In this section, the detail fabrication process technology is discussed.
In the first step, on the RCA cleaned bare silicon wafer is deposited a 2.0µm thickness SiO2 layer by PECVD. The deposition time of the 2.0µm SiO2 layer with 90sccm N2O and 5sccm SiH4 at 350℃, 400mTorr chamber pressure and 10W RF power is about 29 minutes.
Then on the SiO2 layer is spin coated HMDS and a layer of thick photoresist
AZ4620 as the etching mask for the subsequent oxide RIE for anchors. The minimum thickness of photoresist for etching 2.0µm thick SiO2 in RIE is 2.0µm. In this process, the thickness of photoresist is about 7.0µm. The spread cycle is 1000 rpm for 5 seconds. The spin cycle is 4500 rpm for 25 seconds. The temperature of soft bake is 90 ℃ for 3 minutes. The expose time is 2 to 2.5 minutes with the filter.
AZ300 is used for AZ4620 development. The development time is 2 minutes, followed by a rinse in DI water. After exposing, developing and checking the pattern by an optical microscope, the hard bake is 120℃ for 10 minutes. If the pattern is not good, acetone is used for reworking.
Next, the etching rate of RIE of the SiO2 layer grown by SiH4 and N2O is about 1000 Å per 30 seconds with the recipe of SF6 flow rate = 30sccm, CHF3 flow rate = 10sccm, pressure (p) = 50mTorr and 100W RF power. It takes 10 minutes to etch the thickness of 2.0µm SiO2. In this step, wet etching by B.O.E. is another choice. But the undercut is a problem which makes the pattern larger and sidewall not perfect.
After the RIE, the photoresist is cleaned by H2SO4 : H2O2 =3 : 1.
Then the dimple mask is patterned by FH6400 with spread cycle speed = 1000 rpm, spin cycle speed = 4000 rpm, soft bake time = 1.5 minutes, exposure time with filter = 42 seconds, development time = 20 seconds and hard bake time = 6 minutes.
The thickness of photoresist is about 0.7µm. The 0.75µm dimples are etched by RIE for 3 minutes and 45 seconds with the same recipe as the anchor etch. After the etching step, the PR is removed by H2SO4 : H2O2 =3 : 1 .
After etching the anchors and dimples, the thickness of 2.0µm poly1 is deposited by LPCVD. Before deposition, the wafer is cleaned by H2SO4 : H2O2 =3 : 1 for 15 minutes and dipped in B.O.E. for 1 to 3 minutes. Dipping the wafer in B.O.E. can clean the unnecessary SiO2 layer that is produced during cleaning. But dipping in B.O.E. makes the anchor pattern larger. Because the poly is the structure layer, the
stress is an important issue in this layer. Here the low stress poly is deposited by LPCVD for 400 minutes with the recipe of SiH4 flow rate = 40sccm, temperature (T)
= 585 ℃, pressure (p) = 300mTorr and annealed at 1050℃ for 1 hour. To anneal all poly layers at the end also works.
After depositing poly1, AZ4620 is spin coated with spread cycle speed = 1000 rpm for 5 seconds, spin cycle speed = 6000 rpm for 25 seconds, soft bake time = 3 minutes, exposure with filter time = 2 minutes, development time = 2 minutes and hard bake time = 10 minutes. The thickness is about 5µm, and the minimum thickness of photoresist used to etch 2.0µm thick poly1 in RIE is 2.0µm.
Subsequently, the poly1 layer is etched by RIE for 8 minutes with the same recipe as etching the first SiO2 layer.
After poly RIE, the second 2.0µm thick SiO2 layer is deposited by PECVD and patterned by RIE with the same recipe as the first one.
Then the second 1.5µm thick poly layer is deposited by LPCVD for 300 minutes, annealed in furnace with N2 at 1050℃ for 1 hour, and then patterned by RIE for 6 minutes and 30 seconds with the same recipe as poly1. Before the deposition of the 1.5µm poly2, the wafer is cleaned and dipped in B.O.E. for 1 to 3 minutes.
Next, two kinds of residual stress layers are used to produce the force to flip up the plate of lens： Si3N4 and gold. The Si3N4 is deposited by LPCVD with the recipe of NH3 flow rate = 105sccm, SiHCl2 flow rate = 35sccm, temperature = 800℃, pressure = 140mTorr. It takes 35 minutes to deposit the thickness of 0.35µm Si3N4
film. Before depositing, the initial clean and dipping in B.O.E. is necessary. Then on the Si3N4 film is spin coated FH6400 with the recipe of spread cycle speed = 1000rpm for 5 seconds, spin cycle speed = 3000 rpm for 25 seconds, soft bake time
= 1 minute and 30 seconds, exposure time = 25 seconds, development time = 2 minutes. The nitride layer is then etched by RIE for 2 minutes with the same recipe
as etching SiO2 and poly.
The gold layer (3000Å gold with 100Å Cr as the adhesion layer) is deposited by E-beam evaporation and patterned by lift-off. First, the wafer is coated with AZ5214E. Its side wall is sloped at a reentrant angle, which allows the metal to be deposited on the surfaces of the wafer and the photoresist, but provides breaks in the continuity of the metal over the reentrant photoresist step. The photoresist and unwanted metal (atop the photoresist) are then removed in the acetone bath. The recipe and steps of spin coating AZ5214E is spread cycle speed = 1000 rpm, spin cycle speed = 3500 rpm, soft bake time = 1.5 minutes, the first exposure time = 6 seconds, hard bake time = 2 minutes, the second exposure time = 57 seconds and development time in AZ300 = 30 seconds. The thickness of photoresist is about 1.7µm. Before metal deposition, the initial clean and dipping in B.O.E. is needed, too.
Then the SU8 is spin coated with the recipe of spread cycle speed = 1000rpm for 5 seconds, spin cycle speed = 3000 rpm for 30 seconds, soft bake: step1. 65 ℃ for 1 minute and step2. 90 ℃ for 1 minute, exposure time = 5 seconds, post exposure bake time = 2 minutes in 90 ℃, development in SU8 developer = 1 minute, and hard bake
= 6 minutes in 200 ℃.
Finally, the sacrificial layer (SiO2) is etched by immersing the chips in a 49% HF or B.O.E. solution, and then the structure is released.