第八章化學機械拋光

1、第八章化學機械拋光第1頁，共56頁。集成電路微細化的追求：(1)高集成度 (2)更快的器件運行速度其中一個主流技術是二維構造電流(多層化金屬布線)?；瘜W機械拋光的重要性。第2頁，共56頁。在追求構造微細化與多層布線的趨勢中，存在幾個典型的技術難題。(1)分辨率和焦深的折衷 (2) 表面平坦化 減小臺階，提高鍍膜效果。2,kkRNANA第3頁，共56頁。 表面平坦化的問題第4頁，共56頁。舉例：用0.25 mm工藝制作256 MB DRAM，在運用兩層鋁金屬導線的情況下，在存儲單元及外圍電路部分，高臺(存儲單元)和低地(外圍電路)兩者的高度將相差約1 mm，這對曝光機的聚焦效果將造成影響。因此必

2、須表面平坦化。第5頁，共56頁。（3） 銅比鋁具有更佳的導電率，是亞微米線寬的電極布線的理想材料。 銅布線的等離子體刻蝕很難實現(xiàn)，且效果較差。 CMP技術是實現(xiàn)大馬士革結構銅布線層的關鍵工藝。第6頁，共56頁。Definition of Planarization Planarization is a process that removes the surface topologies, smoothes and flattens the surface The degree of planarization indicates the flatness and the smoothness

3、 of the surface第7頁，共56頁。Definition of Planarization第8頁，共56頁。Definition of Planarization第9頁，共56頁。Degrees of Planarization第10頁，共56頁。Planarization Smoothing and local planarization can be achieved by thermal flow or etchback. Global planarization is required for the feature size smaller than 0.35 mm, w

4、hich can only be achieved by CMP.第11頁，共56頁。Thermal Flow Dielectric planarization, PSG or BPSG High temperature 1000 Become soft and start to flow due to surface tension Smooth and local planarization第12頁，共56頁。As DepositedAfter PlanarizationProcess:第13頁，共56頁。Etch Back Reflow temperature is too high f

5、or some materials, such as aluminum Two type: Sputtering etch back Photoresist etch back第14頁，共56頁。Sputtering etch back Ar sputtering etchback chip off dielectric at the corner of the gap and taper the openings Subsequent CVD process easily fills the gap with a reasonable planarized surface Reactive

6、etchback process with CF4/O2 chemistry further planarized the surface第15頁，共56頁。CVD (I)Process:第16頁，共56頁。Sputtering Etch BackProcess:第17頁，共56頁。CVD (II)Process:第18頁，共56頁。Reactive EtchbackProcess:第19頁，共56頁。Photoresist Etchback Planarized solid thin film on wafer surface Plasma etch process with CF4/O2

7、chemistry Adusting CF4/O2 ratio allow to 1:1 of oxide to PR selectivity Oxide could be planarized after etchback第20頁，共56頁。After Oxide DepositedProcess:第21頁，共56頁。Photoresist Coating and BakingProcess:第22頁，共56頁。Photoresist EtchbackProcess:第23頁，共56頁。Necessity of CMP Photolithography resolution R=K1/NA

8、To improve resolution, NA or DOF=K2/2(NA)2, both approaches to improve resolution reduce DOF Here we assumed K1=K2, =248 nm (DUV), and NA=0.6, Resolution for 0.25 mm DOF =and 150 nm for 0.18 mm resolution208 nm第24頁，共56頁。Necessity of CMP 0.25 mm pattern require roughness 0.35 mm, other methods can be

9、 used第25頁，共56頁。Advantage of CMP Planarized surface allows higher resolution of photolithography process The planarized surface eliminates sidewall thinning because of poor PVD step coverage Uniform thin film deposition Reduce defect density, improve yield第26頁，共56頁。Disadvantage of CMP Introduce defec

10、ts of its own Need appropriate post-CMP cleaning第27頁，共56頁。Application of CMP STI formation Dielectric layer planarization Tungsten plug formation Copper interconnection (Damascene method)第28頁，共56頁。Application of CMP第29頁，共56頁。Copper Deposited第30頁，共56頁。CMP Copper and Tantalum第31頁，共56頁。Chemical Mechani

11、cal Polishing System Polishing pad, Wafer carrier, Slurry dispenser第32頁，共56頁。第33頁，共56頁。Orbital Polishing第34頁，共56頁。Wafer carrier第35頁，共56頁。Polishing Pad Porous, flexible polymer materials Pad directly affects quality of CMP Pad materials: durable, reproducible, compressible at process temperature Soft

12、 pad: better within wafer uniformity, poorer topographical selectivity Hard pad: high removal rate, longer conformality range Process requirement: high topography selectivity to achieve surface planarizationToothbrush第36頁，共56頁。Pad Conditioner Sweeps across the pad to increase the surface roughness r

13、equired by planarization Removes the used slurry第37頁，共56頁。CMP Slurries Chemicals in the slurry react with surface materials, form chemical compounds that can be removed by abrasive particles Particulate in slurry mechanically abrade the wafer surface and remove unwanted coating Slurry impact removal

14、 rate, selectivity, planarity and uniformityToothpaste第38頁，共56頁。Slurry Polishing Mechanism Different polishing process need different slurries: Oxide removal: alkaline solution with silica Metal removal: acidic solution with alumina (Al2O3)Silica Particulate第39頁，共56頁。Oxide Removal Mechanism OH- bond

15、s on both film and silica surface Form hydrogen bonds of silica and film surface Form molecular bonds of silica and film surface Mechanical removal of the particles bonds with wafer surface Tear away atoms from film or wafer surface第40頁，共56頁。Oxied CMP, Hydrogen Bond第41頁，共56頁。Oxide CMP, Molecule Bond

16、第42頁，共56頁。Oxide CMP, Removal of Oxide第43頁，共56頁。Metal Removal Mechanism Metal CMP process第44頁，共56頁。Example: Copper CMP Difficult to plasma etch copper Lack of volatile inorganic copper compounds Hydrogen generate easily cause explosion Copper CMP key process in copper metallization process H2O2 or HN

17、O4 can be used as oxidant Alumina particulate is used for a abrasion第45頁，共56頁。Copper CMP Cu was oxided into CuO, CuO2 or Cu(OH)2 by oxidant (H2O2,HNO4,NH4OH) Oxidation layer was removed by abrasion The oxidant continue to oxide new Cu layer. CuO2 is porous and cant form a passivation layer to stop f

18、urther copper oxidation Additive (NH3) is needed to enhance pasisivation to reduce wet etch effect第46頁，共56頁。Dishing effect第47頁，共56頁。Dishing effect Usually happens at a larger opening area Large metal pads More materials are removed from the centre Cross-section view looks like a dish It is related t

19、o the removal selectivity Copper CMP with high selectivity of Cu to USG can cause copper dishing effect第48頁，共56頁。CMP parameters Removal rate Uniformity Selectivity Defects第49頁，共56頁。Removal Rate Mechanical removal rate R was found by Preston The Preston equation can be expressed as 第50頁，共56頁。 Preston

20、 equation works very well for the bulk film polishing processes The protruding portions on a rough surface have higher polishing pressure Removal rate of protruding parts is higher This helps to remove surface topography and planarize the surface第51頁，共56頁。Uniformity Usually 49-points, 3 standard dev

21、iation as the definition of the uniformity for the CMP process qualifications Changes of the film thickness before and after CMP process is monitored For the production wafers, uniformity after CMP process is monitored Normally use 9 or 13 points measurement第52頁，共56頁。Selectivity Ratio of removal rat

22、es of different materials Affect CMP defects, such as erosion or dishing The slurry chemistry is the primary factor that affects removal selectivity of CMP STI oxide CMP require high oxide to nitride selectivity, from 100:1 to 300:1 Tungsten CMP, selectivity to oxide and TiN is very important第53頁，共56頁。 Slurry chem