Mechanical stress investigation after technological

Microelectronics Reliability xxx (2015) xxx–xxx

MR-11711; No of Pages 5

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Mechanical stress investigation after technological process in Deep Trench TerminationDT2 using BenzoCycloButene as dielectric material

H. Arbess a, F. Baccar a, L. Theolier a, S. Azzopardi b, E. Woirgard a

a University of Bordeaux, IMS Laboratory, CNRS UMR 5218, F-33400 Talence, Franceb Bordeaux Institute of Technology, IMS Laboratory, CNRS UMR 5218, F-33400 Talence, France

http://dx.doi.org/10.1016/j.microrel.2015.07.0190026-2714/© 2015 Elsevier Ltd. All rights reserved.

Please cite this article as: H. Arbess, et al., MBenzoCycloButene as dielectric material, Mic

a b s t r a c t
a r t i c l e i n f o
Article history:Received 24 May 2015Received in revised form 22 June 2015Accepted 2 July 2015Available online xxxx

Mechanical residual stress after technological process has been investigated on 1200 V Deep Trench TerminationDiode. The dielectric used to fill the trenches was the BenzoCycloButene (BCB). Four years after the fabricationand storage in a clean roomwithout any electrical or thermo-mechanical stress, cracks in the BCB and delamina-tion at the silicon-BCB interface have been observed. In order to understand such phenomena, the mechanicalstress in this diode has been studied with the help of finite element simulator (Sentaurus Process TCAD tool).2D and 3D structures have been simulated in order to evaluate the stress level and its localization. Several opti-mizations of the trench topology have been proposed in order to minimize the stress level while maintaining aconstant breakdown voltage.

© 2015 Elsevier Ltd. All rights reserved.

Fig. 1. Cross sectional schematic view of the DT2 diode.

1. Introduction

The deep trench structure is one of the candidates for edge termina-tion design of future high voltage power semiconductor devices to re-duce the chip area and improve the breakdown voltage [1–3]. Indeed,as all power semiconductor devices, a junction termination is neededin order to spread the equipotential lines, to reduce the electric field atthe edge of device and to reach the theoretical breakdown voltage.Several junction terminations have been explored for the diodes.Some of the most common ones are the guard ring [4], the metal fieldplates extending over an insulating surface layer [5] and the junction-termination extension (JTE) [6]. All these terminations consume alarge semiconductor area which induces a problem in the miniaturiza-tion of the power chips. The deep trench has a high efficiency as a termi-nation and a smaller area comparing to the other ones.

Depending on its depth, many dielectric materials can be used to fillthe trench such as the polyimide [7], the BenzoCycloButene (BCB) [3]and the silicon oxide [8].

In this study, the BCB was used to fill the trench termination of thediodes. The BCB is a resin normally used as a passivation layer [9] oran interlayer material betweenmetal interconnection [10]. This materi-al has been chosen because of the high depth of the trenches termina-tion (≈100 μm) and its electrical properties [11].

In the present case, delaminations and cracks have been observed onthe fabricated diodes after 4 years of storage. These cracks and delamina-tions have appeared over the time without any electrical or thermo-mechanical stress. Past studies [12,13] have focused on the CriticalAdhesion Energy (CAE) between BCB and silicon or on the residual stressthat the BCB generates on silicon in a simple BCB deposit. Up to now, nostudy related to the mechanical stress in the deep trench terminations

echanical stress investigatioroelectronics Reliability (201

can be found in the literature. In this paper, the mechanical residualstress in the DT2 diode [14] has been studied in details for the first timeusing 2D and 3D Sentaurus Process modeling. The chosen dielectric ma-terial filling the trenches is the BCB. Following for this result, trenchstructure optimizations to decrease the stress in the diodehavebeenpro-posed and optimized using Sentaurus Process and Sentaurus Device.

2. DUT description and main problem

The structure under study is a DT2 diode depicted in (see Fig. 1) witha trench depth equals 100 μm and a width equals 70 μm.

The technological process steps are described in [3]. The trench real-ization process started by the silicon substrates Deep Reactive-Ion Etch-ing (DRIE) applying an adhesion promoter, then, the BCB deposit andthe annealing. Another layer of BCB was requested because the first

n after technological process in Deep Trench Termination DT2 using5), http://dx.doi.org/10.1016/j.microrel.2015.07.019

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Fig. 2. SEM photograph of the BCB-filled trench termination after CMP.

2 H. Arbess et al. / Microelectronics Reliability xxx (2015) xxx–xxx

one did not fill the entire trench. The second layer is applied and curedin the same conditions as the previous one. After trench filling, ChemicalMechanical Polishing (CMP) of BCB is realized using a Logitech CDP(CMP) system. The parameters for the CMPprocess are provided in [15].

Fig. 2 presents a SEM photograph of the BCB-filled trench termina-tion after CMP where no delamination at the Si–BCB interface is ob-served. Fig. 3 shows the measured and the simulated J–V reversecharacteristics of the DT2 diode in the same year of fabrication. Both,simulation and experiment present a breakdown voltage of 1200 Vwith a maximum leakage current density of 10−5 A·cm−2.

Four years after fabrication and storage in a clean room, cracks in theBCB and delaminations at the Silicon–BCB interface have been observedin several devices as illustrated in Fig. 4. It can be possible to observe

Fig. 3. Measured and simulated J–V reverse characteristics of the DT2 diode at the sameyear of fabrication.

Fig. 4. (a) Delamination at the silicon-BCB interface in the DT2 diode. (b) Crack at the BCBcorner in the DT2 diode.

Please cite this article as: H. Arbess, et al., Mechanical stress investigatioBenzoCycloButene as dielectric material, Microelectronics Reliability (201

that, high stress level is located in the tagged zones. Electrical character-izations have beenmade for several degraded devices in order to estab-lish the impact on themain electrical parameters. Results show that, theleakage current levels are not the same for the various selected devices,likewise for the breakdown voltage.

3. Simulated structure and simulator calibration

The study of the residual mechanical stress caused by the fabricationprocess has been realized using finite element simulations withSentaurus Process tool.

The library of Sentaurus Process does not contain the BCB materialproperties. At first, it was necessary to implement the properties ofthe BCB material in the simulator. Table 1 presents the mechanicaland thermal properties of the used BCB (Cyclotene 4000 resin series)[11]. The implemented properties correspond to lines 1 to 4 of this table.

To simulate the process of the deep trench filled by the BCB, an an-isotropic etching of the silicon followed by an isotropic deposit of BCBhas beenmade, then the polymerization process (by annealing) that in-duces an increase of BCB density. The final process was the CMP.

During the polymerization process, the BCB loses almost 30% fromits initial volume [11]. If this percentage is taken into account duringsimulation by increasing the BCB density, the simulated stress value atthe Si–BCB interface and at the corners will be very high (N20 GPa).

Table 1 and other experimental studies [12] show that the residualstress caused by the BCB deposit on silicon at 25 °C is equal to 28 MPa.Taken as a starting point, several simulations have been carried out inorder to calculate the density variation of the BCB during polymeriza-tion. These simulations have been made using a silicon wafer withouttrenches and an isotropic BCB deposit. The thickness of the final onehas no impact on the calculated stress as illustrated by the simulation.The result depicted in Fig. 5 shows that 28 MPa is obtained with an in-crease in BCB density equals 2%. This value has been used for all the sim-ulations of this study.

Table 1Thermal and mechanical properties of BCB (Cyclotene 4000 resin series) [11].

Property Value

Young's modulus 2.9 GPaPoisson's ratio 0.34Specific heat capacity 2180 J/Kg·KThermal expansion coefficient 42 ppm/°CTensile strength 87 ± 9 MPaStress on Si at 25 °C 28 ± 2 MPaTg N350 °CElongation at break 8 ± 2.5%

stre

ss o

n s

ilico

n (

Pa)

0.E+00

1.E+07

2.E+07

3.E+07

4.E+07

5.E+07

6.E+07

7.E+07

8.E+07

0 1

Increase of BC Bdensity (%)

2 3 4 5

Fig. 5. Variation of the stress on silicon versus the increase of BCB density.



3H. Arbess et al. / Microelectronics Reliability xxx (2015) xxx–xxx

4. Results of stress simulation

The simulated structure is presented in Fig. 6 with its dimensions.The total dimensions of the silicon are 130 μm × 130 μm, and those ofthe trench are 70 μm × 100 μm as Fig. 6 shows.

Fig. 7(a) shows the simulation results. The stress areas are located inthe silicon near the silicon–BCB interface on the top of the structure, andnear the corners in the bottom of the trench. In Fig. 7(b), the stress dis-tribution along horizontal axis through the critical areas is presented. Itcan be noticed that, the maximum stress is located in a very small areaand its value is very high and can be themain cause of the delaminationbetween silicon and BCB. In real case, it is very difficult to have a 90°

Fig. 6. Simulated structure.

1.0E+06

5.0E+08

1.0E+09

1.5E+09

2.0E+09

2.5E+09

3.0E+09

3.5E+09

0 50 100 150

Mis

es s

tres

s (P

a)

X (µm)

Axis 1

Axis 2

(b)

(a)

Fig. 7. (a) 2D stress simulation caused by trench termination creation. (b) Stress distribu-tion along horizontal axis pass from the critical points (see Fig. 6).

Fig. 8. 3D simulation of the stress in deep trench filled by BCB.


angle at the corners. Therefore, other simulations have been madewith curvature radius at corners equal to 1 μm. The results haveshown that the maximum stress is equal to 0.8 GPa. Higher the curva-ture radius, lower the maximum stress level. This result can explainwhy some cells can be found with delaminations and other without de-laminations. More simulations have been realized with a BCB depositmade in two steps. This variation shows a minor effect.

The stress evolutionwith temperature has also been studied and hasshown a decrease with the temperature rise. It becomes negligible atabout 250 °C (polymerization temperature of BCB). This last result con-firms that the chosen percentage of the variation of the BCB density dur-ing polymerization (2%) matches with the real one.

3D simulation with the trench real dimensions was very difficult toachieve. Indeed, after each technological process, automaticmesh is cre-ated by Sentaurus Process tool to make the results more accurate. Withthe real dimensions of the trench, the required random access memory(RAM) exceeds the used one in this study (32 GB). To resolve this prob-lem, a 3D simulation has been made with one fifth of structure dimen-sions. Then, these results are more qualitative than quantitative andthe aimof this simulation is just to highlight the localization of the stressand to compare with several structures, more or less optimized. Fig. 8depicts the 3D simulation result. The highest stress level is also localizedat the corners. This result can explain why the cracks are located in theBCB corner.

5. Trench structure optimization

To decrease themaximum stress level at the silicon–BCB interface, amodification of the trench structure has been investigated. The idea is tobe closer to the horizontal deposit, which means a minimum stress.Fig. 9 shows the modified structure. The α angle has been changed

Fig. 9. New simulated trench topology.



Fig. 10. Impact ionization distribution with equipotential lines at breakdown for the devi-ated trench structure (a) and normal trench structure (b).

0

100

200

300

400

500

600

0 10 20 30 40 50

Str

ess

(Mp

a)

Angle (degree)

Y = 3.0 µm

Y = 2.5 µm

Y = 2.0 µm

Y = 1.5 µm

Y = 0.0 µm

Fig. 12. Variation of the stress in terms of trench deviation angle with different depths ofvertical part of the trench (Y) (scale 1/10).

4 H. Arbess et al. / Microelectronics Reliability xxx (2015) xxx–xxx

from 0° to 45°. As previously explained, 3D simulation with real dimen-sions cannot be performed. Therefore, dimensions for this structurehave been divided by ten as indicated in Fig. 9 for all simulated struc-tures dedicated to the stress investigation hereafter and have beenused for comparison purpose. The simulations have been carried outwith curvature radius at corners equal to 0.1 μm. The results showthat the stress level decreased from 500 MPa at α = 0° down to180 MPa at α = 45°. Considering this decreasing (around 65%), theprobability to have a delamination at the silicon–BCB has drasticallybeen reduced.

To study the impact of the structure variation on the electrical char-acteristics of the DT2 diode, new simulations using Sentaurus Devicehave been performed. The edge position of the field plate is conserved.The breakdown voltage decreases from 1250 V at α = 0° down to650 V at α = 45°. Indeed, larger the deviation angle, higher the

Fig. 11. New adjusted simulated trench topology.


attraction of the potential lines upwards by the field plate beforereaching the trench as shown in Fig. 10. This deviation changes thefield plate distribution and the breakdown is now at the corner betweenthe P+/N junction and the trench.

Fig. 10 presents the difference of the impact ionization distributionbetween the new trench structure and the conventional one. It can benoticed that the equipotential lines are attracted upwards below thefield plate. Therefore, the electric field is greater in the corner (P+/N−/BCB). This electric filed peak leads to punctual impact ionization andthen a premature breakdown.

With the high evolution of the silicon etching technology [16], anew trench structure can be proposed as an optimized solution forthe breakdown voltage reduction. Fig. 11 illustrates the new adjustedstructure with a new parameter (Y). The simulation is now per-formed with two parameters (α and Y) as depicted in Fig. 12. Itcan be observed that there is no impact of Y in the calculated stressif this last one is not negligible. Based on this structure, the stressdoes not decrease more than 300 MPa (40%) and it is not necessaryto increase the angle more than 30° due to the constant valuereached by the stress.

The impact of Y on the breakdown voltage is not negligible. Thelargest Y corresponds to the highest breakdown voltage as shown inFig. 13. Considering this structure, the stress level is decreased by 40%with a breakdown voltage decreasing by only 10% (Y = 30 μm andα ≈ 30%).

500

600

700

800

900

1000

1100

1200

1300

0 10 20 30 40 50

Bre

akd

ow

n v

olt

age

(V)

Angle (degree)

Y = 30 µm

Y = 25 µm

Y = 20 µm

Y = 15 µm

Y = 0.0 µm

Fig. 13. Breakdown voltage variation versus trench deviation angle with different depthsof vertical part of the trench (Y).



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6. Conclusion

In this paper, the mechanical stress in Deep Trench Termination DT2

diode is simulated using BenzoCycloButene as a dielectric. The simula-tion method using Sentaurus Process is described in detail. This is thefirst time that the mechanical stress after technological process in apower semiconductor device is simulated. Several simulations havebeen carried out in a comparison purpose due to the structure dimen-sions problems. The reduction of the stress level with a low impact onthe breakdown voltage decrease has been successfully proposed.

Therefore, this study can point out important issues for the futurepower semiconductor device technologies.

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Mechanical stress investigation after technological