fig3 illustrates a cross - sectional view of an embodiment of a wafer carrier assembly including a carrier ring comprising a retaining ring 22 and guard ring 24 , according to the present invention . wafer carrier head 13 is rotated and forced downwardly by wafer carrier support frame 13 a during a polishing process . a wafer , referred to as w , is arranged on a lower surface of wafer carrier head 13 . additionally , a resilient member ( e . g ., a backing film , not shown ) can be positioned between the lower surface of wafer carrier head 13 and the wafer w . especially , wafer carrier head 13 can be equipped with means for applying vacuum to the wafer , for the purpose of firmly holding the wafer . the present invention includes retaining ring 22 and guard ring 24 as a carrier ring . here , retaining ring 22 may function to fasten the wafer to wafer carrier head 13 , and it is attached on the lower surface of wafer carrier head 13 . retaining ring 22 retains the wafer w , or prevents the wafer from moving or departing outwardly , during the rotational motion of the wafer carrier head 13 . preferably , retaining ring 22 has a substantially annular form that can be adjustable to the periphery of the wafer . more preferably , an inner wall 22 a of retaining ring 22 may be arranged to be in contact with the periphery of the wafer , thus effectively preventing the departure of the wafer from wafer carrier head 13 . in addition , a bottom surface of retaining ring 22 is preferably in substantially the same plane as a polished surface of the wafer . in other words , it is preferable to minimize the clearance and / or the compression between the bottom surface of retaining ring 22 and polishing pad 12 . when the clearance is too large , the wafer may depart from wafer carrier head 13 through a gap between retaining ring 22 and polishing pad 12 . when the compression is too great , a ripple may form inside of the retaining ring 22 and overpolish en edge region or periphery of the wafer . thus , retaining ring 22 is preferably configured to have a thickness substantially equal to the thickness of the wafer and ( if present ) any resilient member or backing film on the wafer carrier head 13 . guard ring 24 is attached on the lower surface of wafer carrier head 13 , a predetermined distance from retaining ring 22 . in other words , the inside diameter of guard ring 24 is larger than the outside diameter of retaining ring 22 . the present invention includes guard ring 24 as another carrier ring , as well as retaining ring 22 for fastening the wafer . thus , two carrier rings ( i . e ., retaining ring 22 and guard ring 24 ) generally work together to prevent the periphery of the wafer from being overly polished due to the deformation of polishing pad 12 . the deformation of polishing pad 12 mostly occurs in the vicinity of guard ring 24 , particularly when guard ring 24 has a thickness such that its lower surface extends slightly beyond the bottom surface of retaining ring 22 . therefore , polishing pad 12 rarely deforms in the vicinity of retaining ring 22 and the wafer . more specifically , as shown in fig3 , when wafer carrier head 13 is forced down by wafer carrier support frame 13 a , guard ring 24 primarily presses down polishing pad 12 . as a result , the protrusion portion 12 a of polishing pad 12 , owing to the pad bounding effect or deformation of the pad , occurs intensively in the vicinity of guard ring 24 . in this case , the pad bounding phenomenon rarely occurs in the vicinity of retaining ring 22 and the wafer , thus the pressure working on the wafer becomes uniform over the periphery and the center of the wafer . therefore , the polishing rates according to the chemical and mechanical polishing actions can be maintained constantly over the entire surface of the wafer . meanwhile , a gap ( referred to as g ) between retaining ring 22 and guard ring 24 can be determined or controlled , based on the elasticity of polishing pad 12 , to minimize the pad bounding effect on the wafer . in the case where the polishing pad material has a large elasticity , a region where the pad bounding phenomenon occurs may be wide . in this case , the gap g is preferably relatively large for the pad bounding region not to reach the wafer . in addition , forming a width w 1 of guard ring 24 larger than a width w 2 of retaining ring 22 can minimize the extent of polishing pad &# 39 ; s deformation in the inner region of guard ring 24 . it is preferable to form a protrusion height h of guard ring 24 larger than or equal to a protrusion height h of retaining ring 22 , in order to concentrate the pad bounding phenomenon in the vicinity of guard ring 24 . in the case where the protrusion height h of retaining ring 22 is larger than the protrusion height h of guard ring 22 , the pad bounding phenomenon may occur in the vicinity of retaining ring 22 . on the other hand , if the protrusion height h of guard ring 24 is unacceptably larger than the protrusion height h of retaining ring 22 , the wafer may become out of close contact with polishing pad 12 . accordingly , it is preferable to appropriately control the protrusion heights of retaining ring 22 and guard ring 24 . here , the protrusion height h or h is defined as a height based on a top surface of the wafer or the lower surface of wafer carrier head 13 in contact with the top surface of the wafer , not a real or actual height of retaining ring 22 or guard ring 24 . in one embodiment , the difference in the protrusion heights of ( h − h ) retaining ring 22 and guard ring 24 is less than the height of pas protrusion 12 a under one or more given sets of polishing conditions ( e . g ., the downward force applied by wafer carrier head 13 , the elasticity and / or compressibility of polishing pad 12 , the width of gap g , the width [ s ] of retaining ring 22 and / or guard ring 24 , and / or the amount of and / or formulation of the slurry , etc .). the wafer carrier assembly , according to the present invention , includes a retaining ring for retaining a semiconductor wafer , and a guard ring as an additional carrier ring . the pad bounding phenomenon is generally concentrated in the vicinity of the guard ring . in other words , when the pad bounding phenomenon is prevented in the vicinity of the wafer , a pressure applied to the wafer can be maintained constantly and uniformly over an entire surface of the wafer to be polished . therefore , a cmp apparatus including a wafer carrier assembly according to the present invention can prevent a periphery or edge region of a wafer from being overly polished , because a polishing rate is uniform over an entire surface of a wafer . as a result , the yield and reliability of semiconductor devices can be improved ( in some cases , significantly so ). while the invention has been shown and described with reference to certain preferred embodiments thereof , it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims .