Design of fine geometry 3kV edge termination technique

compatible to CMOS design rules

Author: YunHao Feng

Affiliation: The University of Sheffield

Degree: MEng Electronic and Electrical Engineering

Submission: April 2015

Word Count: 5783

Supervisor: Professor Shankar Narayanan Ekkanath Madathil

Second Marker: Dr Gavin Williams

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Abstract

The purpose of this project is to design a termination structure which able to support power device working on 3000 volts by using two techniques which are ‘Field Limiting Ring’ (FLR) and ‘Variation in Lateral Doping’ (VLD). Furthermore, two designed structures are compared in the termination area length and find out which one has higher area efficiency.

This report is aim to introduce and investigate these two techniques and the researches for each technique is mainly from three aspects: basic introduction to termination structure, the relationship between key parameters and breakdown voltage, the theoretical explanation for these relationships.

For the ‘field limiting ring ‘structure, the distance between rings, the number of rings and the fixed interface charges are investigated. For the aim of blocking capability around 3000 volts, the optimum ring spacing is 0.8 micro metres and the ring number is 30. Moreover, the interface oxide charges need to be controlled no more than 3e11.

For ‘variation in lateral doping’ technique, the window number, the drift length and the fixed interface charges are investigated. For the same target, the widow number is 30 and the drift length is 1000μm. Moreover, the fixed interface oxide charges need to be controlled no more than 3e11 as well.

Comparing from two optimized structures, termination structure by using VLD technique has higher area efficiency than FLR. The structure with above parameters is able to support the power device to work on 3000 volts with the termination length of 1000μm.

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Contents

Abstract …………………………………………………………………………………………………………………………. I

Contents ………………………………………………………………………………………………………………………... I I

Section 1 Specification ……………………………………………………………………………………………. 1

Section 2 Introduction ……………………………………………………………………………………………. 2

Section 3 Basic Theory …………………………………………………………………………………………… 3

3.1 Introduction ……………………………………………………………………………………………………. 3

3.2 Avalanche Breakdown ……………………………………………………………………………………. 3

3.2 Depletion region under reverse bias ……………………………………………………………... 4

Section 4 Field plate structure …………………………………………………………………………….. 5

4.1 Introduction ……………………………………………………………………………………………………. 5

4.2 Field plate ……………………………………………………………………………………………………….. 5

Section 5 FLR technique and VLD technique ………………………………………..………… 7

5.1 Introduction……………………………………………………………………………………………………… 7

5.2 Field Limiting Ring (FLR)…….…………………………………………………………………………… 7

5.2.1 Structure………………………………...…………………………………………………………………. 7

5.2.2 Optimum spacing r………………………………...………………………………….……………… 10

5.2.3 Fixed interface charges 𝐐𝐟 …………………………...……………………..…………………….. 15

5.2.4 Ring number……………………………….…………………..……………………………………….. 16

5.2.5 Summary………………………………...……………………..………………………………………… 19

5.3 Variation in Lateral Doping(VLD)………………………………………………………………....... 19

5.3.1 Introduction and Structure……………………………..………………………………………… 19

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5.3.2 Window width 𝐖𝐢 and Drift length 𝐋𝐝𝐫………………………………………………………. 21

5.3.3 Fixed interface charges 𝐐𝐟 ………………………………...………………..……………………. 23

5.3.4 Summary……………………………...……………………..………………………………………..….. 19

5.4 Comparison between FLR and VLD………………………………………………..……………..... 24

Section 6 Conclusion……………………………………………………………………………….……..………... 25

Section 7 Recommendations…………………………………………………….……..……………………. 26

References ………………………………………………………….……..…………………………………..……………. 27

Appendices …………………………………………………………….……..……………………………………….…….. 28

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Section 1: Specification

Understand the principle of breakdown in power devices

Understand the role edge termination plays in a chip

Understand the current technology used (FLR)

Understand ring spectrum/spacing

Understand how the electric field is profiled with parameters

Understand the principle behind VLD

Understand the doping of N upon the field profile

Obtain an simulation of a high voltage VLD structure

Time Chart for the Project H: holiday

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Section 2: Introduction

Power devices are widely used in integrated circuits as a function of switch. One of the most important features is their high voltage blocking capability which means the device is able to operate on a high voltage.

The structure of a typical power device has three main components which are gate pad (A), active area (B) and edge termination area (C) shown in Figure 1.1

The active area is the main component and has the function of switch. The edge termination area plays another important role which used to support the switch to work under the high reverse bias shown in Figure 1.2.

For the purpose of high breakdown voltage, the edge termination techniques have been developed very rapidly in the last 50 years. They are used to design termination structures such as Floating field limiting ring (FLR), Variation in lateral doping (VLD), Junction termination extension (JTE) and Spiral junction termination (SJT). However, from the perspective of design, the termination area is also considered and expected to be as small as possible so that wafers can be used more efficiently thus saving cost for the company.

This project focuses on the termination area design by using the first two techniques which are FLR and VLD. The objective of optimized termination structure is able to support the devices to work on 3000 volts while the termination area is as small as possible.

The report begins with the basic p-n junction theories and avalanche breakdown is first introduced in section 3.

A

C

B

Fig.1.1: Typical design

of a power device

Fig.1.2: high breakdown

voltage is required

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Section 3: basic theory

3.1 Introduction

This section focuses on basic p-n junction theories. Avalanche breakdown is first introduced. This is one of the most common reasons why p-n junctions breakdown in the high electric fields. The distribution of the electric field in depletion region is researched next. From the electric field distribution profile, the maximum electric field is appeared always in the curved portion of the depletion region, which means that a breakdown will occur here firstly.

3.2 Avalanche breakdown

Generally, avalanche breakdown is the iterative process of impact ionisation in high electric f ield. The basic process of avalanche breakdown is shown in Figure 3.1.

The first electron is accelerated in the electric field and then collided with the first atom. If the colliding speed attains the thermal drift velocity [1] (107cm sec⁄ for electrons and 6.5 × 106cm sec⁄ for holes) the atom will split into a hole-electron pair. The first and second electrons are continued accelerating and then collided with new atoms again. As a result, more and more hole-electron pairs are generated and passing through the junction. More importantly, a great deal of heat is created in this process and damages the power device.

Fig.3.1: Hole-electron created by impact ionisation (After Ben G. Streetman [1])

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3.3 Depletion region under reverse bias

When N type material and P type material are combined, a p-n junction is formed and reached to thermal equilibrium. One of the most important features is the depletion region. There are no free carriers such as electrons and holes but many donor ions 𝑁𝑑+ and acceptor ions 𝑁𝑎− in the depletion region. Moreover, there is an electric field built up by the donor ions and acceptor ions.

If the junction is under reverse bias, the width of the depletion region layer will be larger. The maximum electric field occurs at the meeting point of two types of ion indicated by the blue line shown in Figure 3.2. Moreover, the magnitude of the maximum electric field is increased indicated by the red line compared to the electric field with no bias indicated by the black line in Figure 3.3.

More importantly, it is found that there is an electric field crowding in the curved region, which is shown in the red circle in Figure 3.2. Hence the electric field here is much larger than in other regions. The reason is electric fields are the vector and there is a superposition of the electric field. Therefore, from the point of depletion shape, it can be concluded that the electric field in the portion with high curvature is bigger than in other portions with lower curvature.

3.4 Summary

From the above analysis, avalanche breakdown will firstly occur in the highest electric field region which is the portion with high curvature in depletion region. That is the reason why edge termination techniques are used which essentially are used to change the distribution of electric field and make p-n junctions be able to sustain higher voltage.

Fig.3.2: Depletion region of p-n

junction with reverse bias

Fig.3.3: Electric field magnitude

with no bias and reversed bias

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Section 4: Field plate structure

4.1 Introduction

The key idea to increase the breakdown voltage of power device is the improvement of electric field distribution in depletion region. The simplest way to change the depletion distribution is by using ‘Field Plate’ structure. By controlling the bias of field metal, depletion region, electric field and the breakdown voltage is altered.

4.2 Field Plate

Field plate structure was proposed by A.S Frove, O. Leistiko and W.W. Hooper in 1967. [2] The basic structure is shown in Figure 4.1. Specially, there is a metal at the surface of the oxide layer with a separate bias.

A normal depletion region with reversed bias is shown in the grey area. The inner and outer boundaries of the depletion layer are indicated by the green and yellow line respectively. According to the boundary condition, the potential of inner boundary can be regarded as –Ve and potential of outer boundary approximately equals to zero.

If there is a positive voltage on the field plate, electrons in n-type substrate will be attracted towards it. Some of them are recombined with donor ions and further developed to the donor atoms and others are attracted to the surface of oxide layer. As a result, the depletion region will be shrunken and the outer boundary is changed to be like in the red

.

Fig.4.1: Field Plate Structure with forward, reverse and no bias (After Grove et al. [2])

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line. For the maximum electric field of total depletion region, the position is changed and becomes to occur in the region in the red circle. Considering the upper depletion region (in red circle), the width of the depletion region is altered to be narrower indicated by the purple line. However, the difference of the potential between two depletion boundaries is not much changed which means same voltage is supported in a narrower junction now.

The change of electric field is roughly shown in Figure 4.2 and the electric field in upper

region is indicated by the red line. The total area of triangle shows the total voltage

supported which is not changed, however, the width of depletion region is become

narrower and hence the peak electric field value is increased. Therefore, the breakdown will

be firstly occurred in the upper region and the value will be decreased.

The negative bias has the opposite effect. The depletion region is expanded and the

curvature is reduced which is shown by the blue line in Figure 4.1. For the upper and curved

region (curved region means the crowing electric field region shown in Figure 3.2), the

maximum electric field is changed to be much smaller. Therefore, the maximum electric

field of total depletion region is decreased and the breakdown voltage is increased.

However from the structure, it is impractical to provide a separate bias on the field plate.

Instead, the field plate is created merely by extending the junction metallization over the

oxide [3]. By contrast, the edge termination techniques can be used to optimize the

depletion region in a different way without the separate bias.

Fig.4.2: the Comparison of electric field between upper and

normal junction regions

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Section 5: Field Limiting Ring technique and Variation

in lateral Doping technique

5.1 Introduction

There are several edge termination techniques used to design power devices such as Field Limiting Ring (FLR), variation of Lateral Doping (VLD) and Spiral Junction Termination (SJT). This project focuses on the research of the first two techniques which are FLR and VLD. The structures and theories of them are first introduced especially in the explanation why two structures are able to increase the breakdown voltage. Moreover, the simulation results of two structures are given. Key parameters such as fixed oxide charge ‘𝑸𝒇’ and ring spacing ‘𝒓’

are adjusted and the relationship between them and breakdown voltage are investigated. Furthermore, the results are compared with theoretical situation and errors are analysed as well. Finally, the best structures with optimized parameters for both techniques are obtained and compared.

5.2 Field Limiting Ring (FLR)

5.2.1 Structure

The Field Limiting Ring technique basically is a design of termination structure and proposed by Y.C. kao et al in 1967 [4]. A basic structure is shown in Figure 5.1.

Fig.5.1: Conventional floating field limiting ring structure (After Y.C.Kao [4])

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One of the most significant features is the ‘ring’ component. There are a number of rings in

the termination area which are used to improve the shape of depletion region and reduce

the effects of junction curvature. When the reverse bias is applied and increased, the

depletion region will be extended towards outside from the main junction which is shown

by the green dash arrow in Figure 5.1. By contrast, the depletion width of rings is a constant

which decided by the doping level of rings and shown by the green arrow.

When the out layer boundary of the depletion is extended to meet with the out layer of

rings, two out layer boundaries will be punched through and merged. With the continue

increasing of the reverse bias, the depletion region will continue extending until to meet the

next ring. The ‘merging’ phenomenon is shown in Figure 5.1 and the final depletion layer is

indicated by the red dash line.

A real simulated example of multiple floating field rings structure is given in Figure 5.2 which

has 30 rings and under 3000 V bias. The red line in the figure shows the final outer boundary

of the depletion region.

Fig.5.2: FLR structure under 3000 V bias

(Parameters: Ring number: 30, Increased distance r: 0.8𝛍𝐦 ; Surface charges: 1e11#)

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From the perspective of electric field, assuming that point A is located in the crowded field

region of the main junction (Fig.5.1) and attains maximum electric field 𝜶 (𝑉/𝑚) that

junction can withstand, breakdown will occurs here firstly if the reverse bias continue

increasing. However, by using rings and after merging, the position of maximum electric

field will be changed and may occur in the region B and the value 𝜶 will be decreased. This

result implies these ‘rings’ in termination area can support the device operated in the higher

reverse bias because the peak electric field is mitigated and so that the junction is harder to

breakdown. The theoretical sketch of electric field profile before and after using floating

field rings are compared in Figure 5.3. The field crowing at main junction is mitigated after

using the floating field rings.

Considering the potential distribution, every distance between rings can be regarded as a

small resistor and thus the total potential of reversed bias is shared by them. Therefore, the

potential distribution is shown like a stepladder which is shown in Figure 5.4.

Fig.5.4: Potential distribution of FLR structure under 3000 V bias

Fig.5.3: Comparison of the electric field crowding for planar junction with

(right) and without floating field ring (left)

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5.2.2 Optimum spacing 𝐫

Several researchers have found that optimal spacing between rings progressively increases

as distance from the as main junction increases as shown in Figure 5.2. [5] The increased

spacing 𝐫 is a crucial parameter for determining the performance of breakdown voltage. If

the spacing 𝐫 is too small, there will be a crowing electric field at the last field ring which

means potential shared by previous rings are inefficiently due to the premature punch

through [6]. Electric field and potential distribution for r = 0.6μm (too small) is shown in

Figure 5.5.

Fig.5.5 (a): FLR structure under 3000 V bias for 𝐫 = 𝟎. 𝟔𝛍𝐦

Fig.5.5 (b): Electric field profile under 3000 V bias for 𝐫 = 𝟎. 𝟔𝛍𝐦

𝛄 = 𝟎. 𝟔

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On the other hand, with the increasing of the spacing 𝐫, rings will work more efficiently.

However, if the spacing is too large, the breakdown will be occurred before the merging

with the next ring which means rings behind are not in used and have no contribution to

potential sharing. The distribution of the electric field and potential for r = 2μm (too large)

are shown in Figure 5.6.

Fig.5.5 (c): Potential distribution under 3000 V for 𝐫 = 𝟎.𝟔𝛍𝐦

Fig.5.6 (a): FLR structure under 3000 V bias for 𝐫 = 𝟐.𝟎𝛍𝐦

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More generally, it is found that breakdown voltage is increased as the spacing initially and

then decreased. The relationship between breakdown voltage and the spacing 𝐫 is obtained

in Table 1 and plotted in Figure 5.7.

1

Fig.5.6 (b): Electric field profile under 3000 V bias for 𝐫 = 𝟐.𝟎𝛍𝐦

𝛄 = 𝟎.𝟔

Fig.5.6 (c): Potential distribution under 3000 V for 𝐫 = 𝟐.𝟎𝛍𝐦

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There is a data irrational which is when 𝐫 equals to 1μm, there may be some problems with

meshing in the simulation and therefore this point is ignored in above figure. The Maximal

breakdown voltage is obtained when r equals to 1.2μm.

It is worth to point out that from the design standpoint, the length of termination area is

considered as well. The termination length is indicated in the x axis and expected as small as

possible. If the breakdown voltage required for the device is 3000 volts, the optimized

spacing 𝐫 is 0.8μm. In this situation, the best performance of punching through is obtained

because all rings are just merged under the 3000 volt bias while the termination length is

relative small and around 1200μm.

The depletion region and potential sharing profile when 𝐫 equals to 0.8μm are shown in

Figure 5.8.

Spacing 𝐫 (𝛍𝐦) Breakdown Voltage (V)

0.6 3350 0.8 3600 1.0 3540 (X)

1.20 3800 1.25 3760 1.5 3480

1.75 3450 2.0 3240

Table 1: The breakdown voltage versus the spacing 𝐫

0.6

0.8

1.2 1.25

1.5 1.75

3300

3400

3500

3600

3700

3800

0.5 0.7 0.9 1.1 1.3 1.5 1.7 1.9

bre

akd

ow

n v

olt

age

(V

)

ring spacing (um)

Fig.5.7: The relationship between breakdown voltage and ring spacing

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Fig.5.8 (a): FLR structure under 3000 V bias for 𝐫 = 𝟎. 𝟖𝛍𝐦

1

Fig.5.8 (b): Electric field distribution profile under 3000 V bias for 𝐫 = 𝟎.𝟖𝛍𝐦

𝛄 = 𝟎.𝟔

Fig.5.8 (c): Potential distribution profile under 3000 V for 𝐫 = 𝟎.𝟖𝛍𝐦

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5.2.3 Fixed interface charges 𝐐𝐟

Normally, there are interface charges generated in oxide layer during the grown of the oxide

layer on silicon when fabrication [7]. The present of positive charges will attract electrons in

substrate and then these electrons will compensate the donor ions in depletion region.

Therefore, the depletion region will be shrunken and the curvature of outer boundary will

be increased. The negative charges will cause an opposite effect and the depletion boundary

will be extended. The effects are shown in Figure 5.9. Basically, different charges in the

oxide can be regarded as different bias and the principle is quite similar with the field plate

structure introduced in section 4.

However, in the case of cylindrical junction, there are only positive charges in the

passivation layer (oxide layer) and affects the distribution of potential.

The floating field ring structure with different number of fixed interface charges are

investigated and simulated next and the relationship between it and breakdown voltage is

shown in Table 2 and plotted in Figure 5.10. (Key parameters of the structure: r = 0.8μm ;

ring number=30)

Fig.5.9: The Effects on depletion region by positive and negative oxide interface charges

Fixed interface charges Breakdown Voltage (V)

1e10 3720 5e10 3650 1e11 3600 5e11 3110 1e12 1810

Table 2: The breakdown voltage versus the fixed interface charges

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It is obviously that breakdown voltage is decreased as the fixed interface charges and the number is expected to be zero. However, interface charges cannot be avoided in the fabrication. Instead, the value is structure dependent and here controlled between 1e11 to 3e11.

5.2.4 Ring number

The purpose of using rings is to let them to share more potential. However in practice, the efficiency of rings will be increased as rings initially and then decreased. That is the reason why electric field profiles are like an ‘n’ shape. From the design point of view, the small length of termination area means the cost saving of power devices. Therefore, the high efficiency of rings is desired and meanwhile required breakdown voltage is achieved.

Next, the relationship between ring number and breakdown voltage is investigated and the results are shown in Table 3 and Figure 5.10. Moreover, the length of termination area is compared as well and the optimized value is obtained and analysed.

1500

2000

2500

3000

3500

4000

0.00E+00 5.00E+11 1.00E+12

Bre

akd

ow

n v

olt

age

(V

)

Fixed interface charges (#)

Fig.5.10: The relationship between fixed interface charges and breakdown voltage

Number of rings Breakdown Voltage (V)

10 1030 20 2970 25 3560 30 3600 35 3610

Table 3: The breakdown voltage versus the number of rings

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It is obviously that there is a saturation region for the breakdown voltage and voltage begins

to increase slower when the number of ring reaches to 30. From the distribution of

potential, the last five rings in ‘35 rings structure’ have less contributions to potential

sharing.

Two FLR structures which are with 30 and 35 rings are compared in Figure 5.11 as well as

their electric field profile and potential sharing distribution.

Saturation region

Fig.5.11 (a): The comparison of depletion region

30 rings 35 rings

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The ring spacing is 0.8μm for two structures and both under 3000 volt bias. From the

depletion region graph, it is shown that the last five rings in ‘35 rings structure’ are not

merged because the breakdown is occurred before the merging with the 31th ring. From the

electric field profile, there is no electric field in the last five rings and the maximum electric

field is occurred at someone of rings which is shown in the black circle and thus the

breakdown is firstly occurred here. Comparing from the potential sharing, the last five rings

are located at the last 200μm of termination area but have no contribution to potential

sharing. Therefore for the cost saving, these termination areas can be removed.

30 rings 35 rings

Fig.5.11 (b): The comparison of electric field

30 rings 35 rings

Fig.5.11 (c): The comparison of potential sharing

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5.2.5 Summary

5.3 Variation in Lateral Doping (VLD)

5.3.1 Introduction and Structure

Although, this technique is widely used, a satisfactory theory to aid in understanding has not been reported in the literature. [8] During this project, there were very limited theoretical introductions can be found especially in the explaining why this technique works more efficiently. This report is focused on the basic introduction and simulation of VLD structure. The performance will be compared with floating field ring structure and analysed from the perspective of simulation results.

Variation in lateral doping technique is proposed by R. Stengl and U. Gosele in 1986. [9][10] Essentially, the key idea of increasing breakdown voltage is quite similar with the FLR. By creating rings, depletion regions of main junction and rings are merged and thus the curvature is reduced which further causes the increasing of breakdown voltage. For the main difference in structure, the spacing between rings in FLR is varied by the r value. By contrast, the spacing in VLD structure is a constant but the width of rings is in the linear relationship with the location.

The rings are created by the masked implantation of n type material and the mask structure is shown in Figure 5.12.

Considering from both high breakdown

voltage and cost saving, the optimized

ring spacing is 0.8μm and ring number is

30. By using this termination structure

and under the 3000 volt bias, the last ring

is just merged and all rings share the

potential efficiently. Besides, the fixed

interface charges are controlled around

1e11 so that the breakdown voltage will

be more stable. Totally from the

perspective of power device, above

floating field ring termination structure is

able to support the power device to work

on 3000 volt as the function of switch.

Table 4: The optimum parameters for floating field ring structure

Ring spacing 0.8μm

Ring number 30

Fixed interface

charge 1e11#

Termination Length 1200μm

Breakdown Voltage 3600 V

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The slits based on oxide layer are used for implantation of n type material which will further forms rings. Xi is the centre location of slit and indicates the location of rings as well. The value a is the characteristic length indicating the centre to centre distance between two adjacent rings. W𝑖 is the window width and the value is in the linear relationship with the

location which can be calculated by the formula Wi =𝐚𝐱𝐢

𝐋𝐝𝐫

where 𝑳𝒅𝒓 is the drift length(i.e:

termination length). It is worth to point out that ‘variation in lateral doping’ is a lateral device technique. However in this project, the simulation was based on the vertical device thus drift length here was effectively the wafer thickness.

The key idea of this technique is the ‘variation in lateral doping’. The doping level is increased as the window width. In the implantation, high temperature is required and anneal time is controlled. All rings will be merged more completely with the increasing of anneal time and thus the depletion length in N type region is increased with axis which is shown in Figure 5.13.

Fig.5.12: The mask structure for variation in lateral doping technique

Figure 5.13: Schematic cross-sectional view of IGBT with a VLD drift region

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There are three parameters adjusted which are window number 𝐖𝐢, drift length 𝐋𝐝𝐫 and the

fixed interface charge 𝐐𝐟. For the simulation, there is a big problem with the prototype structure. The window width is required in the linear relationship with the location however it is actually decreased in the prototype structure which is shown in Figure 5.14.

As a result, the depletion region is quite similar with the FLR structure and the depletion layer width is decreased in the x axis. However, the overall tendencies of the results are corresponding to the theoretical results and some errors will be analysed in each section.

5.3.2 Window width 𝐖𝐢 & Drift length 𝐋𝐝𝐫

There are three groups of data and each group is based on one drift length. The window number is adjusted from 5 to 60 and the drift length are 1500 μm, 1000 μm 750μm respectively. Theoretically, the breakdown voltage for different drift lengths as a function of the implantation does. For each drift length, the maximal breakdown voltage is occurred in a different implant dose and this value is increased with drift length. [11] The theoretical relationship between breakdown voltage and implant does for different drift lengths are shown in Figure 5.15. By contrast, the simulation results are obtain in Table 5 and Figure 5.16.

Figure.5.14: The imperfect prototype structure of mask used in simulation

Figure 5.15: Theoretical relationship between breakdown voltage and implantation dose

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There are three data seems irrational and thus be ignored in above diagram. The window number here can be qualitatively expressed as the implantation does. From the figure, the overall tendencies are corresponding to the theoretical relationship and there are three main features can be concluded. The breakdown voltage is increased as the window number initially and then decreased. The maximal breakdown voltage is increased as the drift length. The optimum value of window number to achieve maximal breakdown voltage is increased with drift length as well.

Drift length 1500 𝛍𝐦 Drift length 1000 𝛍𝐦 Drift length 750 𝛍𝐦

Window number

Breakdown Voltage (V)

Window number

Breakdown Voltage (V)

Window number

Breakdown Voltage (V)

5 769 5 869 5 1040 10 1860 10 1870 10 2090 15 2410 15 2850 15 2910 20 2970 20 3590 20 3230 25 3390 25 x 25 x 30 3480 30 3630 30 2140 35 3650 35 2220 35 1390 40 3230 40 1650 40 1140 60 3090 60 1300 60 x

Table 5: The breakdown voltage versus the window number

for different drift length

(um)

(um)

(um)

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5.3.3 Fixed interface charges 𝑸𝒇

The formation and impact of fixed interface charges have been introduced in the section 6.2.3. Generally, it is generated in the fabrication of oxide layer. Positive charges will reduce the breakdown voltage and negative charges have the opposite effect. In this section, interface charges are adjusted from 0 to 311and how that affect the breakdown voltage is investigated. The simulation results are shown in Table 6 and Figure 5.17.

Theoretically, the breakdown voltage is decreased with the number of fixed interface charges. However, from the simulation results, there is an increase stage at the beginning for drift length 750μm and 1000μm which are indicated by the red circle. This phenomenon cannot be explained by the report but probably because of the imperfect prototype structure. Moreover, for the ideal results, the breakdown voltage will be more stable with the increasing of drift length. In other words, the breakdown voltage will not be decayed rapidly as the fixed interface charges. However, from the result, the breakdown voltage for drift length 1000μm is most stable and the reason cannot be explained.

Drift length 1500 𝝁𝒎 Drift length 1000 𝝁𝒎 Drift length 750 𝝁𝒎

Fixed interface charges

Breakdown Voltage (V)

Fixed interface charges

Breakdown Voltage (V)

Fixed interface charges

Breakdown Voltage (V)

0 3690 0 3470 0 2130 1.00E+10 3670 1.00E+10 3510 1.00E+10 2140 5.00E+10 3600 5.00E+10 3670 5.00E+10 2180 1.00E+11 3510 1.00E+11 3630 1.00E+11 2130 3.00E+11 2780 3.00E+11 3220 3.00E+11 1870

Table 6: The breakdown voltage versus the number of fixed

interface charges

(um)

(um)

(um)

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From the perspective of breakdown voltage, there is a similar performance for drift length 1000μm and 1500μm and the breakdown voltage is around 3575 volt when there is 1.00E+10 charges. However, from the perspective of termination area, there will be 33.3% termination area saved if by using drift length 1000μm structure.

5.3.4 Summary

5.4 Comparison between FLR and VLD

Comparing the performance of two techniques from Table 4 and Table 7, the fixed interface charges are controlled at 1e11 for both structures. The performances of breakdown voltage of FLR and VLD are quite similar and around 3600 volts. However, considering of the termination length used, VLD technique is more competitive which occupies 1000μm compared to the FLR technique which is 1200μm. There is 20% edge termination area saved if by using VLD technique for the same breakdown voltage of 3600 V. Moreover, comparing from Figure5.10 and 5.17, VLD technique is less sensitive to the oxide interface charges and the performance will be more stable.

Table 7: The optimum parameters for variation in lateral doping structure

Window number 30

Drift length

(Termination length) 1000μm

Fixed interface charge 1e11#

Breakdown Voltage 3630 V

For the data from drift length 1000μm group shown in Table 4, the optimum window width is chosen to be 30 , in this situation, the maximum breakdown voltage is achieved and equals to 3630 volts. Besides, the fixed interface charges are needed to be controlled and around 1e11 so that the performance of breakdown voltage will be more stable. As a whole, the termination area by using this VLD structure is able to support power device to work on 3000V as well.

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Section 6: Conclusion

In this project, there are mainly two objectives are achieved. Firstly, the edge termination structures have been optimized by using two techniques which are ‘Field limiting ring’ and ‘Variation in lateral doping’. The optimized structures are able to support power devices working on 3000 volts. Secondly, two techniques are compared and the conclusion is VLD structure can save more termination area than FLR for the same breakdown voltage achievement.

This report was aimed to introduce a basic idea of termination techniques especially in the explanation why they are needed and how they can be used to increase the breakdown voltage for power devices. For getting a better comprehension, basic theories were introduced step by step. The report was begun with the basic theory of avalanche breakdown and the breakdown was proved always occurred in the highest electric field. The pn junction under reverse bias was introduced then and the breakdown is firstly occurred in the region with high curvature. For mitigating the impact of high curvature in depletion region, field plate structure was introduced next which is by altering the potential on oxide layer. Because of the impossibility of the separate bias at surface of oxide layer, the report was focused on the introduction of two edge termination techniques which are FLR and VLD. Several parameters were adjusted which are ring spacing, ring number and fixed interface charges for FLR and window number, drift length and fixed interface charges for VLD. The simulation results were analysed and the best parameters were obtained for each technique. Finally, two optimized structures were compared in the same limited conditions and the performance of VLD technique is more stable and more competitive in the cost saving.

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Section 7: Recommendations

In the real design of field limiting ring structure, the width and depth of rings are considered as well. If the depth of rings is too small, p type depletion region in the ring will be easily punched through which means there will be less potential shared by rings.

For the variation in lateral doping technique, the mask for prototype structure is not perfect and need to be improved as in the Figure 17. Moreover, anneal time should be increased in the simulation so that rings will be merged more completely shown in Figure 18. For the perspective of design, there are three main parameters affecting the resistivity which are window size, implant concentration and final junction depth. The last two parameters are needed to be considered and controlled in the simulation as well.

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References

1. Ben G. Streetman, ‘‘Solid State Electronic Devices’’, Prentice-Hall, Inc., Englewood

Cliffs, N.J., U.S.A, 1990.

2. A.S. Grove, O. Leistiko and W.W. Hooper, ‘‘Effect of Surface fields on the breakdown

voltage of planar silicon p-n junctions’’, IEEE Trans. Electron Devices, Vol. ED-14, pp.

157-162, 1967.

3. J.V Subhas Chandra Bose, ‘‘Novel Edge Termination Technqiues For MOS Controlled

Power Devices’’, pp.15-17, 2001.

4. Y.C. Kao and E.D. Wolley, ‘‘High-voltage planar p-n junctions’’, Proc. IEEE, vol. 55, pp.

1409-1414, 1976

5. R.D Kenneth, ‘‘Two-Dimensional Simulation of the Effects of Total Dose Ionizing

Radiation On Radiation On Power-MOSFET Breakdown’’, pp.51-53,1989.

6. J.V Subhas Chandra Bose, ‘‘Novel Edge Termination Technqiues For MOS Controlled

Power Devices’’, pp.20-24, 2001.

7. B.Jayant Baliga, ‘‘Fundamentals of Power Semiconductor Devices’’ Springer, pp. 127-

132, 2008.

8. M.Steve, ‘‘Arbitrary Lateral Diffusion Profiles,’’ IEEE Trans. Vol.42, pp2226-2227,

1995.

9. R.Stengl and U. Gosele, ‘‘Variation of Lateral Doping – A New Concept to Avoid High

Voltage Breakdown of Planar Junctions,’’ International Electron Devices Meeting, pp.

154-156, 1998.

10. R.Stengl, U. Gosele, C. Fellinger, M. Beyer and S. Walesch, ‘‘Variation of lateral

doping as a field termination for high voltage power devices,’’ IEEE Trans. Electron

Devices, Vol.33, pp. 426-428, 1986.

11. E.M. Sankara Narayanan. ‘‘Realizing High-Voltage Junction Isolated LDMOS

Transistors With Variation in Lateral Doping,’’ IEEE Trans. Vol.51,pp. 2223-2227, 2004.

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Appendix

1. Simulation results for VLD

Because of the imperfect prototype structure of VLD, the simulation results are not shown in the report but given here. The optimized structure is shown in the Figure I as well as its electric field and potential sharing profile. (Key parameters: Window number:30; Fixed interface charges:1e11; Drift length:1000μm)

Fig. I (a): Depletion region of structure under 3000 V bias

𝛄 = 𝟎.𝟔

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Fig. I (a): Electric field distribution profile under 3000 V bias

𝛄 = 𝟎.𝟔

Fig. I (c): Potential sharing profile under 3000 V bias

𝛄 = 𝟎.𝟔

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Individual Project Report 2

Final Year Project Title

Former:

Design of fine geometry Edge termination technique compatible to CMOS design

rules for 1.2kV to 4.5kV.

Now (upgraded):

Understand three termination techniques which are “Floating field limiting ring

structure (FLR)”, “Variation of Lateral Doping (VLD)” and “Double RESURF

technique”. Comparing these three techniques and work out which technique can

reduce more area for the same breakdown voltage.

Brief description

Power device is expected to be operated in the high voltage. In this project, several

techniques will be researched to achieve this target.

In the report 1, the main target is to understand a kind of Edge termination technique

which is “Floating field Ring” mainly including how that can increase the breakdown

voltage and why there is an optimum increasing space between rings.

When “FLR” was researched, other two techniques were introduced by the

supervisor and they are “VLD” and “Double RESURF technique”. Next and similarly,

these two techniques should be researched in the same way. The difference is the

three techniques will be compared then and decide which can reduce more edge

termination area for the same breakdown voltage. That means there would be more

power devices on the one wafer so that more profit can be gotten by the company.

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Progress

The first technique which is “Floating field limiting ring structure’’ was researched

basically in the first term especially in the theory and simulation. There are three

significant steps were done and two about the theory and one about the simulation.

They are listed below with a brief introduction.

1. FLR Operation principle

Briefly, the Floating field limiting ring will support some voltage from the main

junction which may increase the high voltage blocking capability of main junction.

2. The research about the effect of Increasing space of rings

The increasing space between rings will affect the performance of power device.

The truth is the space is too large or too short will decrease the breakdown

voltage. Therefore, there is an optimum increasing space should be determined

to achieve the maximum BV.

3. Familiar with the simulation software & simulation

The software was installed in the supervisor’s desktop, therefore I need to remote

access his computer and launch the software by using linux system. Two

important parameters which are potential and electric field distribution were

simulated with different increasing space and different voltage. The result was

matched with the theory and the optimum space was gotten around 0.8

micrometers.

The main problems I came across so far are mainly about the theory principle and

the operation of simulation software but these questions can be solved after the

meeting with the supervisor. I think it’s not important and have no necessary to be

detailed in the report 2. Besides, the time chart 2 modified was given below. It should

be noted that there are only 18 semester weeks on the chart but much progress

would be finished in the Christmas holiday.

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Time Chart in Report 1

Time Chart in Report 2 (H: holiday)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Theory √ √ √ √ √ √ √

Design √ √ √

Processing √ √ √

Testing √ √ √

Report √ √ √

Week

Component

1 2 3 4 5 6 7 8 9 10 11 12 H 13 14 15 16 17 18

Tech

Research

√ √ √ √ √ √ √ √ √ √ √ √ √ √ √ √ √

Familiar soft √ √ √ √ √

Simulation √ √ √ √

Tech

Research

√ √ √ √ √

Tech

Research

√ √ √ √ √

Simulation √ √ √

Report √ √ √ √ √ √ √

Week

Component 1

st

2nd

3rd

2nd

&3rd

1st

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Detailed Specification

Understand the principle of breakdown in a power device.

Understanding the role edgetermination plays in a chip.

Understand the current technology used (FLR).

Understand of ring spectrum/spacings.

Understand how the electric field is profiled with the parameters.

Understand the principle behind VLD.

Understand the doping of N upon the field profile.

Obtain simulation of a high voltage VLD structure.

Understand the principle of Double Resurf.

Understand how double resurf parameters effect electric field shaping.

Pass

2.2, 2.1

1st

(If time

allowed)

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Third Year mid-summary

On the whole, I think the third year modules are easier than the second year but

there is a personal project which is quite challengeable for me.

For the courses, I think all modules are fine; ACS342 is going quite well and the

difficult modules are the mathematics and the finance. I’m going to review in the

Christmas holiday for the ACS342, MAS381,MGT388 exam.

More significantly, I’m developing an app with my doctor friends and hopefully that

can be successful. :) Besides, I’m going to begin my career in the summer and find a

job to get some experience.