the drawings as referred to throughout the description of the present invention are for illustration only , but not drawn according to actual scale . referring to fig1 a for the first embodiment of the present invention , a zero - layer wafer substrate 11 is provided , which for example can be a silicon wafer so that the process is compatible with a standard cmos process . next , transistor devices can be formed by standard cmos process steps as required ( not shown ), followed by deposition , lithography and etch steps to form interconnection including a contact layer 12 a , a first metal layer 13 a , a first via layer 12 b , a second metal layer 13 b , and a second via layer 12 c . in one embodiment , the contact layer and the via layers can be made of tungsten ; the metal layers can be made of aluminum ; and the dielectric layer can be made of oxides such as silicon dioxide . other conductive or dielectric materials can be used to replace what are suggested above , and the structure can include more or less number of metal layers . the pattern of each of the layers 12 a - 12 c and 13 a - 13 b is thus that a sacrificial region 14 a and an isolation region 14 b are formed therein . the detailed pattern of each layer is not shown for simplicity of the drawings . the regions 14 a and 14 b may be made of the same or different materials ; in this embodiment they are both made of an oxide , such as silicon dioxide . the sacrificial region 14 a is provided so that the suspension structure of the mems device to be manufactured is separated from the silicon substrate ( also referring to fig1 d ). the isolation region 14 b is provided to isolate the mems device region 100 from the other circuit region 200 ; its cross sectional pattern need not be exactly as shown but can be of any other shape . referring to fig1 b , next via layers 12 d - 12 f and metal layers 13 c - 13 f are deposited to form the mems structure . the number of the via layers and metal layers can be modified ; what is shown is only one example . a sacrificial region 14 c is formed to separate the suspension structure of the mems device from another part of the mems device . according to the design of the mems device structure , in this embodiment , a sacrificial region 14 d is also formed to separate an upper structure of the mems device from a lower structure thereof ( also referring to fig1 c ). this sacrificial region 14 d can be omitted if the suspension structure of the mems device is one integrated body . the sacrificial regions 14 c and 14 d can be made of oxides such as silicon dioxide . the sacrificial regions 14 a , 14 c and 14 d and the isolation region 14 b can be formed concurrently with the patterns of the via layers and the metal layers . referring to fig1 , in one embodiment , a mask 16 is formed and patterned ; and an etch step is performed to remove the oxides in the sacrificial regions 14 a , 14 c and 14 d ( for better identification of these sacrificial regions , the sacrificial region 14 c is also referred to as the first sacrificial region , the sacrificial region 14 d is also referred to as the second sacrificial region , and the sacrificial region 14 a is also referred to as the third sacrificial region ). the mask 16 shields the regions other than the sacrificial regions to protect those other regions from etching . the etch for example can be hf ( hydrogen fluoride ) vapor etch , or boe ( buffered oxide etch ) by immersing the whole wafer in an acid tank . the mask 16 for example can be a photoresist layer patterned by lithography , or made of other single or composite materials , such as metal or amorphous silicon . since a purpose of the mask 16 is to protect the other circuit region 200 , a passivation layer which is often used in the circuit region may be used as the mask 16 . the passivation layer may be a nitride layer or a bi - layer structure including an oxide layer and a nitride layer . referring to fig1 d , after the sacrificial regions are removed , a surface micromachining step is taken to etch the zero - layer silicon substrate from its top side . the etch step for example is an isotropic etch by xef2 gas . if the mask 16 is a photoresist layer which has not yet been totally consumed , it can be removed after the step of fig1 d . if the mask 16 is a layer with some other function , such as the passivation layer described above , then the process stops at the step of fig1 d , and a desired mems device is obtained . now referring to fig2 a for the second embodiment of the present invention , a zero - layer wafer substrate 11 is provided , which for example can be a silicon wafer so that the process is compatible with a standard cmos process . next , transistor devices can be formed by standard cmos process steps as required ( not shown ), followed by deposition , lithography and etch steps to form a contact layer 12 a , via layers 12 b - 12 f , and metal layers 13 a - 13 f . similar to the previous embodiment , the contact layer and the via layers can be made of tungsten ; the metal layers can be made of aluminum ; and the dielectric layer can be made of oxides such as silicon dioxide . the suspension structure of the mems device in this embodiment is different from that of the previous embodiment , so it is not necessary to form a sacrificial region 14 a ( but can be formed if desired ); however an isolation region 14 b is preferably formed to isolate the mems device region 100 from the other circuit region 200 . a sacrificial region 14 c is also formed depending on the design of the mems device . the isolation region 14 b and the sacrificial region 14 c can be made of oxides such as silicon dioxide . the sacrificial region 14 d in the previous embodiment is not shown in the figure , but it can be provided if required . next referring to fig2 b , to form the desired mems structure , an anisotropic etch step is taken from the back side of the substrate , forming a recess region 15 at the back side of the substrate to reduce the thickness of the substrate in this region . the etch step for example can be an icp ( inductively coupled plasma ) etch . this step is part of the bulk micromachining process . alternatively , this step can be taken after the step of fig2 c . referring to fig2 c , in one embodiment , a mask 16 is formed and patterned ; and an etch step is performed to remove the oxide in the sacrificial region 14 c . the mask 16 shields the regions other than the first sacrificial region to protect those other regions from etching . the etch for example can be hf ( hydrogen fluoride ) vapor etch , or boe ( buffered oxide etch ) by immersing the whole wafer in an acid tank . the mask 16 for example can be a photoresist layer , or other single or composite material layers , such as metal layer , amorphous silicon layer , or passivation layer which is often used in the circuit region . the passivation layer may be a nitride layer or a bi - layer structure including an oxide layer and a nitride layer . referring to fig2 d , after the sacrificial region 14 c is removed , an etch step is taken to etch the zero - layer silicon substrate , by icp etch for example , to complete the mems device . similar to the previous embodiment , if the mask 16 is a photoresist layer which has not yet been totally consumed , it can be removed . if the mask 16 is a layer with some other function , it can be kept . this embodiment is different from the previous embodiment in that the suspension structure 101 of the mems device includes the silicon substrate . it can be readily understood by those skilled in this art from the foregoing description of the embodiments that the front end process of the first and second embodiments are identical ( fig1 a - 1b and fig2 a , and even fig1 and fig2 b ), except the layout may be different . thus , according to the present invention , a wafer may be processed by the same steps to form a basic structure , and thereafter subject to surface and bulk micromachining processes at different locations on the same wafer . such flexibility does not exist in prior art , and therefore the present invention is better . although the present invention has been described in detail with reference to certain preferred embodiments thereof , the description is for illustrative purpose and not for limiting the scope of the invention . for example , the materials , number of metal layers , etch details , and so on , can be modified without departing from the spirit of the present invention . one skilled in this art can readily think of other modifications and variations in light of the teaching by the present invention . in view of the foregoing , it is intended that the present invention cover all such modifications and variations , which should interpreted to fall within the scope of the following claims and their equivalents .