A Study of Temperature Distribution in SOI- Smart Power Devices in by warrent


									            A Study of Temperature Distribution in SOI-
            Smart Power Devices in Transient Conditions
                     by Optical Interferometry
                    a           a             a          a           a                 b
         N. Seliger , D. Pogany , C. Fürböck , P. Habaš , E. Gornik and M. Stoisiek
                        Institute for Solid State Electronics, TU Vienna
                             Floragasse 7, A-1040 Vienna, Austria
                  Siemens Corporate Research and Development ZFE T KM6
                       Otto-Hahn-Ring 6, D-81739 Munich, Germany

              Thermally induced changes in the optical reflectivity of the
              Fabry-Perot resonator formed by the SOI layer structure are
              used to evaluate the temperature variation in SOI power devices
              under switching bias conditions. Measurements of the
              temperature distribution inside and outside the active device
              region in the SOI well are presented and compared to thermal
1. Introduction

   Silicon-On-Insulator (SOI) by Direct Wafer Bonding [1] has become an attractive
technique for the fabrication of smart power devices. Self-heating effects in such structures
are, however, more critical compared to bulk devices due to a reduced heat removal across
the buried and trench oxides [2,3]. Power dissipation could cause localized temperature
increase in the device active region, which may influence the device performance and
reliability. In monolithic chip technology, the amount of heat laterally spread from the power
devices across the sided trench oxides is also important as it may influence the
characteristics of a nearby CMOS control circuit. In this paper we present an analysis of the
transient temperature variations inside and outside the SOI well of smart power devices
obtained by optical interferometry measurements.

2. Measurements

  The measurements have been carried out on trench isolated Lateral Double-diffused
(LD)MOSFETs [4]. The SOI wafer structure consists of a highly doped p+-substrate, the
buried oxide of 3µm, the Si layer of 20µm thickness and the SiO2 passivation layer. The
temperature variation in the Si layer is monitored by the intensity change of an infrared laser
beam (λ=1.3µm) which is focused on the device from the top side (Fig.1a). A homogeneous
temperature increase ∆ T in the Si layer of the thickness L causes an increase in the optical
thickness ∆ LOPT due to the thermooptical effect:
      ∆LOPT =     ⋅ ∆T ⋅ L ,                                                           (1)
   ≈ 1.5·10-4 K-1 [5,6] is the temperature coefficient of the refractive index. Light
absorption in the highly doped substrate causes that only the temperature-induced variation
                                                 in the optical thickness of the SOI layer is
                                                 detected via the reflectivity changes of the
                                                 Fabry-Perot (F-P) resonator [7] formed by
                                                 SiO2 passivation-layer/Si-layer/SiO2 buried
                                                 oxide/Si-substrate (Fig.1). A homogeneous
                                                 vertical temperature distribution in the well
                                                 is assumed, which is reasonable for
                                                 positions far from the MOS channel.
                                                 Heating in the Si active layer is induced by
                                                 applying short pulses of 20µs to 100µs
                                                 from 0 to 12V to the gate under various
                                                 drain-to-source biases. The duty cycle is
                                                 chosen sufficiently long to assure device
                                                 cooling between the pulses. In addition,
                                                 the temperature evolution within and
                                                 around the well is calculated by finite-
                                                 element-simulation [8] using the measured
 Fig.1: a) Cross section of LDMOSFET with the laser beam indicated. The
 device is symmetric with respect to the drain metallization. b) Top view of
 the device with marked laser beam measurement positions (beam diameter
 8µm, positions 1 to 6 are located in the center of the n--drift region).

power dissipated in the device as input.

3. Results and discussion

       Fig.2: Intensity signal measured on different positions (see Fig.1b) inside and outside the
       LDMOS device.
  The intensity signal during heating pulse on and off (Fig.2) was measured at different

                                                                      2,0                                                  LDMOS: P=16W for 100µ s

                                       Intensity signal (a.u.)
                                                                      1,5                                                    9

                                                                      0,5                     7                                              position 3
                                                                                  Heating         Cooling
                                                                              0             100                      200         300                400
                                                                                                                      Time (µ s)
positions (see Fig.1b) of the laser beam inside and outside the well. The transient
temperature increase within the device was calculated from the measured intensity signal
using the properties of the F-P reflectivity-temperature function (a result from a calculation
is shown in the inset of Fig.3).

                                                                                      T(t)=5.77 x (t-6.97x10-3 t2)                                            200
    Fig.3: Intensity signal measured on LDMOSFET and the corresponding temperature function

                                                                                                                                                                    Temperature increase (K)
    evaluated from the experiment. The inset shows the calculated Fabry-Perot reflectivity as
    function of temperature for the SiO2/Si/SiO2-multilayer inherent in LDMOSFET.160
            Intensity signal (a.u.)

                                                                              140                                                 LDMOS: P=16W for 100µs
                                                   Temperature increase (K)

                                                                              120                                                            position 1
                                                                              100                                   0,6                      simulation pos.1

                                                                                                                                             position 7
                                                                              80                                    0,4
                                                                                                                                             position 9
                                      0,5                                     60                                    0,2                                       40
                                                                              40                                    0,0
                                                                                                                       0    50 100 150 200 250 300 350 400
                                                                                                                               Temperature increase (K)
                                      0,0                                     20                                                                               0
                                                                 0                    10       20       30      40                            50             60
                                                                                  0                  Time (µ s)
                                                                                        0         100                       200           300          400
                                                                                                                           Time (µ s)

     Fig.4: Temperature increase extracted from measured intensity signals in Fig.2. The time
     evolution of the temperature at position 1 is compared to results obtained from finite element
The time evolution of the temperature at different measurement positions (Fig.4) is then
found from the corresponding intensity functions. The temperature increase obtained from
optical measurements is in good agreement with the simulation results (Fig.4). The decay of
the temperature after pulse turn-off is governed by the thermal time constant of the system,
which is found from measurements to be about 70µs, in good agreement with simulation.
                                                                                                  Maximum temperature increase (K)
Maximum temperature increase (K)
                                   160                                                                                               160
                                                       2          3                                                                                         LDMOS: P=16W for 100µs
                                   140                                              4
                                                                                                                                     120                          measurements
                                   120                                                                                                                            simulation
                                         position 1                                     5                                            80 position 1
                                    80                                                                                               40
                                                                                                                                                                   8     9        10
                                         LDMOS: P=16W for 100µ s heating pulse
                                         0       100       200     300       400            500                                            -50     0       50     100    150     200    250
                                                Distance from device center (µ m)                                                             Lateral distance from trench oxide (µm)

                  Fig.5: Peak temperature measured at the end of the                                                                 Fig.6: Peak temperature measured and simulated
                  heating pulse (at time 100µs) at different positions                                                               at different lateral positions (see Fig.1b) outside
                  along the device length (Fig.1b). The laser beam is                                                                the well.
                  located at the center of the drift region.

From spatial measurements along the device length we found an almost homogeneous
distribution of the peak temperature in the well (Fig.5). A significant decrease in the well
temperature is only measured in the vicinity of the trench oxide, as there is no heat
generation at the device ends and the heat is also removed across the trenches. Optical
measurements outside the well for a typical pulse width of 100µs (Fig.6) show that the
maximum temperature at a distance 45µm and 90µm far from the trench oxide is reduced by
a factor of 5.5 and 10, respectively, in comparison with the temperature inside the well.

4. Conclusions

   Time domain reflectivity measurements provide a simple, contactless and non-invasive
(no sample preparation is needed) method to study transient heating effects in thick-film SOI
smart power devices. The thermal time constant and the temperature distribution inside and
outside the trench-isolated device obtained from the experiments are in a good agreement
with the results of numerical simulation.


This work is financially supported by the Austrian Federal Ministry of Science and the
Society for Microelectronics, Austria.

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