CZ和FZ方法介绍以及硅片的生产步骤2-USB迷|专注于互联网分享

2024年6月13日发(作者：针绿凝)

International Scientific Colloquium

Modelling for Electromagnetic Processing

Hannover, March 24-26, 2003

Modeling in Industrial Silicon Wafer Manufacturing - From

Crystal Growth to Device Processes

Th. Wetzel, J. Virbulis

Abstract

Silicon wafer manufacturing is one of the key processes that determine the yield and the

profitability in semiconductor device production. The present paper gives an overview on vari-

ous applications for numerical modeling in the wafer manufacturing process. It starts with ther-

mal and convection models for the crystal growth process, both by the Czochralski (CZ) and

the Floating Zone (FZ) method. Within this field, in particular modeling of electromagnetic

field influence on melt flow has become an an indispensable means for the puller design and the

process development for both methods. A further chapter is devoted to model approaches for

predicting crystal defects, like grown-in voids, self-interstitial aggregates or oxygen

precipitates. The defect modeling connect the crystal growth directly to the device

manufacturing processes, as crystal defects may be detrimental or beneficial to microelectronic

devices, produced on the silicon wafer. A last chapter points briefly to more recent applications

of numerical modeling in auxiliary processes, like wafer heat treatment steps, epitaxial growth

or wafer cleaning.

Introduction

During the last five decades, the technology for growing silicon monocrystals from the

melt, and the subsequent manufacture of silicon wafers, have become highly developed. The

quality, purity and size of today's silicon crystals and wafers have reached an outstanding level.

However, it remains time consuming and expensive to develop new processes that meet the

crystal and wafer quality requirements for devices with further decreasing design rules. The

current transition to a wafer diameter of 300 mm creates further challenges for the wafer

industry.

One of the main driving forces for the use of numerical modeling in the wafer industry

are the extremely high costs for growth furnace design and growth process development.

Thermal simulation of the global heat transfer in crystal growth furnaces contributes substan-

tially to keeping the development costs at an acceptable level. In particular with the transition

to 300 mm diameter CZ crystal growth, magnetic fields have become an important additional

means to control heat and mass transfer in the large melt volumes. Numerical modeling

facilitates the suitable design of inductor systems and helps to understand the effect of the

magnetic fields on the melt flow. However, the growth conditions do not only affect yield or

pull rate, but also the quality of the crystals. Grown-in defects like voids, self-interstitial

aggregates or oxygen precipitates can be detrimental or beneficial in subsequent process steps.

Therefore, defect engineering, i.e. the control of the formation of such defects, is an important

part of the whole wafer production. Numerical modeling helps to predict the defect formation

and behavior through crystal growth and for subsequent process steps, up to the device

processes for microelectronic components in the chip manufacturer’s production line.

Beside the success of the crystal growth and the defect simulation, there is an

increasing number of new applications of numerical modeling in the wafer manufacturing

industry, e.g. in process steps like wafer polishing, epitaxial layer growth or wet cleaning of the

wafers.

1. Crystal Growth

There are two major methods for producing silicon monocrystals from the melt. With

the Czochralski (CZ) method, today crystals of up to 300 mm are grown. The Floating Zone

(FZ) technology is currently scaled up to grow 200 mm crystals.

1.1 Czochralski Growth

Fig. 1 shows a longitudinal cut through a CZ growth furnace. The prediction of the

temperature field in the whole furnace and in the crystal requires a global thermal simulation,

taking into account the heat transfer by

conduction, convection and radiation.

Software tools for the conduction and

radiation calculation along with the pre-

diction of the melt-crystal interface are avai-

lable from different institutes [1, 2, 3]. They

are based on 2D axisymmetric models, em-

ploy view factor approaches for the

radiation treatment and are based on the

Finite Element Method (FEM) or the Finite

Volume Method (FVM). Some of these

codes allow the reproduction of transient

growth processes [2].

With increasing melt volumes and

crystal diameters, the consideration of the

melt convection has become more im-

portant. The melt flow in large diameter CZ

crucibles - today diameters of up to 32” with

loads of up to 450 kg are used - is characte-

rized by time dependent, three dimensional

processes. The time needed for a fully

transient, three-dimensional simulation of

Fig. 1: Schematic layout of an industrial CZ

melt flow in a crucible of the mentioned size

puller and simulated temperature distribution

however, still prevents their application for

(left)

industrial engineering purposes. Therefore,

2D axisymmetric steady state models have been developed, that reproduce the most important

features of the 3D flow, but do still provide reasonably short calculation time [4, 5]. One of the

most critical parts in any CZ melt flow simulation, both 2D and 3D, is the consideration of

turbulence. The 2D simulation tools usually employ k-ε or LowRe k-ε or k-ω models. Such

tools require careful experimental verification and modification based on comparisons with

experiments [5, 6, 7]. A model proposed in [8] provided good agreement of the simulated

melt-crystal interface shapes with measured ones for 200 mm crystals. 3D time dependent

simulations with LES turbulence models [9] and simulations close to DNS [10, 11] are used to

further develop the understanding of the flow behavior and to improve less time consuming

simulation models. One goal of such attempts is the global modeling of oxygen transport in the

CZ furnace, which is extremely sensitive towards any lack of precision in the turbulence

modeling.

Another aspect, closely related to reliable melt flow prediction, is the modeling of

magnetic field influence. In 300 mm CZ growth, different types of alternating (AC) magnetic

fields as well as static (DC) magnetic fields are used. The incorporation of the melt flow

models into the global heat transfer simulation tools facilitates direct consideration of the effect

of such fields on the temperature distribution in the crystal and all coupled phenomena, like

point defect dynamics. A 2D axisymmetric model approach for the AC and DC magnetic field

effect on the silicon melt is described in [5]. The model has been verified with experimental

results from a CZ model setup with various magnetic fields [7, 12].

1.2 Floating Zone Growth

Fig. 2 shows a longitudinal cut

through an FZ growth setup [13]. For the

numerical simulation of Floating Zone

growth there is a complete model chain

necessary, starting with the prediction of the

phase boundaries at feed rod, molten zone

and crystal, based on RF field influence and

heat transfer [14]. A second part of the

model chain is covering the prediction of the

transient melt flow and heat transfer based on

2D [15] and 3D [16] approaches. Based on

the melt flow calculation results, the dopant

transport can be simulated, yielding finally

the macroscopic and microscopic resistivity

distributions in grown wafers. The current

development of 200 mm FZ crystal growth

processes has been substantially supported by

the use of such simulation models.

Beyond these applications, the

influence of additional low frequency

Fig. 2: Schematic layout of a Floating Zone

inductors has been modeled [14] and

setup with simulated features and FEM mesh

considerations about the stress induced dislocation generation have been started [17]. A

recently developed improved phase boundary simulation model is presented in [18].

3. Defect Modeling

There are different types of defects in silicon wafers, which are related to the crystal

growth conditions and the thermal history of the crystal and wafer. During the solidification

process, intrinsic point defects, namely vacancies and silicon self interstitials, are incorporated

into the crystal. These intrinsic point defects form grown-in defects during cooling down from

melting to room temperature. Impurity atoms can also be incorporated into the growing

crystal, later forming precipitates or other defects. The most common of these impurities is

oxygen, which is dissolved from the silica crucible in CZ processes and transported through the

melt to the crystallization front.

Three types of defects in CZ crystals will be

mentioned here specifically: grown-in octahedral voids [19]

formed by the aggregation of vacancies (Fig. 3), oxide

precipitates [20] and the OSF ring [19].

Whether vacancy or self-interstitial related defects

are found in a CZ crystal or crystal region, depends on the

value v/G in that region at the crystallization interface during

solidification (v being the pull rate, G the temperature

gradient at the solid/liquid interface) [19, 21]. In addition to

determining these quantities from the thermal simulation,

models are used to directly describe the point defect

dynamics, i.e. the diffusive and convective transport of these

defects during crystal growth as well as their annihilation by

recombination [22]. The combination of such simulation

Fig.3: Octahedral void in an

models with the thermal simulation results, in particular the

as-grown silicon crystal [19]

transient temperature field in the crystal, allows to design

the growth furnace and the growth process in such a way, that grown crystals show a desired

defect species.

Furthermore, especially for vacancy rich crystals, that are for the production

of memory chips, it is necessary to ensure a specific size and density of the grown-in voids.

Simulation models have been developed, that describe the formation of such defects from

vacancies [19]. The size of the grown in voids depends strongly on the cooling rate in a

specific temperature interval during cooling down. Therefore, these models do also need the

thermal history of the crystal as well as the results of the point defect models. Fig. 4 shows

simulated size distributions of voids in fast and slow cooled CZ crystals. The void formation

models are also used to predict the impact of wafer heat treatment steps on size and density of

the voids.

An important part of defect modeling is devoted to the behavior of oxygen in CZ

silicon. As oxide precipitates are used as centers for internal gettering [20], models have been

developed to predict the size and in particular the density of such defects in CZ crystals and

wafers [23]. Another well known defect phenomenon is the so called OSF ring, appearing on

the wafer surface after

wet oxidation. The

nature and formation

of the OSF ring has

been studied by many

authors (see [19] for

further ref.). How-

ever, there is ongoing

research activity in

this field, as there are

complex interactions

of oxygen precipi-

tation with other point

defects or impurity

atoms in the growing

crystal, e.g. Nitrogen,

Fig. 4: Simulated size distributions of voids in crystals with different

that the

cooling rates

width of the OSF ring

[24].

3. Modeling for Wafer Processing Steps

Following crystal growth, there are several process steps in a wafer production line. It

starts with cutting of the ingots, goes through slicing, etching, polishing and ends with final

cleaning of the wafers. Inbetween there can be several other steps like lapping, thermal

treatment or epitaxial layer growth. In particular in the areas of etching, cleaning and epitaxial

layer growth there are several simulation activities based on Computational Fluid Dynamics

(CFD) codes, extended with chemistry and mechanical models. Many questions in this field can

be handled already with standard CFD tools, like flow distribution in cleaning baths. Other

examples for such simulations are the prediction of temperature distributions in annealing

furnaces or in RF heated epi reactors.

These more recent applications will gain increasing attention as they open a similar

potential for improved physical understanding of the processes as well as for saving con-

siderable amounts of time and money for process and equipment development. As in the crystal

growth and defect modeling activities, the close collaboration of industry and research

institutes will be extremely important for such new modeling topics, to finally have them at the

same scientific level and with the same impact on industrial day to day research and

development work.

Conclusions

Numerical modeling is an integral part of today’s industrial silicon crystal growth and

wafer manufacturing R&D activities, with a variety of applications. Crystal growth simulation

with thermal and convection models, both for the CZ and the FZ method is the most well

established application. Within this field, in particular modeling of electromagnetic field

influence on melt flow has become an indispensable means for the puller design and the process

development for both methods. The defect distribution in a wafer is extremely important for

the yield and profitability of microelectronic device production lines. Therefore, various model

approaches for predicting crystal growth and wafer treatment related defects, like grown-in

voids or oxygen precipitates, have been developed and are successfully used in the industry.

New applications of numerical modeling in auxiliary processes, like wafer heat treatment steps,

epitaxial growth or wafer cleaning are being developed and successfully used in industrial

equipment and process design.

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Authors

Dr.-Ing. Wetzel, Thomas

Wacker Siltronic AG

Numerical Modeling

P.O. Box 1140

D-84479 Burghausen

E-mail: @

Dr.-Phys. Virbulis, Janis

Center for Processes’ Analyses

PAIC

Zellu Str. 8

LV-1002 Riga, Latvia

E-mail: janis@