ADVANTAGES OF 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 low 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 énhanced 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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`SAMSUNG ET AL. EXHIBIT 1049
`Page 1 of 8
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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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`T
`HF GROUND
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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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`I95
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`SAMSUNG ET AL. EXHIBIT 1049
`Page 2 of 8
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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 rate, 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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`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 bombardment/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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`CONTROL OF FILM smess
`STRESS (X 10E9 D/CM2l
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`”""é'é§:l'.1i§’é"$5.°3E'i§f“¢’s"fi."§£‘§.é’éI?3E°'°’
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`THICKNESS ON METAL (Thousands)
`--1-
`-—
`-r
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`1
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`17
`16
`15
`14
`13
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`scmaeuuz THICKNESS (Thousand!)
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`'
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`6'0
`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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`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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`slopes between SiN, SiOxNY and SiO2 are the result of differences in atomic
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`SAMSUNG ET AL. EXHIBIT 1049
`Page 3 of 8
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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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`ETCH RATE (7:1 BOE)
`TEOS SlO2
`IMIN
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`I
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`2°
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`7°
`5°
`5°
`‘°
`3°
`rsncsm u= rowan
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`5°
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`W
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`SILICON NITRIDE
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`ZWOIA
`I 900
`1800
`1 700
`1800
`1500
`I 400
`1S00
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`1200
`'0
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`waWM|Nl
`300
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`200
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`I00
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`0
`10
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`20
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`SILICON NITRIDE
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`srasss Ix new menu
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`_,
`.
`° '5 M
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`age 0.: use 0.4 0.45 0.5 can us one
`TD‘|IL. HF POWER (W/GA!)
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`33111535 (x fllfl arena)
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`55! LF
`0.4 WIOM2
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`:12 3.3 3.4 3.5
`2.5 2.7 2,3 2.9 3.0 3.:
`nsrosanouwnssunstronn)
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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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`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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`SAMSUNG ET AL. EXHIBIT 1049
`Page 4 of 8
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`The change in film density is particularly important for TEOS S102.
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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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`PECVD TEOS oxide is gradually being introduced in ILD applications
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`because of the superior step coverage [12]. Silicon nitride and oxynitrides 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 [l4][l5].
`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 coverage.
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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 S502
`SIDEMALL STEP covsmxos (as)
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`T503 siog
`SIDEWALL STEP covsnncs (-1.)
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`90
`80
`70
`G0
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`40
`30
`20
`10
`
`
`
`
`
`
`
`
`
`20
`
`so
`
`0
`
`1
`
`
`
`0.7
`
`
`0.6
`
`
`
`0.9
`
`
`
`1.0
`
`
`
`1.1
`
`
`
`
`* SINGLEFREQUENCY
`
`0.6
`0.5
`
`
`
`ASPECT RATIO
`
`
`
`
`.0
`
`0.1
`
`
`
`0.2
`
`
`
`
`
`0.3
`
`
`
`0.4
`
`
`
`00
`
`5°
`5°
`‘°
`PERCENT LF POWER 0:.)
`
`7°
`
`5°
`
`9°
`
`‘W
`
`U DUALFREDUENCY
`
`
`Figure 5-
`
`
`
`
`
`
`
`
`
`TEOS Sioz Sid¢W3“ Step
`
`
`
`
`
`COVCYHSC 85 3 function 0f
`
`
`
`percent LF power.
`(Aspect
`
`
`
`ratio = 0.35)
`
`
`
`
`Figure 7.
`
`
`
`
`
`
`
`Sidewall step coverage for
`
`
`
`
`single and dual frequency
`
`
`
`
`
`deposited TEOS SiO2 as a
`
`
`
`
`function of aspect ratio.
`
`
`
`
`
`(Metal height = 0.8 um).
`
`SAMSUNG ET AL. EXHIBIT 1049
`Page 5 of 8
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`

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`Figure 6 shows the step coverage of TEOS SiO2 as a function of low
`
`
`
`
`
`
`
`
`
`
`
`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
`
`
`
`
`
`
`
`
`
`
`
`coverage of TEOS SiO2 deposited in a single frequency and dual frequency
`
`
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`
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`
`
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`mode as a function of aspect ratio. For silicon nitride, similar results have been
`
`
`
`
`
`
`
`
`
`
`obtained.
`In this case,
`the typical
`improvement
`in step coverage is
`
`
`
`
`approximately 10 to 12%.
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`
`
`
`
`FILM COMPOSITION
`
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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
`
`
`
`
`
`
`
`
`
`
`
`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
`
`
`
`
`
`
`
`
`
`
`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 %. This is achieved
`
`
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`
`
`
`
`
`
`
`by optimizing the gas composition rather
`than trying ‘to optimize the ion
`
`bombardment.
`
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`
`
`SILICON NITRIDE
`
`H CONTENT (‘hi
`
`
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`
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`
`
`50
`
`PERCENT LF POWER
`
`
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`
`
`Figure 8.
`
`
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`
`
`Changes in SiH and NH content
`
`
`
`
`
`
`in PECVD SiN as function of
`
`
`
`
`
`the amount of low frequency.
`
`
`
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`
`
`
`
`
`
`
`
`frequency can be used to improve
`In the case of doped oxides, dual
`
`
`
`
`
`
`
`
`
`oxidation of the hydrides and effectively eliminate Si-H bonds.
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`SAMSUNG ET AL. EXHIBIT 1049
`Page 6 of 8
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`

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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
`
`
`
`
`
`
`
`
`
`
`
`
`tensile stress will adsorb water and increase their SiOH content over time.
`
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`
`
`
`
`CONCLUSIONS
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`The use of dual frequency for PECVD of dielectrics provides increased
`
`
`
`
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`
`
`
`
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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
`
`
`
`
`
`
`
`
`
`
`substrates are subjected during deposition.
`Increasing the low frequency power
`
`
`
`
`
`
`
`
`
`
`
`
`the plasma potential and the amount of
`ions
`following the low
`increases
`
`
`
`
`
`
`
`
`
`
`
`frequency RF field (< lMHz).
`The resulting low energy ion implantation
`
`
`
`
`
`
`
`
`
`
`
`
`occurring during deposition causes a change in the intrinsic film stress from
`
`
`
`
`
`
`
`
`
`
`tensile to compressive,
`increases
`film density and improves
`the chemical
`
`
`
`
`
`
`
`
`
`reactions.
`In addition,
`the low energy bombardment enhances the surface
`
`
`
`
`
`
`
`
`
`
`
`
`mobility of adsorbed TEOS and Si(NH2)x thus improving the step coverage of
`
`
`
`
`
`
`
`
`
`
`TEOS SiO2, standard silicon nitride, UV transparent nitride and oxynitrides.
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`However,
`if the ion energy is too high,
`the step coverage deteriorates due to
`
`
`
`
`
`
`premature decomposition of the reactive species.
`
`
`
`REFERENCES
`
`
`
`[1]
`
`[2]
`[3]
`
`[4]
`[5]
`
`[6]
`
`[7]
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`C. Adams; Proc. Reduced Temp. Processes for VLSI, Electroc. Soc. (1986),
`
`
`p. Ill
`
`
`
`
`
`
`
`
`
`
`
`
`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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`
`
`
`J.T. Yue, W. P. Funsten, R. V. Taylor; Proc. 23rd IRPS (l985) abstr. 5.1
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`
`
`
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`
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`
`
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`
`
`F. C. Hsu and K. Y. Chiu, Proc. VLSI Symposium Kobe, Japan (I985), p.
`108
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`
`
`
`
`
`
`
`
`
`
`
`A. Tsukune, N. Nishimura, et al; Proc. Electrochem. Soc. Mtg. San Diego
`
`
`
`(1986), p. 580
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`
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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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`
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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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`
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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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`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 al.; J. Electrochem. Soc. V. 137 (Feb. 1990), p.
`
`614
`
`
`
`
`
`
`
`
`
`
`
`
`
`B. L. Chin, E. P. van de Ven; Solid State Techn. (April 1988), p. 109
`
`
`
`
`
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`
`[13]
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`[14]
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`[15]
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`SAMSUNG ET AL. EXHIBIT 1049
`Page 8 of 8