Pressure Distribution Control on Surface Conformable Polishing in Chemical Mechanical Planarization

An original polishing mechanism with a triple layer pad was developed as a surface conformable polishing mechanism in CMP (Chemical Mechanical Planarization). According to the polishing mechanism, removal proﬁle can be controlled by base plate curvature through an assist pad made of elastic material. A pad system supported by an envelope plane composed of ﬁve wafers contributestomakeawaferlevelparalleledtoapadlevelrelativelyduringpolishing.Astheresult,astaticpressuredistributioncanbetransferredpreciselytoadynamicpolishingproﬁleregardlessofstrongshearingforcebypolishing.Inordertoverifythepolishingremovalcontrollability,anoriginalpressuredistributionanalysiswasdevelopedbyanactualpressuredistributionmeasurementaroundanentirepadsurface.Theanalyzedpressuredistributionproﬁlewasingoodagreementwiththeactualpolishingproﬁle.Fromtheresultthatthestaticpressureproﬁlecanbetransferredtothepolishingproﬁleprecisely,itwasveriﬁedthatthepolishingmechanismisreasonableforsurfaceconformablepolishingsysteminCMP.©TheAuthor(s)2015.PublishedbyECS.ThisisanopenaccessarticledistributedunderthetermsoftheCreativeCommonsAttribution4.0License(CCBY,http://creativecommons.org/licenses/by/4.0/),whichpermitsunrestrictedreuseoftheworkinany

CMP (Chemical Mechanical Planarization) has become a key process in device planarization process. Both local planarizing polish and global uniform polish are required in CMP. In particular, patterned wafer subject to CMP process has inherent warp or inherent thickness variation. Therefore, uniform removal across a wafer has been needed with conforming to surface height variation as it is. For uniform removal across a wafer, entire uniform pressure should be applied to a wafer surface regardless of the inherent warp or thickness variation of a wafer.
In a traditional polishing system, a polishing mechanism using wafer back side pressure was widely used as a uniform polishing system used in CMP process. However, it is difficult for the wafer back side pressure mechanism to control polishing pressure uniformly across a wafer. 1 Fig. 1 shows a principle comparison of polishing pressure between a traditional mechanism and developed one about a surface conformable polishing represented by CMP. A neutral plane condition about a wafer is indicated below the pressure principle diagram. The traditional mechanism is shown in a left side. A uniform pressure is applied by a membrane sheet from a wafer back side. The wafer is put in position between the back side membrane sheet and polishing pad surface during polishing. As you know, both the membrane sheet and the polishing pad are composed of elastic materials. This means that a reference base plane for polishing cannot be defined during polishing. A wafer surface can be curved easily under down force for polishing. Accordingly, no matter how uniformly the polishing down force is provided to a wafer, the interface pressure between a wafer and a pad never lead to uniform distribution. 1 It is quite difficult to control polishing profile in the traditional mechanism. The pressure distribution within a wafer depends on wafer stiffness also. Therefore, the wafer stiffness is not negligible for the polishing pressure distribution formation. 2 In particular, in a case of hard material wafer eg SiC or sapphire, the influence of wafer stiffness becomes remarkable. In addition, the wafer stiffness causes excessive removal called 'edge effect' at wafer edge under uniform pressure from wafer backside. 3 Furthermore, each wafer has some extent of inherent curvature. 4 Therefore, the pressure controlled wafer by wafer is needed to compensate for the inherent wafer curvature in the traditional mechanism. To solve the traditional potential problems, a developed mechanism is shown in right side. The wafer is sucked by a flat wafer chuck. Therefore, a dynamic neutral plane of a wafer is fixed with the curz E-mail: Fujitata@accretech.jp vature eliminated during polishing. Under a condition that reference surface is fixed, an original composition of a pad provides controlled pressure to a wafer surface. In the former report, it was demonstrated that developed polishing mechanism was satisfied with both local planarizing polish and global uniform polish by making use of bending stiffness of surface layer pad regardless of wafer curvature. 5 In this report, the developed polishing mechanism was described on the purpose of removal profile control. Pressure distribution is controlled by curvature height of a base plate which is laid under the triple layer pad. Then, an original pressure distribution analysis was developed to guide pressure integration profile from an actual pressure measurement. The pressure integration profile estimated by base plate curvature was compared with the actual removal profile of a wafer by polish. As the result, it was demonstrated that the adjusted pressure profile under a static condition is in good agreement with an actual removal profile by polish.    wafer chuck. Wafer curvature such as warp or bow is eliminated by the sucking force by vacuum. Under the condition, a long range of height variation of the wafer surface corresponds to wafer thickness variation. Uniform removal polishing conforming to the height variation is required under the condition. In addition, by sucking a wafer, a wafer position is fixed at both regular distance and height in a retainer ring, even if the wafer is under shearing force during polishing process.

Configuration of Polishing Mechanism
To fix wafer position in the retainer rigidly leads to controlling wafer edge polishing precisely.

Base plate
Assist pad Wafer Pressure distribution Surface pad Base plane  Fig. 3 shows a pad composition in this system. Compared with normal polishing configuration, this system applies for a wafer face up configuration. The pad is composed of triple layers. 5 Table I shows a physical property of the pad material. First layer is composed of a surface pad. The surface pad is constituted by a normal polishing pad with hydrophilic property such as IC1000 made of porous polyurethane. The first layer pad plays a role to hold some extent of slurry and to polish a wafer surface. Second layer is composed of a support film. The support film is constituted by a polyethylene terephthalate film for commercial use which is an extension proof material under wet condition. The film is pasted entirely on the back side of the surface pad. The support film plays a role to provide stiffness to the surface pad. Therefore, the stiffness of the support film fulfills a local planarization function against fine protruding parts on a patterned wafer. Third layer is composed of an assist pad. The assist pad is constituted by a segmented rubber material which features a property of stable pressure distribution without deformation under repeated stress condition. The assist pad is laid under both first layer of the surface pad and second layer of the support film. The assist pad plays a role to provide a stable pressure distribution to a wafer regardless of wafer inherent thickness variation, thermal deformation of platen or pad thickness variation dependent on the error of pad pasting operation. It is possible for the surface pad to conform to a long range height variation on wafer surface. Fig. 4 shows a base plate curvature for pressure distribution control. The base plate with a small curvature is laid under the assist pad. The base plate plays a role to control polishing pressure distribution to a wafer. The adjustment of small curvature height of the base plate contributes to control the polishing pressure distribution to a wafer. Then, the assist pad plays a role to relieve the curvature height of base plate also. Accordingly, controlled uniform polishing pressure can be applied to a wafer surface with absorbing wafer height variation. In the pad configuration, a pressure distribution applied to the wafer is not affected by the flatness deterioration attributed to a thermal deformation of the platen because the assist pad absorbs the thermal deformation. The curvature profile of the base plate provides a good controllability on a pressure distribution applied to a wafer. As the result, the wafer surface can be polished uniformly as a surface conformable polishing mechanism regardless of the inherent wafer curvature.    5 shows a comparison of relative attitude between a pad and a wafer. In a typical traditional polishing mechanism, polishing head leveling was flexible enough to conform to pad surface by using a gimbal joint mechanism. 6 In some reports, the wafer level change is suggested under a lubrication condition on polishing. 7-8 However, the wafer level fluctuation means a pressure distribution fluctuation from a pad under a dynamic condition. If the wafer level fluctuation is accepted positively under shearing force from a pad, the pressure distribution to a wafer has much influence on various tribological factors from a pad, eg. pad dressing condition, slurry lubrication or pad speed. Under the condition, even if pressure distribution was adjusted carefully under a static condition, the wafer level fluctuation makes the adjusted pressure distribution spoiled. Therefore, in this development, the relative level between a wafer surface and a pad surface was focused on securing a stable pressure distribution during polishing. In order to maintain the pressure distribution during polishing, a wafer level must be paralleled to a pad level consistently even if a strong shearing force works wafer surface by polishing. The consistent parallel level between the pad and the wafer should result in a good correlation between a static pressure profile and a dynamic removal profile. Fig. 6a shows a overview of the polishing machine configuration. Fig. 6b shows a cross sectional view of the polishing machine mechanism. The machine has five heads to hold wafers. Platen with the pad is positioned with the face down. Five wafers are positioned with the faces up. The five wafers move with a planetary gear. In this polishing mechanism, each wafer chuck keeps horizontal leveling by thrust bearing with high stiffness. Relative leveling precision among the five wafer chucks is within 5 microns. The pad system including the platen is supported by an envelope plane consisted of five wafers. According to the polishing mechanism, no matter how strongly the shearing force on the wafer works by polishing, the relationship between the wafer surface and the pad surface is kept to be parallel without tilting the wafer surface. Because the pad system including the platen with a large inertia is supported by a constant envelope plane formed by five wafers chucked rigidly. Therefore, the pressure distribution between the wafer and the pad is constant during polishing.
Next, a pad attachment mechanism is a critical issue to reproduce pressure distribution between a wafer and a pad. In the traditional system, a polishing pad used to be attached to a platen with an adhesive tape. However, it is difficult to attach a polishing pad on a platen uniformly without any non-uniform undulation. Fig. 7 shows a pad attachment mechanism. The surface polishing pad is tightened on an assist pad by lifting an outer ring supporting the pad. Then, the surface polishing pad conforms to a surface profile of the assist pad. In the method, adhesive tape is not necessary on attaching the pad to    base plate which make a pressure distribution. Therefore, the polishing pad attachment mechanism leads to uniform pressure distribution reproducibility to a wafer.

Pressure Distribution Analysis
A correlation between a pressure distribution profile and an actual removal profile by polish is mentioned using an original pressure distribution analysis. At first, the method to transform curvature height of base plate into pressure distribution applied to a wafer is indicated by an original pressure distribution analysis. The pressure distribution analysis was based on an actual pressure distribution measurement. Fig. 8 shows a schematic diagram of pressure distribution measurement. The pressure distribution is measured by pressure distribution measurement system using a ductile sensor sheet which is used as C-Scan model made by Nitta corp. The sensor sheet includes thin conductive ink coating lines of both row ones and column ones. The principle to measure the pressure distribution makes use of resistance variation at the intersection between row line and column line when down force is applied to the intersection point. As the actual pressure distribution measurement, the sensor sheet is placed on a wafer. The sensor sheet on the wafer is pressed down by a polishing pad with actual static load. Then actual pressure distribution can be measured by the sensor sheet with 240 mm × 240 mm size. Fig. 9 shows an example of pressure distribution measurement. The sensor sheet can measure pressure distribution with approximate 10 mm pitch resolution within 240 mm square. Therefore, one measurement area corresponds to a wafer area approximately. Pressure distribu-Y X 240mm 240mm Figure 9. Example of pressure distribution measurement. tion map at entire pad area can be obtained by repeating pressure distribution measurement at various positions. Fig. 10 shows a pressure distribution allocation on an entire pad surface. Each pressure distribution map can be measured at 45 degree intervals on the pad surface. A pressure distribution at an entire pad area can be covered at eight positions around pad surface. Entire pressure distribution at all area of the pad is formed by the pressure distribution result at eight positions. Fig. 11 shows an example of a pressure histogram of the concentric pressure band across an entire pad. The entire pressure distribution of the pad surface is divided into twelve concentric pressure bands at same intervals. Each pressure value within each concentric pressure band is averaged from inner radius to outer one around a pad. Then, the pressure histogram is defined by pressure arrays of 12 bands within the area which ranged from 104 mm to 320 mm in radius.
Next, the pressure histogram of concentric pressure bands across a pad is transformed into a pressure integration profile as a function of a wafer radius. Fig. 12 shows a method to transform the pressure histogram into pressure integration profile on a wafer. A circle with a certain radius r k within a wafer is focused on. The pressure 240x240 Measured pressure distribution accumulation to a certain circle on the wafer can be found by pressure integration from concentric pressure band around a pad. The average pressure of the pressure band with number i is defined as p i . A part of the circle with the radius r k traverses the pressure band p i . The angle δ i,k of the circle which traverses pressure band p i is given by, where θ i,k is given by, where, r k : the radius of a certain circle within a wafer c i : the inner radius of pressure band with number i on the pad, R: the distance between the center of a pad and the center of a wafer About the circle with the radius r k , the ratio of the angle traversing pressure band p i to a round angle is given by 2δ i,k /2π. Pressure applied to the circle from pressure band p i is given by p i δ i,k /π. Therefore, the integration pressure to receive from all pressure bands p i (i = 1, 2, . . . m) is given by, From the use of both Eq. 1 and Eq. 2, Eq. 3 can be transformed as mentioned below, where, P k (r k ) : pressure integration applied to the circle with radius r k The pressure integration P k (r k ) is a function of wafer radius r k . Therefore, the pressure integration can be calculated at each radius r k (k = 1, 2, . . . , n) which ranges from zero to outermost radius. As the result, pressure integration profile can be obtained by pressure integration P k (r k ) as a function of wafer radius r k . In addition, the non-uniformity on pressure integration P NU is defined by, whereP is given by,P Based on a method aforementioned above, pressure integration profile was introduced to compare with actual removal profile.

Correlation Between Pressure Integration Profile and Actual
Removal Profile by Polish Fig. 13 shows actual curvature profile of base plates of both 0 μm convex and 400 μm convex. Both of base plate has a property of symmetry about a wafer center. The base plate profile is a gentle curvature within wafer diameter. The gentle curvature profile contributes to control pressure distribution to a wafer. Fig. 14   pressure histogram using a base plate of both 0 μm convex and 400 μm convex. In a case of 0 μm convex base plate, a pressure distribution became uniform across a wafer. On the other hand, in a case of using the 400 μm convex base plate, the pressure value at the area of a wafer center was relatively higher than the pressure at a peripheral area. A gentle pressure distribution was formed by a gentle base plate curvature profile. In the case, the assist pad served as a curvature absorber of the base plate of 400 μm convex curvature. In the former report, it was demonstrated that pressure distributions within wafer are coincided within 2 kPa, using the total thickness variation of either 9.6 μm convex or 15 μm concave. 5 Either wafer thickness variation or platen deformation dependent on thermal condition is relatively small enough to be negligible, compared with the convex curvature of 400 μm of the base plate. Therefore, the base plate curvature and the assist pad have a good controllability on forming a gentle pressure distribution to a wafer. Fig. 15 shows a pressure integration profile as a function of wafer radius. The pressure integration profile was calculated by the procedure shown in Fig. 12. The base plate of 400 μm convex made wafer center pressure higher relatively than peripheral part pressure.
In the other words, pressure integration profile became a gentle convex profile as well as the base plate profile. The pressure non-uniformity became 9.67% across a wafer. On the other hand, the base plate of 0 μm convex made pressure integration profile uniform across a wafer. The pressure non-uniformity became 2%. In this way, the base plate profile corresponded to the pressure integration profile. Next, a correlation between pressure integration profile and actual polishing profile was evaluated using the base plates of both 0 μm convex and 400 μm convex. Table II shows experimental condition. As a slurry type, diluted fumed silica slurry was used. An oxide film on 8 inch Si wafer was used for the evaluation. For removal evaluation, film thickness was measured at 49 points within a wafer. Fig. 16 shows an actual removal profile as a function of wafer radius using the base plate of both 0 μm convex and 400 μm convex. The actual removal profile is in good agreement with the pressure integration profile  Pressure integration non-uniformity % Removal non-uniformity % Center fast Center slow Figure 17. Correlation between pressure integration non-uniformity and actual removal non-uniformity. mentioned above. The result supports that the polishing mechanism was reasonable for pressure distribution control. In addition, the polishing profile speculation was accurate in the polishing mechanism. Fig. 17 shows correlation between pressure integration non-uniformity and actual removal non-uniformity. The horizontal axis indicates pressure integration non-uniformity and the vertical axis indicates actual removal non-uniformity. As well as pressure non-uniformity, if each actual removal is regarded as R k within a wafer, the removal nonuniformity was given by, whereR is given by,R = 1 n n k=1 R k [8] According to the evaluation between pressure non-uniformity and removal non-uniformity, it was verified that the pressure integration non-uniformity and the actual removal non-uniformity have a strong correlation with a correlation coefficient of 0.99. The result supports that polishing pressure is a dominant factor to determine removal rate strongly. On the assumption that the pressure integration nonuniformity is regarded as X and the removal non-uniformity is regarded as Y, the relationship is given by, [9] In this way, it was verified that pressure non-uniformity is in good correlation with actual removal non-uniformity.

Discussion
In the developed system, the pressure integration profile was a good agreement with the actual removal profile precisely. The pressure profile to a wafer can be controlled by a base plate curvature. Then, in accordance with the pressure distribution control, removal profile could be controlled precisely.
On the other hand, it will be quite difficult for the traditional polishing mechanism using a gimbal head to get such a good agreement between pressure integration profile and actual removal profile. Because the wafer level is not constant to the pad surface. The wafer level is susceptive to small lubrication condition eg. pad conditioning, slurry type and pad surface asperity. 9-10 The wafer level fluctuation leads to pressure distribution fluctuation. That means that actual removal profile depends on not only static pressure distribution but also dynamic and lubrication condition. Therefore, actual removal profile does not always correspond to the static pressure integration in the traditional polishing mechanism in principle.
In contrast, in this system it was demonstrated that the static pressure integration profile corresponded to the actual removal profile accurately. The main causes are considered as follows.
-First, according to the original polishing mechanism, the pad surface is supported by envelope plane of five wafers. Therefore, the polishing mechanism could keep relative level attitude paralleled between a wafer and a pad surface regardless of strong shearing force by polishing. As the result, pressure distribution designed under static condition could be transferred into an actual removal profile accurately regardless of dynamic and lubrication condition fluctuation. -Second, a wafer is held rigidly by vacuum chuck. According to the wafer chuck, wafer inherent curvature is eliminated by the sucking force by vacuum. In addition, the wafer is not curved under down force for polishing. Therefore, a neutral plane of a wafer is maintained as a stable flat plane during polishing. A stable polishing process is performed regardless of wafer type, stiffness and inherent curvature. -Third, an original triple layer pad was introduced. First layer plays a role to polish a wafer. Second layer plays a role to provide extension proof of surface pad and some extent of bending stiffness for local planarization. Third layer plays a role to relieve base plate curvature and to form gentle pressure profile. The original pad composition provides a stable controlled pressure distribution to a wafer regardless of height variation of wafer surface, thermal deformation of platen or pad thickness variation dependent on pad pasting operation.
Consequently, a pressure distribution adjusted in static condition can be transferred to a dynamic polishing profile accurately.

1.
As an ideal surface conformable polishing mechanism, the original polishing mechanism including pad composition was developed for CMP process. The main features of the polishing mechanism are as follows.
Removal profile can be controlled by base plate curvature through an assist pad. The assist pad plays a role to relieve base plate curvature and forms gentle pressure profile.
A neutral plane of a wafer is maintained as a stable flat plane by vacuum chuck regardless of wafer type, stiffness and inherent curvature.
The pad including a platen is supported by an envelope plane formed by five wafers. The mechanism made wafer leveling paralleled to a pad surface under a condition of polishing shearing force. A pressure distribution adjusted under a static condition could be transferred into a dynamic polishing profile precisely. 2. In order to verify the polishing machine mechanism, an original pressure distribution analysis was developed. The analysis is based on an actual pressure distribution around an entire pad surface. A pressure integration profile in a proportion to a base plate curvature was introduced by the analysis. 3. It was clarified that the pressure integration profile is in good agreement with an actual polishing profile precisely with high correlation coefficient of 0.99. The result supported that machine mechanism was reasonable in terms of the correlation between a static pressure condition and dynamic polishing condition precisely.