Evaluation of Thermal Cycle Stress in SiC
Power devices by Raman Spectroscopy
Reduction of thermal stress in power devices is important in terms of device reliability because
power devices are used under extremely wide range of temperature. Raman spectroscopy
can quantitatively evaluate the stress of the devices.
2. Sample

1. Stress evaluation of 4H-SiC by Raman Spectroscopy When the stress is applied to the samples,the force constant between atoms change
and the peak frequency is shifted.
 In the cross section of 4H‐SiC, the stresscomponents σ11 and σ33 are expressed
as follows[1, 2]:𝑏 ∆𝜔𝑏 ∆𝜔
𝜎𝑎 𝑏𝑎𝑐𝑏𝑎𝑐∆𝜔𝑎 ∆𝜔
𝜎𝑎𝑎𝑐𝑏𝑎 𝑏

CompressiveTensile
E2

Intensity ( a. u. )Raman system
Spectrometer: Horiba Jobin Yvon U1000
Excitation source: 457.9 nm of an Ar+‐ion laser
Peak frequency is determined within 0.02 cm‐1.

A1(TO)
Mold resin was removed in Sample2.
Cu plate was removed in Sample3.

E1(TO)

760

780
800-1
Raman shift (cm )820
Mold resin

An example of Raman spectra of4H‐SiC (a‐ plane).

, 𝑏 , 𝑎 , 𝑏 and 𝑐 are phonondeformation potentials. (Constant values)

3. Temperature-dependent stressevaluation of SiC MOSFETs

SolderSiC
Lead frame

𝜎Optical microscopic image of the cross section of the SiC MOSFET.

 Point A Point B

 𝜎 （Vertical）

Temperature‐controlled heating and coolingstage was used.
Cycle range: ‐40 ℃ ⇔ 140 ℃0Δσ11 (MPa)2000
Stress(MPa)𝜎B

A

4. Stress change before and after the thermal cycle50

 𝜎 （Horizontal）

Commercially available 1200V SiC MOSFETs with TO‐247
discrete package was used in this experiment.
The cross section was made by mechanical polishing.

-200

The horizontal stress at PointA in Sample
2 showed no significant change. Thisresult indicates that the stress at Point A
is mainly resulted from the mold resin.

-50-100-400

PointA
PointB-200
0 5 10 15 20 25 30 0 5 10 15 20 25 30-600-50 050 100 150 -50 0

Temperature (℃)

The stress change of Sample 3 wassmallest at Point B. This result indicatesthat the stress at Point B is mainly
resulted from the solder and Cu plate.

Sample1
Sample2
Sample3-150

Cycle number50 100 150

Temperature (℃)

The horizontal stress was largely changed by thetemperature. This tendency was remarkably
observed at Point B which is located near the solder.

Cycle number

SiC

SolderThe crack was generated near theinterface between the SiC and thesolder after 30 cycles. The stressrelaxation was occurred by this crack.

Crack

SEM image near the interface between
the SiC and the solder after 30 cycles.

5. The degradation process of the device through the thermal cycles
 The stress in the package was changed through the thermal cycles.
It indicates that the irreversible change was occurred by the temperaturechange in the package.

Examples of the irreversible change : Delamination, Degassing, Crack, etc.

 In the SiC MOSFET, the residual stress increased after the thermal cycles, especially near the solder.
The crack was generated near the interface between the SiC and the solder and the stress relaxation wasoccurred by this crack.
 The mold resin affects to the stress at the edge of the chip.
Thermal stress can be occurredby the differences in CTEs.
*CTE : Coefficient of Thermal Expansion

Raman spectroscopy can measure the stress in devices and evaluate the degradation degree of the devices quantitatively.
[1] R. Sugie and T. Uchida., J. Appl. Phys. 122, 195703 (2017).
[2] R. J. Briggs et al., Phys. Rev. B 13, 5518 (1976).
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