our reult ugget that thi produce a net reduction of the bond denity acro the interface and hence a reduction of the adheion energy compared with an OSG urface that wa expoed to jut an He plama. If the He treatment i performed after the NH 3 treatment, the He * pecie in the plama trip the amine group from the OSG urface. The reult i an improvement in the adheion energy to a value that i omewhat larger than for ample with jut the He treatment, preumably becaue of the additional OSG denification that take place during the NH 3 treatment. Fig. 7 how the adheion energy of the OSG/SiCN interface a a function of water diffuion time, along with the adheion energy obtained from the diffuion model. Agreement between the experimental meaurement and the model i very good over the entire range of diffuion time. The effect of the plama treatment i clearly reflected in the reult; the a-prepared plama-treated film tack have adheion energie that are nearly a factor of two larger than the untreated film. The effect of the Fig. 6. Linear relationhip between the energy releae rate to drive an interfacial crack at a given velocity and the natural logarithm of the water partial preure in the environment for OSG/SiN x and OSG/SiCN film tack. Fig. 7. Degradation of the adheion energy for OSG/SiCN film tack with plama-treated interface caued by water diffuion. Dicrete point repreent experimental meaurement; olid line the model prediction.

7 Y. Lin et al. / Acta Materialia 55 (2007) plama treatment i greatly reduced, however, after allowing water to diffue into the film tack; the adheion energy for a aturated film tack i comparable to that of an untreated film tack that i fully aturated. Thi large reduction in adheion energy i a direct conequence of it enhanced dependence on water partial preure for plama-treated film tack a illutrated in Fig. 6. Thi finding ha obviou conequence for the effectivene of thi type of plama treatment in the preence of water. Moreover, the reult how that it i imperative to avoid any expoure to water when comparing plama treatment with a goal of enhancing the adheion. The reult in Fig. 7 ugget that the time cale for adheion degradation i independent of the precie interfacial treatment the film tack received. Indeed, the diffuion coefficient obtained from the model and which are ummarized in Table 2 are the ame for each of the plama treatment. Furthermore, they have the ame value a the diffuion contant for untreated film tack. Thi obervation ugget that the diffuion path i the ame for all the film tack and that it i not the OSG/SiN x or OSG/ SiCN interface Diffuion coefficient and diffuion path The quetion remain what the precie diffuion path of the water molecule in the OSG film tack i. Table 3 provide an overview of water diffuivitie available in the open literature for quartz, amorphou ilica, and other relevant material ytem. It i evident that the diffuion coefficient for water in mot of thee material i many order of magnitude maller than the value found in thi tudy. Even the diffuion coefficient for hydrogen ilequioxane (HSQ), which i probably the dielectric mot comparable to OSG in term of ma denity and tructure, i three order of magnitude maller. Therefore, the value of the diffuion coefficient meaured in thi tudy ugget that water diffuion through the OSG i not reponible for the adheion degradation proce. Further inight can be gained if the temperature-dependence of the diffuion coefficient i conidered. An Arrheniu graph of the diffuion coefficient yield a diffuion activation energy of 26 kj mol 1, or 0.27 ev, and a pre-exponential factor of Fig. 8. Deuterium concentration in OSG/SiN x film tack after expoure to D 2 O for variou period of time m 2 1. Hence, the activation energy for water diffuion in the OSG film tack i at leat a factor of two maller than the activation energie for bulk diffuion lited in Table 3. The activation energy i conitent, however, with interfacial diffuion. For intance, the activation energy for diffuion along the TiN/SiO 2 interface i reported to be 0.21 ev [37]. Since the diffuion coefficient i independent of the precie nature of the OSG/capping layer interface, it eem logical that the fat diffuion path for the OSG film tack ued in thi invetigation lie along the OSG/Si interface. Thi i confirmed by the compoition profile obtained from the SIMS analyi of the OSG/SiN x film tack that were expoed to deuterium oxide. Fig. 8 how the deuterium concentration a a function of depth for four film tack: one a-depoited reference ample and three ample expoed to D 2 O for 1, 7 and 15 day, repectively. The deuterium concentration in the SiN x capping layer i very low it correpond to the natural abundance of deuterium in the hydrogen preent in the SiN x film and it i independent of whether the ample wa expoed to D 2 O. Thi confirm that SiN x i a good barrier to D 2 O diffuion. The deuterium concentration in the OSG, by contrat, how a ignificant variation with expoure time: the concentration in the reference ample i equal to what one would expect baed on the hydrogen content of the OSG and the natural incidence of deuterium, but the other Table 3 Overview of diffuivitie in SiO 2 and relevant material ytem Material Solute D at 25 C (m 2 1 ) Activation energy (ev) Temperature range ( C) Method Reference Quartz D 2 O, H 18 2 O Nuclear [33,34] reaction Silica D 2 O, H 18 2 O Nuclear [33,34] reaction Silica D 2 O, HTO, [35,36] H 2 O, H 18 2 O (extrapolated) PECVD ilica D 2 O, H 18 2 O (7±2) SIMS [37] HSQ H 2 O Quartz crytal [38] microbalance TiN/SiO 2 interface D 2 O, H 18 2 O (6±2) SIMS [37]

8 2462 Y. Lin et al. / Acta Materialia 55 (2007) three ample how elevated level of deuterium. Furthermore, the ample that were expoed for 1 and 7 day how ditinct concentration gradient through the OSG film thickne, while the ample expoed for 15 day how a relatively flat profile. Thi i preciely the profile one expect if the OSG/Si interface i the fat diffuion path. It hould be noted that the ocillation in the D concentration are not the reult of noie in the meaurement, but reflect the fact that the OSG coating were depoited in ix dicrete layer. Having demontrated that the water molecule diffue primarily along the OSG/Si interface, it i now poible to derive bound for the diffuion coefficient of water in OSG. In order to degrade the adheion of the OSG/SiN x or OSG/SiCN interface, water molecule need to diffue through the film from one interface to the other. Since thi tep i not rate limiting, we find that D OSG=Si D OSG h 2 D OSG=Si; ð7þ b where h i the film thickne and b the ample width. Eq. (7) implie that the diffuion coefficient in OSG at room temperature i larger than m 2 1, which i reaonable given the data in Table 3. The equation further implie that if the film thickne i increaed ufficiently, diffuion through the OSG may become rate limiting. The diffuion of water through amorphou ilica i undertood in term of a mechanim where water diffue in molecular form and react with the ilicon oxygen network of gla to form SiOH group [35]: Si O Si þ H 2 O 2SiOH: ð8þ At high temperature, the reaction repreented by Eq. (8) proceed relatively quickly and the mobile water molecule are in equilibrium with the nearly immobile hydroxyl group; at low temperature the reaction i low and the water molecule are not necearily in equilibrium with the hydroxyl group. The reult preented here do not provide information on the precie diffuion mechanim of water along the Si/OSG interface or in the OSG, but they are certainly conitent with a imilar mechanim for water diffuion in OSG. The adheion degradation proce i well decribed by Fick law; moreover, the data preented in Fig. 4 demontrate that the adheion degradation proce i entirely reverible under relatively mild annealing condition. Thi reult ugget that the diffuion of mobile water molecule dominate over reaction: at the low tet-temperature ued in thi tudy, the reaction between water molecule and Si O bond i either negligible in the abence of a tre field or fully reverible at the time cale involved in the experiment. expoed to water for ome time and may caue delamination of the film tack. For pecific geometrie, it i poible to olve the diffuion equation and hence to predict how the adheion energy varie with expoure time. In thi cae it i appropriate to ue the adheion energy for zero crack velocity, i.e. the equilibrium interfacial fracture reitance. It i alo poible to determine how the crack extenion force change a a function of flaw ize by olving the appropriate boundary value problem. By combining both piece of information, one can devie a map that predict when a film tack fail a a function of expoure time and initial flaw ize. Conider, for intance, the cae of film delamination tarting at an edge. Fig. 9 how the correponding geometry. The energy releae rate for an interfacial crack in thi geometry ha been calculated by Yu et al. [39]: it i zero for a crack of vanihing length and quickly approache a teady-tate value G : G ¼ ð1 m2 f Þr2 re t f ð9þ 2E f a few film thicknee away from the edge. In thi expreion, t f i the film thickne, E f and m f are Young modulu and Poion ratio of the film, and r re i the reidual tre in the film. The reitance to fracture i given by the model dicued in thi paper combined with the expreion for diffuion into a emi-infinite olid to account for the different geometry. Fig. 10a how the energy releae rate a a function of crack length along with the fracture reitance for the pecial cae where the reitance of a dry film tack i large enough to prevent failure, but the reitance of a wet tack i not. Initially, the entire fracture reitance curve lie above the curve for the energy releae rate, i.e. flaw of any length are table. After expoure to water for a period of time, part of the fracture reitance curve drop below the energy releae rate curve and crack with length in thi range become untable. Thi reult i ummarized in the critical crack length map in Fig. 10b. The olid line in thi figure give a a function of expoure time the critical crack length at which crack tart growing; the dahed line give the maximum length to which an untable crack will grow; if the crack length exceed the dotted line, the crack will grow indefinitely. The map in Fig. 10b i valid for plane-train crack, but i readily extended for more realitic flaw geometrie. One limitation of the map in Fig. 10b i that it implicitly aume that diffuion through the film tack i the main water tranport mechanim. If there i an exiting crack when the film tack i firt expoed to water, tranport of water along the crack face would 4.4. Adheion degradation map The adheion degradation that occur a a reult of water diffuion can lead to delayed fracture of a film tack. For intance, a flaw that i too mall to caue fracture in a dry film tack may be untable after the film tack i Fig. 9. Schematic of the geometry under conideration.

9 Y. Lin et al. / Acta Materialia 55 (2007) of water, but thi enhancement i lot almot completely upon expoure of the film tack to water. The kinetic of the degradation proce i independent of the plama treatment uggeting that water doe not diffue preferentially along the OSG/SiCN interface. Diffuion experiment with deuterium oxide ugget that the Si/OSG interface i the main diffuion path for the film tack in thi tudy. Even though the diffuion path in actual integrated circuit are likely to be different and much lower than in thi tudy, adheion degradation i an iue for integrated circuit becaue length cale in integrated circuit are alo much maller than thoe conidered in thi tudy. The incorporation of porou low-k dielectric in integrated circuit in the near future i expected to further increae their uceptibility to diffuion-induced adheion degradation. Acknowledgement The author acknowledge funding from the Semiconductor Reearch Corporation (2005-KC , KJ-1339); they are grateful to Ofer Shopen for help with ample preparation and to Dave R. Clarke for inightful dicuion. The OSG film tack were provided by Texa Intrument Incorporated. Reference Fig. 10. (a) Energy releae rate and fracture reitance a a function of crack length and ditance to the edge of the film, repectively, for the geometry hown in Fig. 9. (b) Adheion degradation map howing critical crack length a a function of time. W f i defined a the train energy denity per unit area of film, W f ¼ð1 m 2 f Þr2 re =2E f. The parameter ued in the derivation of the map are typical for a Si/OSG/SiN x film tack with delamination occurring at the OSG/SiN x interface. preumably occur much fater than for diffuion through the film or along an interface. Hence the map i trictly valid only for flaw that do not allow water tranport or that are introduced right after water expoure. 5. Concluion When OSG/SiN x and OSG/SiCN film tack are expoed to water, the adheion energy decreae with time a a reult of the in-diffuion of water. Experimental meaurement of adheion energy a a function of expoure time are in good quantitative agreement with an analytical model that couple water diffuion and ubcritical crack growth. When the film tack i aturated with water, the adheion reduction can be quite large. The degradation i completely reverible under mild annealing condition. Thi obervation ugget that in the abence of a tre field the reaction between water and the Si O bond in the OSG i either negligible or fully and readily reverible. Plama treatment of the OSG/SiCN interface reult in a ignificant increae in the adheion energy in the abence [1] Grill A, Neumayer DA. Structure of low dielectric contant to extreme low dielectric contant SiCOH film: Fourier tranform infrared pectrocopy characterization. J Appl Phy 2003;94:6697. [2] Mabboux PY, Gleaon KK. Chemical bonding tructure of low dielectric contant Si:O:C:H film characterized by olid-tate NMR. J Electrochem Soc 2005;152:F7. [3] Lin Y, Tui TY, Vlaak JJ. Octamethylcyclotetrailoxane-baed lowpermittivity organoilicate coating: compoition, tructure and polarizability. J Electrochem Soc 2006:153. [4] Morgen M, Ryan ET, Zhao JH, Hu C, Cho TH, Ho PS. Low dielectric contant material for ULSI interconnect. Ann Rev Mater Sci 2000;30:645. [5] Maex K, Baklanov MR, Shamiryan D, Iacopi F, Brongerma SH, Yanovitkaya ZS. Low dielectric contant material for microelectronic. J Appl Phy 2003;93:8793. [6] Magbitang T, Lee VY, Miller RD, Toney MF, Lin ZL, Briber RM, et al. Templating organoilicate vitrification uing unimolecular elforganizing polymer: Evolution of morphology and nanoporoity development with network formation. Adv Mater 2005;17:1031. [7] Lewi HGP, Edell DJ, Gleaon KK. Puled-PECVD film from hexamethylcyclotriiloxane for ue a inulating biomaterial. Chem Mater 2000;12:3488. [8] Deai TA, Hanford DJ, Leoni L, Eenprei M, Ferrari M. Nanoporou anti-fouling ilicon membrane for bioenor application. Bioenor Bioelectron 2000;15:453. [9] Moudgil S, Ying JY. Organoilicate biomaterial: from monolith to nanoparticle. Abtr Pap Am Chem Soc 2002;224:U503. [10] Nichol MF. The challenge for hermetic encapulation of implanted device a review. Crit Rev Biomed Eng 1994;22:39. [11] Cype SH, Saltzman WM, Gianneli EP. Organoilicate-polymer drug delivery ytem: controlled releae and enhanced mechanical propertie. J Contr Releae 2003;90:163. [12] Lau KKS, Lewi HGP, Limb SJ, Kwan MC, Gleaon KK. Hot-wire chemical vapor depoition (HWCVD) of fluorocarbon and organoilicon thin film. Thin Solid Film 2001;395:288.

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