Investigation of Thermal Stretching in BCB Bonding

Manigandan Sivasubramanian

Photonic Integration Department of Electrical EngineeringEindhoven University of Technology

Supervisor’s:1. Dr. Jos Van der Tol.2. Dr. Yuqing Jiao.

Organization

Introduction

Bonding Procedure

Parameters Considered

Analysis and Results

Conclusion

• Integration of electronic chips(CMOS) with an added optical

layer (IMOS) will help in realizing making a great variety of

Application specific Photonic Integrated circuits.

• Many advantages of IMOS

− Smaller Photonic Devices

− Reduced Packaging cost (Generic

Integration Process)

− Realization of both active and passive

functions.

Introduction

2

BONDING PROCEDURE

Bonding Procedure – Preparation Stage

• PECVD deposition of Silicon Oxide on top of Si

- 1000 micro meter of Silicon oxide

• Removal of Photo resist layer on InP layer by

acetone followed by isopropanol alcohol.

• InP wafer is subjected to cleaning for removal

of Organic residuals by plasma cleaning.

• PECVD deposition of Silicon Oxide on top

of InP – 50 nm of Silicon oxide.

Silicon / CMOS

SiO2

InP

Photoresist Layer

InP

SiO2

Bonding Procedure – BCB Bonding

• Silicon wafer is then coated with BCB solution

using the spinning machine.

• After the BCB layer the wafer is subjected to

heating till the temperature reaches 180

degrees using the baking oven.

• Next step is to flip the InP on top of silicon

wafer and it requires precise positioning of

wafers.

• Then the wafer is carefully kept inside the

bonding machine.

Silicon with SiO2

BCB Coating

Silicon , SiO2, BCB

InP , SiO2

Bonding procedure – Substrate Removal of InP

• Once the bonding is over, they need to be taken out carefully and cleaved

• Then they are etched – wet chemical etching is done in two steps – first they are introduced to a solution of 10 H20: 1 H2S04: 1 H202.

• Then the layer is etched using 4 HCL: 1 H20 solution at 35 degrees.

Fig 2a: Wafer after bonding Fig 2b: Wafer after removal of substrate.

Hypothesis: Thermal Stretching

• Materials undergo molecular vibration when heat is applied.

• As a result, thermal expansion occurs. Stretching is due to difference in thermal expansion in different materials. (Si and InP in this case)

• Thermal expansion is the tendency of a matter to change its shape, area and volume in response to change in temperature through heat transfer.

• In the bonding process heating takes place during the curing of the BCB

Si

InPBCB

Si

InPBCB

Application of heat

Application of heatExpansion

Fig 3a: Before application of heat - before bonding

Fig 3b: After application of heat - end of bonding.

PARAMETERS CONSIDERED

Parameters Considered

• In order to measure the stretching, we need to know the

relative error before and after bonding in the wafer.

• So, before the start of the bonding we had printed marker

points on the InP wafer which will enable to us to find the

relative error before and after bonding.

• Once the bonding is over, we measure the distance

between the points.

• Then calculating the relative error will give us an idea about

the stretching that takes place.

Fig 4: Marker pointsOn the wafer.

Parameters Considered

Figure 5: Representation of points and distance between them

Parameters Considered

• Experiment was designed to repeat

according to the chart .

• The chart is the extent of BCB cure

released by the manufacturer.

• Two types of bonding was done, on 6

different wafers in various temperature

and time.Figure 6: Extent of BCB cureagainst time and temperature

Parameters Considered

Parameters Considered

• Once the points are noted down , as said earlier their distance were calculated.

• The distance between the points will help us find the relative error between the

original points and the after bonded points.

• The graphs were plotted with Relative error in percentage in the x-axis and

the number of occurrences in the y-axis.

ANALYSIS AND RESULT

Analysis and Result

Fig 7: Relative error at 280 degreesfor 1 hour

Fig 8:Relative error at 310 degreesfor 6 min

Analysis and Result

Fig 9: Relative error at 240 degreesfor 10 hour

Fig 10:Relative error at 200 degreesfor 1 hour

Fig 11: Relative error at 240 degreesfor 1 hour

Analysis and Result

• The previous graphs shows the relative

error for various bonding and

various conditions.

• The relative error’s are almost in the

range of 0.025-0.030 percentage.

• The next graph shows the average of the

relative error for all wafers. Fig 12:Average of relative error

Analysis and Result

 A standard deviation close to 0 indicates that the

data points tend to be very close to the mean 

of the set, while a high standard deviation indicates

that the data points are spread out over a wider

range of values.

Fig 13: Standard deviation of relative error

Conclusion

• We investigated the thermal stretching during the bonding process of Silicon with

III-V semiconductor material.

• The suspect we thought was the BCB layer which is added to enable the bonding.

• Found the relative error between them and plotted in the form of graph.

we tried different BCB thickness, and varied the temperature and time of bonding.

•  They all show fairly same behavior in all the cases which is conclusive proof that

BCB is not the main cause of thermal stretching.

• The future work needs to be on the lines of find the cause of stretching leaving the

BCB out of loop.

QUESTIONS?

THANK YOU!!