General Semiconductor Packaging Process Flow Wafer Backgrinding, Die Preparation, Die Attach, Wire Bonding, Die Overcoat, Molding, Sealing, Marking, DTFS, Lead Finish, Electrical Testing, Tape & Reel, Dry Packing, Boxing and Labeling. DMT 243 – Chapter 3 M.Nuzaihan

General Semiconductor Packaging Process Flow Wafer Backgrinding Is the process of grinding the backside of the wafer to the correct wafer thickness prior to assembly. Also referred to as 'wafer thinning.' Wafer back grinding has not always been necessary, but the drive to make packages thinner and thinner has made it indispensable. Most package types in the semiconductor industry today would require a wafer thickness ranging from 8 mils (~200 µm) to 20 mils (~500 µm). Stacked die technology would require less than 100 µm. Improves the transfer of heat from the front device side of the wafer to the backside, thus help to improve reliability. DMT 243 – Chapter 3 M.Nuzaihan

General Semiconductor Packaging Process Flow Wafer Backgrinding There are several methods that are presently being used for thinning wafers, the most popular being the well-established mechanical backgrinding and polishing technique. This process is preferred in many cases because it is faster and less costly than the newer chemical or plasma etching processes that have been recently developed. However, it does have the disadvantages of applying mechanical stress and heat during the grinding process and of causing scratches on the backside of the wafer. These scratch patterns and the depth of the scratches on the surface of the wafer are directly proportional to the size of the grit and the pressure exerted on the wafer during the grinding process. The depth of the scratches and the backside surface roughness of the semiconductor die have a direct correlation to the strength of the die, so it is critical that the finished backside surface of the wafer be as smooth (or polished) as possible. DMT 243 – Chapter 3 M.Nuzaihan

The Backgrinding Process General Semiconductor Packaging Process Flow The Backgrinding Process Surface lamination Coarse Grinding Fine Wafer Cleaning de-laminate Three equipment designs for coarse and fine grinding of multiple wafers. DMT 243 – Chapter 3 M.Nuzaihan

The Backgrinding Process General Semiconductor Packaging Process Flow The Backgrinding Process Wafers normally undergo a cleaning and surface lamination process prior to the actual backgrinding process. Surface lamination involves the application of a protective tape over the surface of the wafer to protect it from mechanical damage and contamination during backgrinding. Backside grinding of the wafer using a two-step process – coarse grinding followed by fine grinding. During coarse grinding, typically 90% of the back grind is completed, significantly reducing the thickness of the wafer. Coarse grinding will cause microcracks and damage the silicon lattice. Fine grinding completes the back grind process and removes part of this damage, but still leaves some silicon flaws behind. To remove debris from the wafer while backgrinding, the wafer is usually washed continuously with D/I water while undergoing backgrinding. Lastly, the protective tape on the surface is removed (delaminate). DMT 243 – Chapter 3 M.Nuzaihan

General Semiconductor Packaging Process Flow Important Parameters Spindle speed - the rotational speed of the grinding wheel Spindle coolant water temperature and flow rate D/I water temperature Initial and final wafer thickness Feed speeds - the feed rate of the grinding wheel toward the wafer. DMT 243 – Chapter 3 M.Nuzaihan

General Semiconductor Packaging Process Flow Process Parameters Silicon wafers backgrinded with a) 2000 grit and b) 1200 grit grinding wheels (30X). Average roughness Die strength Wafer breakage DMT 243 – Chapter 3 M.Nuzaihan

Wafer Backgrinding Failures General Semiconductor Packaging Process Flow Wafer Backgrinding Failures Scratches After backgrinding, the wafer will exhibit a scratch pattern on the backside. The depth of these scratches will depend on the size of grit of the wheel and the amount of vertical pressure applied during grinding (A finer grit results in smaller and shallower scratches.) Die scratches DMT 243 – Chapter 3 M.Nuzaihan

Die scratches (optical) General Semiconductor Packaging Process Flow Wafer Backgrinding Failures Die scratches (optical) Die scratches (optical) Die scratches (SEM) DMT 243 – Chapter 3 M.Nuzaihan

Other Approaches in Wafer Thinning General Semiconductor Packaging Process Flow Other Approaches in Wafer Thinning (Dicing before grinding) Process Step: (Mechanical Polishing) The wafer is cut in half (dicing saw) The tape is laminate the surface The backside of the wafer is grinding The wafer is then mounted on the wafer mounting The laminated surface is peeled DMT 243 – Chapter 3 M.Nuzaihan

DMT 243 – Chapter 3 M.Nuzaihan

Other Approaches in Wafer Thinning General Semiconductor Packaging Process Flow Other Approaches in Wafer Thinning Atmospheric Downstream Plasma (ADP) Carbon tetrafluoride (CF4) Dry etch process that removes material isotropically (same is all direction) from the Si wafer surface. The process uses a combination of argon (Ar) and carbon tetrafluoride (CF4) plasmas. The thinning mechanism occurs when radicals of the reactive gas, produced in the plasma, causes chemical reactions on the Si wafer surface. This process can selectively remove silicon from the wafer backside. It has highest throughput but highest capital investment as well. DMT 243 – Chapter 3 M.Nuzaihan

Other Approaches in Wafer Thinning General Semiconductor Packaging Process Flow Other Approaches in Wafer Thinning Wet chemical etching A common technique to remove material stress free. To thin a whole wafer, a spin-spray etching mechanism has been developed. The process involves spraying an etching agent onto the surface of a rotating wafer periodically. Different chemical compositions of etching agents can give a wide variety of surface finishes, from a mirror-like surface finish to a very rough surface finish. The most common isotropic etching agent used for thinning Si wafers is a mixture of hydrofluoric acid (HF) and nitric acid (HNO3). It has very fast etching rate in silicon, unfortunately 20 um thick silicon has to be removed in order to reach good thickness variation performance. So the throughput is not the highest among these three. DMT 243 – Chapter 3 M.Nuzaihan

Other Approaches in Wafer Thinning General Semiconductor Packaging Process Flow Other Approaches in Wafer Thinning Polishing Types BG Plasma Etching Chemical Etching Machine Cost Good No Good Running Cost Throughput Fair Extra Environment Facility None Need DMT 243 – Chapter 3 M.Nuzaihan

Other Approaches in Wafer Thinning General Semiconductor Packaging Process Flow Other Approaches in Wafer Thinning DMT 243 – Chapter 3 M.Nuzaihan

General Semiconductor Packaging Process Flow Die Preparation Die preparation is the process by which the wafer is singulated into individual dice in preparation for assembly. Die preparation consists of two major steps, namely, wafer mounting and wafer saw, (other than wafer saw, people also use diamond scriber/Die cutting/Dicing) DMT 243 – Chapter 3 M.Nuzaihan

General Semiconductor Packaging Process Flow Die Preparation Wafer mounting is the process of providing support to the wafer to facilitate the processing of the wafer from Wafer Saw through Die Attach. Wafer mounting is performed right before the wafer is cut into separate dice. DMT 243 – Chapter 3 M.Nuzaihan

General Semiconductor Packaging Process Flow Die Preparation During wafer mounting The wafer and a wafer frame are simultaneously attached on a wafer or dicing tape. The wafer frame may be made of plastic or metal, but it should be resistant to warping, bending, corrosion, and heat. The dicing tape (also referred to as a wafer film) is just a PVC sheet with synthetic adhesive on one side to hold both the wafer frame and the wafer. The adhesive tape on which the wafer is mounted ensures that the individual dice remain firmly in place during dicing (as the process of cutting the wafer). The following concerns must be prevented: wafer cracking or breakage, bubble trapping on the adhesive side of the tape, scratches on the active side of the wafer, and non-uniform tape tension which can result in tape wrinkles. DMT 243 – Chapter 3 M.Nuzaihan

Die Preparation General Semiconductor Packaging Process Flow Wafer mount station Wafer film for mounting Wafer glued on blue tape and cut into pieces Wafer mounted on the wafer frames DMT 243 – Chapter 3 M.Nuzaihan

General Semiconductor Packaging Process Flow Wafer Saw or Die cutting or dicing is a process of reducing a wafer containing multiple identical integrated circuits to dice each containing one of those circuits. During this process, a wafer with up to thousands of circuits is cut into individual pieces, each called a die. In between the functional parts of the circuits, a thin non-functional spacing is foreseen where a saw can safely cut the wafer without damaging the circuit. This spacing is called the scribe or saw street. The width of the scribe is very small, typically around 100 μm. A very thin and accurate saw is therefore needed to cut the wafer into pieces. Usually the dicing is performed with a water-cooled circular saw with diamond-tipped teeth. DMT 243 – Chapter 3 M.Nuzaihan

General Semiconductor Packaging Process Flow DMT 243 – Chapter 3 M.Nuzaihan

General Semiconductor Packaging Process Flow Wafer saw follows wafer mounting is the step that actually cuts the wafer into individual dice for assembly in IC packages The wafer saw process consists of the following steps: The frame-mounted wafer is automatically aligned into position for cutting; The wafer is then cut through its thickness according to the programmed die dimensions using a resin-bonded diamond wheel rotating at a very high rpm; and The wafer goes through a cleaning process using high pressure DI water sprayed on the rotating work piece and then dried by air-blowing. DMT 243 – Chapter 3 M.Nuzaihan

General Semiconductor Packaging Process Flow Important Parameters Important parameters for consideration during wafer saw include the following: cut mode (direction and manner of cutting), feed speed (speed at which the wafer is being introduced to the blade), spindle rev (speed of revolution of the cutting wheel), blade height, and cutting water flow. Important parameters for the washing step include the following: wash time, wash rpm, DI water pressure, dry time, dry rpm, temperature and air flowrate. DMT 243 – Chapter 3 M.Nuzaihan

Lateral Crack on laminated wafer Saw like chipping on bare die General Semiconductor Packaging Process Flow Wafer Saw Failures Lateral Crack on laminated wafer Saw like chipping on bare die DMT 243 – Chapter 3 M.Nuzaihan

General Semiconductor Packaging Process Flow Wafer Saw Failures Crack at the side Minor crack line DMT 243 – Chapter 3 M.Nuzaihan

General Semiconductor Packaging Process Flow After Semester Break Wafer Backgrinding, Die Preparation, Die Attach, Wire Bonding, Die Overcoat, Lab Session + Assignment 2 Molding, Sealing, Marking, DTFS, Lead Finish, Electrical Testing, Tape & Reel, Dry Packing, Boxing and Labeling. TEST 1 – 20/02/2008 Chapter 1 & Chapter 2 & Chapter 3 DMT 243 – Chapter 3 M.Nuzaihan