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`ADVANTAGES 0F DUAL FREQUENCY PECVD
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`FOR DEPOSITION OF ILD AND PASSIVATION FILMS
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`Evert P. van de Ven, I-Wen Connick, Alain S. Harrus
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`Novellus Systems, Inc.
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`3950 N. First Street
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`San Jose, California 95136
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`ABSTRACT
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`Dual frequency PECVD has been explored in R&D for several years.
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`Recently it has been incorporated into production equipment for deposition of
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`silicon nitride, oxynitride and TEOS oxides. The combination of high (13.56
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`MHz) and low (300-400 KHz) frequency RF provides control of film stress and
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`can improve step coverage,
`film density,
`chemical composition and film
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`stability. Optimization of
`these film properties
`is possible by controlling
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`deposition pressure and the ratio of high and 10w frequency RF power.
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`INTRODUCTION
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`Plasma Enhanced Chemical Vapor Deposition has been widely used in the
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`semiconductor industry for
`the deposition of silicon nitride, oxynitride and
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`oxide films [1][2].
`Over
`the last
`few years,
`the increasing complexity of
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`advanced multilevel metal
`technologies has greatly challenged the deposition
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`The
`methods and processes in order to achieve enhanced film quality [3].
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`decrease in metallization thicknesses, channel lengths and design linewidths has
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`exacerbated problems such as stress cracking, stress induced metal voiding [4]
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`and short-channel hot-electron device degradation [5]. The film requirements
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`have therefore become more stringent and flexibility of process control and
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`integration is being emphasized. One approach to improve film quality and
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`process flexibility has been the introduction of dual frequency in PECVD. The
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`benefits of this were first reported by Fujitsu for deposition of silicon nitride
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`[6] and later more elaborated on by Novellus [7] and ASM [8].
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`the
`frequency for
`In this paper, we discuss the advantages of dual
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`deposition of silicon nitride, oxynitrides and TEOS oxide films and propose a
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`mechanism explaining the effects on step coverage,
`film stress,
`chemical
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`composition and film density and stability.
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`June 12-13, 1990 VMIC Conference
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`TH~0325-1/90/0000—0194 $01.00 C 1990 IEEE
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`AMSUNG EXHIBIT 1049
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`Page 1 of 8
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`SAMSUNG EXHIBIT 1049
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`DUAL FREQUENCY HARDWARE DESCRIPTION
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`The PECVD reactor used in this study is described in detail elsewhere
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`[9]. We focus here on the dual frequency configuration. Early work in the use
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`of dual frequency for plasma deposition compared different RF configurations,
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`i.e., high and low frequencies on the same electrode, separate electrodes and in a
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`triode configuration. Using separate electrodes yields better results and allows
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`better control. Figure 1 shows the RF configuration used in this work. The
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`bottom electrode is a heated susceptor connected to a 300 KHz power supply. A
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`matching transformer and a low pass filter ensure maximum efficiency for the
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`RF power
`input and shunts high frequency to ground.
`The matching
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`transformer is used since at
`low frequency the plasma is almost exclusively
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`resistive. The top electrode,
`in a similar fashion,
`is connected to a 13.56 MHz
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`RF generator through a high pass filter and a matching network. In the parallel
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`plate configuration used,
`the electrode spacing is
`fixed and high and low
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`frequency power can be controlled independently. Deposition at high frequency
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`only, low frequency only or a mixture of both is possible.
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`Figure 1.
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`Dual Frequency RF
`Configuration
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`HF
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`NFI
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`HAT N
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`7
`HF GROUND
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`l
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`LOW PASS
`FILTER
`MATCHING
`TRANSFORMER
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`100 - 400 KM: T
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`DUAL FREQUENCY AND ION BOMBARDMENT: MECHANISM
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`At high frequency (13.56 MHz) only the electrons are able to follow the
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`RF field while the ions are "frozen" in place by their heavier mass and inertia.
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`The cross—over frequency at which the ions start following the electric field is
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`between 1 and 5 MHz depending upon the mass of the ions. Consequently,
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`below 1 MHz, the ion bombardment is significantly higher. This has effectively
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`been used in PECVD to obtain high quality films. The ion bombardment not
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`only enhances chemical reactions but also causes a low energy ion implantation
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`which densifies the film and provides an intrinsic compressive stress. However,
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`in a low frequency system control of the ion bombardment is difficult. The
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`SUNG EXHIBIT 1049
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`Page 2 of 8
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`SAMSUNG EXHIBIT 1049
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`the deposition pressure.
`in is
`only parameter which has some impact
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`Unfortunately, changes in pressure also affect the deposition ra’te, uniformity,
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`etc. Another limitation of most
`low frequency systems is
`the sensitivity to
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`substrate resistivity causing deposition rate variations [10].
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`”"".127.J'Jlfé‘fiisfiéiitsé’s"FNMSSSé’étlfiém’
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`--,--v- ~—
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`THICKNESS 0N METAL (Thousands
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`CONTROL OF FILM STRESS
`STRESS (X 10E9 D/CMZl
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`A combination of high and low frequency (13.56 and 50-400 KHz
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`respectively) provides a solution to the above problems.
`The high frequency
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`gives a stable discharge, generates part of
`the reactive species and assures
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`effective coupling to the substrate (Figure 2). The low frequency provides the
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`ion bombardmcnt/implantation. Accurate and independent control
`is possible
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`using a constant
`total RF power and changing the low frequency percentage.
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`Under
`this condition,
`the deposition rate, uniformity and other process
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`parameters are hardly affected. The independent control allows optimization
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`and provides process flexibility as discussed further on.
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`slopes between SiN, SiOxNY and SiO2 are the result of differences in atomic
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`11
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`SCRIBELINE THICKNESS (Thou-and.)
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`18
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`so
`40
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`PERCENT LF POWER
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`Figure 2.
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`Comparison of thickness
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`of dual frequency SiN
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`in scribeline and on
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`metal interconnect.
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`Figure 3.
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`Film stress as a function
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`of percent LF power. The
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`total (HF and LF) power
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`density in this and following
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`figures was kept constant at
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`0.4 W/cmz.
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`STRESS CONTROL
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`Dual
`(Figure 3).
`the film stress
`frequency can be used to control
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`Increasing the amount of LF power increases the ion bombardment to which the
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`growing film is subjected. The effect is a low energy (< 300 eV) implantation
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`or "stuffing" of the film with Si, O or N. This stuffing causes a change in the
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`intrinsic film stress from tensile to compressive and increases the film density.
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`As shown in Figure 3, the change is gradual and easy to control. The different
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`Page 3 of 8
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`SAMSUNG EXHIBIT 1049
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`in the amorphous
`the distance between the atoms
`the shorter
`distance ~-
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`material, the larger the effect of the low energy implantation. Changes in total
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`RF power, deposition pressure or deposition rate do not affect the slope, only
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`the position of the line. Higher deposition rates shift
`the line to the right,
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`lower rates to the left.
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`the
`a
`strong function of
`the as-deposited film is
`The density of
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`deposition temperature, amount of ion bombardment and the deposition rate.
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`Whereas the density of thin films is difficult to determine accurately, wet etch
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`rate and refractive index can be considered a reasonable representation of what
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`is happening with the film, assuming no major changes in chemical composition
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`[1 I]. Figure 5 shows the etch rate of TEOS SiO2 and SiN in 7:1 BOE as function
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`of
`low frequency power.
`Again a
`linear
`relationship is observed which
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`correlates well with the change in stress and changes in refractive index.
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`ETCH RATE (7:1 BOE)
`TEOS Sl02
`[MIN
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`i
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`20
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`70
`60
`5°
`‘0
`30
`PERCENT LF POWER
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`50
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`W
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`2°00 (A
`I 900
`1800
`1 700
`1000
`1500
`I 400
`1$00
`1200
`I0
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`AIMIN
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`wot
`300
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`O
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`20
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`SILICON NITRIDE
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`SILICON NITRIDE
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`STRESS IX ‘IJEU 0/9423
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`.3
`.6 0.2 0.25 0.8 055 0.4 0.45 0.5 0.55 03 015
`rpm HF POWER (W/M)
`a
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`3317135 (x at DIGI2)
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`on LP
`0.4 WISH!
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`2.6 2.7 23 2.9 SD 3.! 32 3-3 8.4 u
`mummm)
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`Figure 4. Dependence of SiN stress on
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`RF power density and deposition
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`pressure.
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`Figure 5. Wet etch rate of TEOS SiO2 and
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`SiN as a function of percent
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`LF power.
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`The influence of total RF power and deposition pressure are shown in
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`Figure 4. The RF power has little effect even on silicon nitride deposition as
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`an increase in ion bombardment
`is canceled out by a higher deposition rate.
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`Deposition pressure, however, has a large influence as it changes the plasma
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`potential and deposition rate in opposite directions.
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`FILM DENSITY AND STABILITY
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`AMSUNG EXHIBIT 1049
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`Page 4 of 8
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`SAMSUNG EXHIBIT 1049
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`The change in film density is particularly important for TEOS 8102.
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`Low density tensile films tend to pick up water and form SiOH groups. This
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`causes degradation of electrical and mechanical properties.
`The water
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`adsorption can be determined by changes in film stress during exposure to water
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`vapor and/or by FTIR analysis.
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`Typically, films deposited with an intrinsic compressive stress are stable
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`and are even able to withstand boiling water without
`increasing the SiOH
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`content or adsorbing water.
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`STEP COVERAGE IMPROVEMENT
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`Improvement in step coverage will only occur when the ions have a low
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`energy.
`Higher energy ions have the exact opposite effect,
`breaking the
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`precursors and increasing the reaction rate on surfaces perpendicular to the
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`flux. The result:
`a higher deposition rate and a decrease in surface mobility on
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`the horizontal surfaces leading to degradation of the sidewall step coverage.
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`TEOS Si02
`TEOS $502
`SIDEMLL STEP COVERAGE ('5) SIDEWALL STEP COVERAGE (‘5)
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`90
`80
`7O
`60
`50
`40
`30
`20
`10
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`PECVD TEOS oxide is gradually being introduced in ILD applications
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`because of the superior step coverage [12]. Silicon nitride and oxynitridcs have
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`long been known to have acceptable step coverage [13].
`In both cases the step
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`coverage is
`the result of a low sticking coefficient of
`the precursors which
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`enhances the surface mobility [14][15].
`The effect of
`low energy ion
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`bombardment on this is significant.
`The low energy ion flux, which is
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`perpendicular to the substrate,
`increases the desorption rate and improves the
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`surface mobility of the precursors on the horizontal surfaces, The result is a
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`better sidewall step caverage.
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`0.7
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`0.8
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`0.9
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`1.0
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`1.1
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`* SINGLEFREQUENCY
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`0.6
`0.5
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`ASPECT RATIO
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`.0
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`0.1
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`0.2
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`0.3
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`0.4
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`D0
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`1°
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`2°
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`3°
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`5°
`5°
`‘0
`PERCENT LF POWER 1%)
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`7°
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`8°
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`9°
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`D DUALFREOUENCY
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`Figure 6-
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`TEOS Sioz Sidewa“ Step
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`coverage as a function of
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`percent LF Dower.
`(Aspect
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`ratio = 0.35)
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`Figure 7.
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`Sidewall step caverage for
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`single and dual frequcncy
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`deposited TEOS Sio2 as a
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`function of aspect ratio.
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`(Metal height = 0.8 um).
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`Changes in film composition have only been observed for silane based
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`processes. The low energy ion bombardment helps to break the rather weak Si-H
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`bond. This
`is most
`noticeable in
`deposition of silicon
`nitride and doped
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`oxides. Figure 8 shows the SiH and NH content of plasma nitride as a function
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`of percent
`low frequency.
`The SiH content decreases with increasing ion
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`bombardment while the amount of NH slightly increases. The latter is probably
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`due to reaction of the released hydrogen with nitrogen atoms on the surface. For
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`UV transparent nitride, the SiH content must be below 2 at 0/9. This is achieved
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`by optimizing the gas composition rather
`than trying 'to optimize the ion
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`bombardment.
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`oxidation of the hydrides and effectively eliminate Si-H bonds.
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`50
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`PERCENT LF POWER
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`Figure 8.
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`Changes in SiH and NH content
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`in PECVD SiN as function of
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`the amount of low frequency.
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`In the case of doped oxides, dual
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`frequency can be used to improve
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`Figure 6 shows the step coverage of TEOS SiO2 as a function of low
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`frequency power. The step coverage improves up to approximately 60 - 70% LF
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`and degrades when the ion energy becomes too high. Figure 7 compares the step
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`coverage of TEOS SiO2 deposited in a single frequency and dual frequency
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`mode as a function of aspect ratio. For silicon nitride, similar results have been
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`obtained.
`In this case,
`the typical
`impr0vement
`in step coverage is
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`approximately 10 to 12%.
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`FILM COMPOSITION
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`SILICON NITRIDE
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`H CONTENT ('1!)
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`the RF
`independent of
`The chemical composition of TEOS oxides is
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`condition. As deposited, all films have the same low SiOH content (< 3%), no
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`detectable H20, and less than 0.5 at % C (the detection limit of SIMS). As
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`mentioned earlier,
`the density is affected and films deposited with an intrinsic
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`tensile stress will adsorb water and increase their SiOH content over time.
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`CONCLUSIONS
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`The use of dual frequency for PECVD of dielectrics provides increased
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`flexibility and process control. The main role of the high frequency RF is to
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`generate the reactive species and provide sufficient electron and ion densities.
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`The low frequency is added to control
`the ion bombardment
`to which the
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`substrates are subjected during deposition.
`Increasing the low frequency power
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`the plasma potential and the amount of
`ions
`following the low
`increases
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`frequency RF field (< 1MHz).
`The resulting low energy ion implantation
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`occurring during deposition causes a change in the intrinsic film stress from
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`tensile to compressive,
`increases
`film density and improves
`the chemical
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`reactions.
`In addition,
`the low energy bombardment enhances the surface
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`mobility of adsorbed T1308 and Si(NH2)x thus improving the step cowrage of
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`TEOS SiOz, standard silicon nitride, UV transparent nitride and oxynitrides.
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`However,
`if the ion energy is too high,
`the step coverage deteriorates due to
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`premature decomposition of the reactive species.
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`
`REFERENCES
`
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`
`
`
`C. Adams; Proc. Reduced Temp. Processes for VLSI, Electroc. Soc. (1986),
`
`
`p. 111
`
`
`
`
`
`
`
`
`
`
`
`
`E. P. van de Ven; Solid State Techn. (April 1981), p. 167
`
`
`
`
`
`
`
`
`Proc.
`IEEE VLSI Multilevel Interconnect Conference (1986, 1987, 1988,
`
`1989)
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`
`
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`
`
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`
`
`
`J.T. Yue, W. P. Funsten, R. V. Taylor; Proc. 23rd IRPS (1985) abstr. 5.1
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`
`F. C. Hsu and K. Y. Chiu, Proc. VLSI Symposium Kobe, Japan (1985), p.
`108
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`
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`
`A. Tsukune, N. Nishimura, et al; Proc. Electrochem. Soc. Mtg. San Diego
`
`
`
`(1986), p. 580
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`R. S. Martin, E. P. van de Ven, C. P. Lee; Proc. IEEE VLSI Multilevel
`
`
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`Interconnect Conf. (1988), p. 286
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`H. Hey, B. Sluijk, D. Hemmes; Solid State Techn. (April 1980), p. 139
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`Equipment Frontiers; Solid State Techn. (Oct. 1987), p. 49
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`W. D. Parlow, B. C. Samuels; J. Electrochem. Soc. V. 134 (July 1987) p.
`
`1740
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`A. K. Sinha and E. Lugujjo; Appl. Phys. Lett. 3_4 (Feb. 1987), p. 245
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`[I]
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`F. Moghadam and R. Shukla; Proc. Electrochem. Soc. Mtg. (May 1990), p.
`
`280
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`
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`7A UN EXHIBIT 01 '
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`of8
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`S
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`J.K. Chu, S. Sachdev, P. K. Gargini; Proc. Electrochem. Soc. Mtg. (Oct.
`
`
`
`1983), p. 510
`
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`D. Smith, A. Alimonda, et a1.; J. Electrochem. Soc. V. 137 (Feb. 1990), p.
`
`614
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`B. L. Chin, E. P. van de Ven; Solid State Techn. (April 1988), p. 109
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`[l3]
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`[14]
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`[15]
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