Lecture 28: Process and device evaluation Contents 1 Introduction 1 2 Electrical measurements 3 3 Thickness measurement 5 3.1 White light interferometry . .5 3.2 Spectrophotometer . .7 3.3 Ellipsometry . .9 3.4 Profilometry . 10 4 Contamination and defect detection 11 1 Introduction Integrated circuit manufacturing is similar to an assembly line manufacturing process, with the wafers moving from one `station' (process step) to the next. The starting point is blank single crystal Si wafer and the various processes to the finished product are carried out in a single fab. There are more than 500 individual steps in current IC manufacturing and the total time for making a single device can be as long as a month. The devices are usually tested (electrical testing) after the process is complete, to check functionality. At this point, it is very hard to trouble shoot the origin of any `killer' defects i.e. a defect that can destroy or significantly reduce functionality. Thus, process control tests are done throughout the process. The disadvantage is that this can can also increase manufacturing time and hence cost. Process or device evaluation is critical to ensure that the chips (or dies) are `working correctly' after each step, without significantly altering total process time and cost. Steps in CMOS manufacturing are shown in figure 1. There 1 MM5017: Electronic materials, devices, and fabrication Figure 1: Various steps in CMOS manufacuturing. The process monitoring steps are marked with M. More the number of monitoring steps, longer is the total process time but error detection and correction will be quicker. Adapted from Semiconductor manufacturing and process control - May and Spanos. are a total of 30 steps in this process, with 14 additional evaluation steps inserted along the process flow. Thus, the overall process involves around 45 steps. Increasing the number of evaluation steps will increase process efficiency but overall time and cost will also increase. On the other hand, less number of evaluation steps can keep cost low but trouble shooting could be difficult and could lead to wastage of time and increased cost, if the process has to be tweaked. The process monitoring is carried out on three types of wafers that are used in the fab 1. Directly on the process wafer 2. On test wafers, which are processed along with the product. There are specific test dies on the process wafer that are evaluated. 3. On blank wafers, which are used to monitor a particular process. These wafers are process specific and do not go to other processes in the manufacturing. These are also called test/monitor wafers. This is apart from process evaluation on the finished wafers. The type of monitoring depends on the nature of the process. For batch processes, like furnace operations, multiple wafers are processed together. 2 MM5017: Electronic materials, devices, and fabrication In such cases, monitoring is carried out on test wafers that are processed to- gether with the process wafers. For serial processes, like etching or polishing, either the process wafers have to be directly monitored or test wafers have to be inserted between runs of process wafers and then the process is monitored using these wafers. Metrology refers to the measurement of physical surface features. In fabri- cation, some examples of metrology are, measurement of pattern widths, film depth, defect locations, and pattern registry errors (in lithography). Metrol- ogy should provide information on whether the wafers are `good enough' to move to the next step. Hence, the evaluation should be fast and conclu- sive. Usually, for every step in the flow, a process window is defined. This provides theacceptable range of parameters, for that process. If process eval- uation can confirm that the wafers fall within this range, then the wafers can move to the next step in the process flow. For a furnace operation (to grow oxides on Si), the parameters are usually oxide thickness and defect density. These are usually measured on test wafers (since furnace process is a batch process). The product wafers will be `waiting' till the metrology is complete and acceptable. Then the process wafers can move to the next step. There are three main measurement types at each step 1. Electrical testing 2. Physical parameter measurement 3. Defect measurement (contamination) As device integration increases and size decreases, these measurements will have to made on smaller regions. This can pose lots of technological chal- lenges, especially if the evaluation has to be fast and conclusive. 2 Electrical measurements Typically, electrical measurements refer to measurement of resistance of the component. Resistance or resistivity can be used to obtain a wide vari- ety of information on the fabrication process. Addition of dopants to the wafer alters electrical conductivity, which can be easily tracked by resistance measurement. Similarly, formation of metal-semiconductor junctions can be probed by I-V and capacitance measurements. These measurements are fast and provide conclusive information on the process and are widely used in the fab. There are also electrical measurements done on the components after the fabrication is done. These tests are more involved and probe a variety 3 MM5017: Electronic materials, devices, and fabrication Figure 2: (a) - (c) Some simple e-test structures for interconnects. These are used for probing effect of defects on the electrical properties of the inter- connects. Mostly resistance and capacitance measurements are carried out. Adapted from Semiconductor manufacturing and process control - May and Spanos. of properties than simply resistance or I-V behavior. There are specially de- signed test structures, which are incorporated in the dies, for these electrical measurements. An example of test structures for interconnects is shown in figure 2. The most widely used setup for resistance measurement is the four-point probe technique. The schematic of the setup is shown in figure 3. The inner probes measure the voltage difference while the outer probes are used to pass a known current. By using a four probe technique it is possible to eliminate any resistance effect due to the circuit (wires). For the arrangement shown in figure 3, the resistivity is related to voltage (V ) and current (I) by 2πsV ρ = for t s I (1) πt V ρ = for t s ln2 I The first condition in equation 1 refers to a thin film case or a doped region in a semiconductor, while the second condition corresponds to a shallow junction. When resistance is measured on a thin doped region (which has a much lower resistance than the wafer) an electrical quantity called sheet resistance, Rs, is preferred. The quantity has units of ohms per square (Ω per ). The sheet resistance is give by π V V R = = 4:53 (2) s ln2 I I For metal films on semiconductors or insulators, the sheet resistance can be used to measure thickness since resistivity of the metal is known. 4 MM5017: Electronic materials, devices, and fabrication Figure 3: Four point probe setup for measuring resistance, used on a pn junction. The inner probes measure the potential difference while current is passed through the outer probes. The probe can be used to measure junc- tion depth by correlating with the resistance. Adapted from Semiconductor manufacturing and process control - May and Spanos. For pn junctions, resistance measurements can be used to characterize the depth of the junction. This is done by fabricating a bevel structure on the wafer and measuring the resistance as a function of depth. This is shown in figure 4. Since resistance is related to dopant concentration, this can be used to measure the junction depth. A more reliable technique for dopant concentration as a function of depth, is secondary ion mass spectroscopy (SIMS) but it is time consuming. SIMS has a sub-nm resolution and can also provide chemical information about the dopants and other impurities in the wafer. 3 Thickness measurement 3.1 White light interferometry Measurement of thickness of thin films is an important step in process eval- uation. A variety of thin films are grown, oxides, nitrides, metals, Si. Thin layers (less than 500 nm) of oxide and nitrides have a natural color that depends on the thickness. This is due to interference of light reflected from the top and bottom of the film, called white light interference. This is shown in figure 5. The interference color depends on 1. Index of refraction i.e. film type 2. Viewing angle 5 MM5017: Electronic materials, devices, and fabrication Figure 4: (a) Bevel structure for measurement of resistance as a function of depth. The bevel has to be fabricated on the wafer and hence, this is a destructive measurement. (a) The resistance is converted and plotted as dopant concentration vs. depth. The dotted lines indicate the junction. Adapted from Microchip fabrication - Peter van Zant. Figure 5: (a) White light interference from a thin oxide film. (b) Changing the thickness changes the color since the path length changes. Adapted from Microchip fabrication - Peter van Zant. 6 MM5017: Electronic materials, devices, and fabrication 3. Film thickness Usually, overhead viewing angle is used. The colors for different thicknesses of silicon oxide are tabulated in a color chart in table 1. Using the color Table 1: Color chart for silica, as a function of thickness. Adapted from Microchip fabrication - Peter van Zant. Film thickness, nm Color (perpendicular illumination, fluorescent light) Order I 50 Tan 75 Brown 100 Violet 150 Light blue 200 Light gold or metallic yellow 250 Orange 300 Blue to violet blue 350 Light green Order II 400 Yellow 450 Violet red 480 Blue violet 500 Blue chart, it is possible to estimate the thickness of the film. This can be used as a rough guide to calculate the time required to grow a specific oxide or nitride layer.