Time Dependent Isolation Capability of High Voltage Deep Trench Isolation

Ralf Lerner; Uwe Eckoldt; Klaus Schottmann X-FAB Semiconductor Foundries AG

Erfurt; Germany ralf.lerner@xfab.com

Steffen Heinz; Klaus Erler; Andre Lange, Gunter Ebest Chemnitz University of Technology; Faculty Electrical

Engineering and Information Technology, Chair Electronic Devices of Micro- and Nanotechnology

Chemnitz, Germany

Abstract— The long term isolation properties of deep trenches in thick SOI have been investigated by current-voltage-characteristics. A strong change of the measured trench leakage current was observed depending on the applied voltage. Further on a marked decrease of the leakage current was observed depending on the duration and polarity of the applied stress. The improvement of the formatted trench isolation was found to be irreversible with time, temperature and voltage polarity. This so-called formation effect can be described by a theoretical simulation model assuming a tunneling process of electrons through the oxide barrier, taking into account the charging and discharging of traps within the trench sidewall oxides. The observed formation effect leads to improved reliability results, wherein the trench is either damaged at the start of the stressing or no dielectric breakdown occurs at all.

I. INTRODUCTION

Dielectric or deep trench isolation is an attractive solution for high voltage Power ICs which require a large number of effectively isolated switches. Features such as bi-directional isolation, temperature tolerance and very small area consumption are some of the advantages of the trench isolation scheme for these applications [1], [2], [3]. Up to now, reliability investigations on deep trench structures have been published, for example, for voltages of up to 30 Volts in epi-wafers [4]. This paper deals with the time-dependent electrical performance and related reliability issues of deep trenches in SOI for voltages above 300 Volts.

II. ELECTRICAL CHARACTERISATION

A. Test arrangement

Test wafers were manufactured using X-FAB’s 650V trench-isolated SOI process [5]. Vertical isolation is accomplished using a 2μm buried oxide. Lateral isolation is provided by a 55μm deep single trench for operation up to 300V, and a double or triple trench structure for voltages above 300V. Figure 1 shows a schematic cross section of a thick SOI wafer with single-trench isolation. A physical cross section of a single trench is shown in Fig. 2.

The isolation structures are composed of two layers of silicon dioxide with a layer of low doped polysilicon embedded between. Since the specific conductivity of the polysilicon is several orders of magnitude higher than that of the oxide, the main voltage drop occurs across the two layers of oxide. With a negligible voltage drop across the polysilicon, the influence of this layer on the static leakage current characteristic can be neglected.

The test structure consists of a square-like trench ring which is contacted in the centre. The second contact is made outside the trench ring. The structure is surrounded by a second trench which isolates the test structures from the adjacent silicon (Fig. 3). The electrical characterisation was carried out by measuring current-voltage-characteristics across

N- doped device wafer

Handle wafer

Buried oxideN+ doped buried layer

Potential 1 Potential 2Trench Cover

N+ sinkerOxidePoly

N- doped device wafer

Handle wafer

Buried oxideN+ doped buried layer

Potential 1 Potential 2Trench Cover

N+ sinkerOxidePoly

Figure 1: Schematic cross section of single-trench SOI isolation Figure 2: Cross section of investigated trench structure

1-4244-1533-0/08/$25.00 ©2008 IEEE 205

Proceedings of the 20th International Symposium on Power Semiconductor Devices & IC's

May 18-22, 2008 Oralando, FL

V/A

V/A

V/A

V/A

Heatable chuck

V/A- Source Monitor Unit

Contact Pads

Trench

V/A

V/A

V/A

V/A

Heatable chuck

V/A- Source Monitor Unit

Contact Pads

Trench

Figure 3: Top-down view (top) and cross section (bottom) of the test configuration

12 3

4

12 3

4

Figure 4: Current-voltage-characteristics of a single-trench structure during 25 voltage sweeps 1: 0…-400V; 2: -400V…0V; 3: 0V…+400V; 4: +400…0V etc.

Break for5 days

Annealing in air14h, T=180°C

Break for5 days

Annealing in air14h, T=180°C

Figure 5: Maximum currents at +400V and -400V, respectively, versus measurement cycle number

the trench, where the voltage was applied to the inner tub.

The measurements were aimed at lifetime investigations to determine the trench reliability. Therefore, test conditions were used significantly outside the normally allowed maximum operating voltage of, for example, 300V for a single trench. The electrical tests were performed on a HP4062UX system together with a Karl Suess PA200 wafer probing system.

B. Results Fig. 4 shows results of 25 voltage sweeps down to -400V

and up to +400V. Each cycle took 30 minutes. The first cycle (indicated by the number “1” in Fig. 4) starts at 0V and decreases to -400V. A strong increase of the trench leakage current was detected for this very first cycle, starting at -150V and 0.2pA with a rise to 1nA at -400V. When the voltage is

elevated back to zero (“2”) a hysteresis can be observed with a decrease of the leakage current down to 0.2pA already at a lower voltage of -250V. A similar effect can be seen with positive voltages. In the positive “up”-cycle (“3”) the trench current increases at +200V, while during the “down”-cycle (“4”) the leakage decrease already starts at +300V. With successive voltage sweeps, the leakage current rise occurs at higher voltages.

To obtain a better view of the behaviour at minimum and maximum voltages, the results of another measurement are shown in a different style. Here, only the maximum values of the leakage current at voltages of -400V and +400V, respectively, are drawn versus the cycle number (see Fig. 5). A steady decrease of the maximum leakage current at -400V and +400V, respectively, can be seen with increasing number of voltage cycles. This lowering of the trench leakage current is called trench formation. A formation effect for positive as well as negative voltages, being more or less independent of each other, is found. The steady trend of this curve continues after a break of 5 days, as well as after annealing at 180°C for 14h.

The general effect of temperature on the trench leakage current is shown in Fig. 6a and Fig. 6b for two wafers manufactured using the same technology. The results represent the measured medium trench currents after 10 minutes formation time, respectively. Adjacent samples were investigated with different applied voltages of (370V, 380V, 390V and 400V). A fairly small exponential temperature dependency of the current could be observed. A rise of temperature of nearly 100K had about the same effect as a voltage increase of about 10V. Although both wafers were produced within the same batch, their characteristics differ slightly from each other due to the influence of technological parameters on the trench leakage current.

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Figure 6a and 6b: Trench leakage current versus temperature, measured after a formation time of 10 min on two different wafers

TABLE I. MAXIMUM NUMBER OF VOLTAGE PULSES WITHOUT BREAKDOWN; “0”= BREAKDOWN AT FIRST PULSE,

“MAX”= NO BREAKDOWN DURING WHOLE TEST;Wafer 21 Wafer 24 Wafer25

Vol

tage

DUT No. of pulses w/o

breakdown

DUT No. of pulses w/o breakdown

DUT No. of pulses w/o

breakdown

+/-4

30V

A4a D7a E7a A5g B5g C5g D5g E5g F5g G5g H5g I5g J5g

00

4924 (max) 0

6227 (max.) 6227 (max.) 6227 (max.) 6227 (max.) 6227 (max.) 3332 (max.) 6227 (max.) 6227 (max.) 4924 (max)

A5a B5a C5a D5a E5a F5a G5a H5a I5a J5a A4a B4a C4a D4a E4a F4a G4a

4347 (max) 4347 (max) 4347 (max) 4347 (max)

00

4347 (max) 30

4347 (max) 4347 (max) 4347 (max) 4347 (max) 4347 (max) 4347 (max)

00

A5a B5a C5a D5a E5a F5a

4924 (max) 4924 (max) 4924 (max) 4924 (max) 4924 (max) 2890 (max)

<430

V

No breakdowns observed, only max. pulse numbers

Figure 7: Maximum currents at +430V and -430V respectively versus measurement time at different temperatures

The effects of polarity and temperature on the trench formation are shown in Fig. 7. The general formation effect shows a steady decreasing current for both polarities and as well for both temperatures. With higher temperatures the current is slightly increased, but the slope of the curve, i.e. the trench formation process, stays the same.

Table 1 summarizes the number of possible pulses before trench breakdown at 430V. It can be seen that the number of pulses is either zero (= breakdown with the first applied pulse)

or the maximum number applied (= no breakdown at all during stress time). With voltages lower than 430V, the maximum possible number of pulses could always be observed. No breakdowns occurred when voltages below 430V were applied.

III. DISCUSSION

A thick thermal oxide was used as the isolation oxide inside the trench. Therefore, we can assume a lower trench oxide quality compared to high-quality thin gate oxides. Possible mechanisms for carrier conduction inside such a thick oxide might be:

A. Mobile ions within the oxide A current transport mechanism based on mobile ions

would have a linear voltage dependency. Because of the observed non-linear current voltage characteristics with its strong nonlinear voltage dependency, we exclude this mechanism.

B. Thermal emission of carriers up to an energy level high enough to cross the potential barrier Schottky emission (thermal excitation of electrons from

the silicon conduction band) as well as Poole-Frenkel emission (thermal excitation of electrons from traps within the insulator) show a relatively small voltage dependency and a strong or medium temperature dependency, respectively. Because of the observed different voltage and temperature dependencies, these mechanisms do not appear to fit with our results.

207

Figure 8: Simulated (line) and measured (dots) trench currents versus time for different voltages

C. Tunneling processes through a triangular potential barrier at higher electric fields (Fowler-Nordheim-tunneling) and/or tunneling of electrons into oxide traps Fowler-Nordheim tunnelling has a large voltage

dependency. The reduction of the barrier thickness at higher voltages leads to an increased tunnelling probability, and thus the observed leakage current rise can be explained.

Positive charges within the oxide close to the silicon lead to an accumulation layer in the conduction band in the silicon close to the interface. At higher temperatures, higher energy levels are occupied by electrons having a smaller barrier and thus a higher tunnelling probability. This leads to the measured small increase of the leakage current with temperature.

Oxide defects – e.g. dangling bonds near the oxide-silicon interface or defects due to built-in dopants – can create positively charged defects. These positive charges inside the oxide lead to a reduction of the potential barrier thickness compared to an ideal defect-free oxide. On the other hand, negative oxide charges (or neutralisation of positive charges) will increase the effective barrier height. Charging or neutralisation of the defects has an impact on the effective barrier height of the oxide, thus changing the leakage behaviour.

Simulated current voltage curves - assuming such a tunnelling process to be the transport mechanism - are in good agreement with the measured characteristics (see Fig 8). The main simulation parameters affecting the current behaviour are the oxide thickness and the effective barrier height.

The formation effect can be explained by a neutralization of positively charged oxide defects close to the trench–silicon interfaces. In the untested -or non-formatted- trench the positive oxide charges close to the interface reduce the effective barrier height, leading to a high tunnel current. Near the cathode side of the trench, the tunnelling electrons have a similar energy level than the oxide traps, and hence they have a high recombination probability, leading to neutralization of the positive charge. With this reduction of the positive charge, the effective barrier width is increased and the tunnel current decreases. On the anode side of the trench, the electrons are at

a higher energy level inside the oxide conduction band. Therefore, the recombination probability is much lower and the formation effect becomes polarity dependent. Simulation results based on this model [6] are shown in Fig. 8. By variation of the model parameters, their influence on the isolation properties can be shown. Changing the oxide thickness, for example, has a strong effect on the trench leakage current. Thus, the deviations of the characteristics between the samples of two different wafers in Fig. 6a and Fig. 6b can be explained.

Due to this formation effect, the typical oxide reliability tests, such as Time Dependent Dielectric Breakdown (TDDB), do not work. Therefore, we defined the maximum operating voltage to be 300V, i.e., at the onset of the leakage current of the formatted trench. In order to verify this value, pulses with alternating voltages far above the specified operating voltage were used. The tests are summarized in Table 1. For voltages below 430V, no oxide breakdowns were observed, while at 430V the isolation has either a dielectric breakdown with the first applied voltage pulse, or the formation effect reduces the tunnel current with ongoing stress time so that no breakdown occurs. The process of formation effectively improves the reliability of the isolation. If the trench survives the first voltage pulse (e.g. during IC probing), then the trench withstands all subsequent voltage pulses, with leakage behaviour even improving.

IV. CONCLUSIONS

The observed reduction of trench leakage current with applied voltage can be simulated with a theoretical model assuming a process of electrons tunnelling through the oxide barrier, taking into account the charging and discharging of traps within the trench sidewall oxides. The “formation effect” of the isolation trench gives rise to reliability behaviour in which the trench is either damaged during the first measurement or survives the complete test.

Improving the oxide quality by a reduction of the positively charged defects close to the trench–silicon interface should be a promising method for further improvement of the isolation capability.

REFERENCES

[1] F. Udrea, D. Garner, K. Sheng, A. Popescu, H.T. Lim, W.I. Milne; SOI power devices; Electronics & communications engineering journal, Feb. 2000, 27-39

[2] U. Heinle; Vertical High-Holtage Transistors on Thick Silicon-on-Insulator, Thesis, Uppsala 2003

[3] B. Murari; F. Bertotti; G.A.Vignola (Eds, Smart Power IC´s – Technologies and Applications, Springer 1995

[4] P. Moens et al.; “Reliability assessment of deep trench isolation structures”; IEEE 43rd Annual International Reliability Physics Symposium, San Jose, 2005; pp. 573-577

[5] XDH10 data sheet at http://www.xfab.com[6] Lange, A.; Heinz, S.; Erler, K.; Ebest, G.; Lerner, R.; Eckoldt, U.;

Schottmann, K.: Modeling the Leakage Current of Dielectric Structures in a High- Voltage Semiconductor Technology, Proc. of IEEE International Symposium on Industrial Electronics, June 4-7, Vigo, Spain, 2007

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