Quality Assurance of IC Memories 1. Views on Quality and Reliability Hitachi products should always meet individual users' purposes and required quality levels, maintaining satisfactory performance for general applications. Hitachi works continuously to assure high reliability standards for our IC memories in actual usage. To meet user needs and to cover expanding applications, Hitachi has defined these goals: 1. 2. 3. 4. Establish reliability by design during new product development. Establish quality at all steps in the manufacturing process. Strengthen the inspection process at all points. Improve product quality based on user data. Furthermore, to reach the highest quality and performance levels, development and production teams cooperate very closely with Hitachi research laboratories. All these methods together make it possible for Hitachi to meet and exceed user requirements. 2. Reliability Design of Semiconductor Devices 2.1 Reliability Targets The establishment of reliability targets is important in manufacturing and marketing, as well as in determining function and price. Practically, the reliability targets cannot be determined from failure rates produced by any single common test condition; they are based on many factors such as equipment characteristics, target system purposes, derating applied during design, operating conditions, and maintenance requirements. 2.2 Reliability Design Factors Timely analysis and execution are essential to achieve performance based on reliability targets. The primary design items of interest are design standardization, device process and structural design, design review, and reliability testing. 1. Design standardization Design standardization requires the establishment of design rules and the specification of parts, materials, and processes. When design rules are being established for the circuit, cell, and layout designs, critical quality and reliability features should also be examined. By doing this effectively, the use of stand-ardized processes or materials, even in newly developed products, should generate much higher reliability (with the possible exception of special requirements or functions). 2. Device process and structural design It is important during device design to consider the total balance of process design, structural design, and circuit and layout design. Especially in the case of applying new processes or new materials, at Hitachi we study the technology in depth prior to any detailed device development. Quality Assurance of IC Memorie 3. Reliability testing by test site The test site is also called the test pattern. It is a useful method for evaluating the reliability of complex ICs and complicated functions. a. The purposes for the test site are: To make clear definitions about mental failure modes To analyze relationships between failure modes and manufacturing processes and/or conditions To analyze failure mechanisms To establish QC points in manufacturing b. The effects of the test site are: Evaluation of common fundamental failure modes and failure mechanisms Determination of predominant failure modes, and comparisons with field experiences Analysis of relationships between failure causes and manufacturing factors Simplification of testing 2.3 Design Review Design review is a method to systematically confirm whether or not a design satisfies the performance required by users, whether it meets all specifications, and whether the technical items accumulated in test data and application data are effectively utilized. In addition, from the standpoint of comparisons to competitive products, a major focus of the design review is to insure the quality and reliability of the product. At Hitachi, the design review is perform-ed as a part of new product development, and when changing existing products. The following items are considered in design review. 1. Describing the product based on specified design documents. 2. Planning and executing each product function and program (such as calculations) by considering the product and its documentation from the standpoint of each participant. Experiments and further investigations are indicated if any results are not exactly as expected. 3. Determining the contents and methods of reliability testing based on design documents and drawings. 4. Checking manufacturing process ability to achieve design goals. 5. Arranging preparations for production. 6. Planning and executing each product function and program of all design changes proposed by individual specialists. Generating tests, experiments, and calculations as needed to confirm the results of each design change. 7. Refering to past performance and failure experiences with similar devices. Confirming the prevention of any repetition of such experience, and planning and executing a test program to prove this level of performance. At Hitachi, design reviews including these steps of analysis and decision are made using individual check lists according to each objective. 2 Quality Assurance of IC Memorie 3. Quality Assurance System of Semiconductor Devices 3.1 Activity of Quality Assurance At Hitachi, these are the general purposes of quality assurance: 1. Problems are resolved within each step, so that by the final stage of production even very small potential failure factors will be removed. 2. Information developed at every step is used in other steps, as indicated, to improve quality in the entire production sequence, and therefore achieve satisfactory levels of reliability and performance. 3.2 Qualification For maximum product quality and reliability, qualification tests are done at each stage of trial production and mass production, based on design reliability as described in Section 2 "Reliability Design of Semiconductor Devices." These are the purposes of qualification at Hitachi. 1. 2. 3. 4. 5. Qualify the product objectively from a customer standpoint (as by a third party). Consider the failure experiences and data provided by customers. Qualify every change in design and process. Qualify, with special emphasis, all final choices of parts, materials, and processes. Establish control points within the production procedure by considering the process ability and factors of manufacturing variance. Figure 1 shows the general outline of design qualification at Hitachi. 3 Quality Assurance of IC Memorie Step Contents Target Specification Design Trial Production Design Review Materials, Parts Approval Characteristics Approval Quality Approval (1) Quality Approval (2) Characteristics of Material and Parts Appearance Dimension Heat Resistance Mechanical Electrical Others Confirmation of Characteristics and Reliability of Materials and Parts Electrical Characteristics Function Voltage Current Temperature Others Appearance. Dimension Confirmation of Target Sepc. Mainly about Electrical Characteristics Reliability Test Life Test Thermal Stress Moisture Resistance Mechanical Stress Others Confirmation of Quality and Reliability in Design Reliability Test Process Check same as Quality Approval (1) Confirmation of Quality and Reliability in Mass Production Mass Production Figure 1 Flowchart of Device Design Qualification 4 Purpose Quality Assurance of IC Memorie 3.3 Quality and Reliability Control in Mass Production In mass production, quality is the functional responsibility of each department, primarily as defined by the manufacturing department and the quality assurance department. The total function flow is shown in Figure 2. Process Quality Control Method Inspection of Material and Parts Inspection on Material and Parts for Semiconductor Devices Lot Sampling, Confirmation of Quality Level Manufacturing Manufacturing Equipment, Environment, Sub-material, Worker Control Confirmation of Quality Level Inner Process Quality Control Lot Sampling, Confirmation of Quality Level 100% Inspection on Visual and Electrical Characteristics Testing, Inspection Sampling Inspection on Visual and Electrical Characteristics Lot Sampling Reliability Test Confirmation of Quality Level, Lot Sampling Material, Parts Material, Parts Screening 100% Inspection Products Products Inspection Warehouse Shipment Customer Quality Information Claim Field Experience General Quality Information Feedback of Information Figure 2 Flowchart of Quality Control in the Manufacturing Process 5 Quality Assurance of IC Memorie 3.3.1 Quality Control on Parts and Materials With the tendency toward higher performance and higher reliability of devices, the quality control of parts and materials becomes more important. Items such as crystals, lead frames, fine wire for wire bonding, and packages and materials required in manufacturing processes like mask patterns and chemicals are all subject to inspection and control. Besides the qualification of parts and materials as stated in Section 3.2, the quality control of parts and materials begins at incoming inspection, which is performed based on purchase specifications, drawings and (mainly) sampling tests based on MIL-STD-105D. Other activities related to quality assurance are as follows. 1. Technology meetings with vendors. 2. Approval and guidance of vendors. 3. Analysis and test of physical chemistry. 6 Quality Assurance of IC Memorie The typical check points of parts and materials are shown in Table 1. Table 1 Quality Control Check Points of Parts and Materials (example) Material parts Important control items Check points Water Appearance Dimension Sheet resistance Defect density Crystal axis Damage and contamination on surface Flatness Resistance Defect numbers Mask Appearance Dimension Resistoration Gradation Defect numbers, scratches Dimension level Appearance Dimension Purity Elongation ratio Contamination, scratches, bend, twist Appearance Dimension Processing accuracy Plating Mounting characteristics Contamination, scratches Dimension level Appearance Dimension Leakage resistance Plating Mounting characteristics Electrical characteristics Mechanical strength Contamination, scratches Dimension level Airtightness Bondability, solderability Heat resistance Composition Electrical charactristics Thermal characteristics Molding performance Mounting characteristics Characteristics of plastic material Fine wire for wire bonding Frame Ceramic package Plastic Uniformity of gradation Purity level Mechanical strength Bondability, solderability Heat resistance Mechnical strength Molding performance Mounting characteristics 7 Quality Assurance of IC Memorie 3.3.2 Process Quality Control Control of process quality is extremely significant in the overall process of device quality assurance. Quality control functions at every stage of production are described below. Figure 3 lists specific process quality control factors. 1. Quality control of products in every stage of production Potential device failure factors should be removed as soon as possible in the manufacturing process. To do this, check points are set up within each process to prevent products exhibiting failure factors to move onto any following process. Especially for devices designed for high reliability, manufacturing lines are rigidly monitored to control process quality. Additionally, we perform very stringent checks on some processes and/or lots, and even 100% inspections in certain critical processes to remove potentially failing items related to unavoidable manufacturing variances. Screening based on high temperature aging or temperature cycling are also part of quality assurance procedures. Controlling quality during processing includes these items: a. b. c. d. e. f. g. Control of conditions of equipment and workers Sampling test of uncompleted products Proposal and implementation of improvements in working conditions Continuous worker education Maintenance and improvement of yields Identification of quality problems, and implementation of countermeasures to them Communication of quality-related information 2. Quality control of manufacturing facilities and measuring equipment Manufacturing facilities have been developed to answer the need for higher device performance and automated production. It is also important to define and accurately measure quality and reliability. At Hitachi, automated manufacturing is used to reduce manufacturing variances. The operation of high perfrmance equipment requires automated control to function properly. Maintenance inspections are carried out daily to ensure proper quality control, and in some instances at other more frequent intervals according to specifications, at every check point. The adjustment and maintenance of measuring equipment is done according to specifications and past experience, and is vigorously monitored to maintain and improve the quality of our products. 3. Quality control of the manufacturing environment and submaterial Final quality and reliability of devices are especially affected by manufacturing processes. We therefore thoroughly control factors of the manufacturing environment, such as gases or pure water. Dust control is critical to achieve higher integration and higher device reliability. To maintain and improve the cleanliness of the manufacturing site, we take great care to keep buildings, facilities, air-conditioning systems, materials, clothes, and all possible elements associated with production as clean and dust-free as we can. We extend this effort to periodically check the ambient air in the manufacturing facility for floating dust, and we check for any minute amounts which might have accumulated on the floor, other surfaces, or on any equipment. 8 Quality Assurance of IC Memorie Process Control Point Purpose of Control Purchase of Material Wafer Wafer Surface Oxidation Appearance, Thickness of Oxide Film Pinhole, Scratch Dimension, Appearance Dimension Level Check of Photo Resist Diffusion Depth, Sheet Resistance Gate Width Characteristics of Oxide Film Breakdown Voltage Diffusion Status Photo Resist Inspection on Photo Resist PQC Level Check Diffusion Diffusion Scratch, Removal of Crystal Defect Wafer Assurance of Resistance Oxidation Inspection on Surface Oxidation Photo Resist Characteristics, Appearance Inspection on Diffusion PQC Level Check Evaporation Thickness of Vapor Film, Scratch, Contamination Wafer Inspection Wafer Thickness, VTH Characteristics Inspection on Chip Electrical Characteristics Chip Electrical Characteristics Evaporation Control of Basic Parameters (VTH, etc) Cleaness of surface Prior Check of VIH Breakdown Voltage Check Assurance of Standard Thickness Inspection on Evaporation PQC Level Check Chip Scribe Prevention of Crack, Quality Assurance of Scribe Appearance of Chip Inspection on Chip Appearance PQC Lot Judgement Frame Assembling Assembling PQC Level Check Appearance after Chip Bonding Appearance after Wire Bonding Pull Strength, Compresion Width, Shear Strength Quality Check of Chip Bonding Quality Check of Wire Bonding Prevention of Open and Short Appearance after Assembling Inspection after Assembling PQC Lot Judgement Package Sealing Sealing PQC Level Check Final Electrical Inspection Failure Analysis Marking Appearance after Sealing Outline, Dimension Marking Strength Guarantee of Appearance and Dimension Analysis of Failures, Failure Mode, Mechanism Feedback of Analysis Information Appearance Inspection Sampling Inspection on Products Receiving Shipment Figure 3 Example of Process Quality Control Factors 9 Quality Assurance of IC Memorie 3.3.3 Final Tests and Reliability Assurance Tests 1. Final tests Lot inspection is done by the quality assurance department for products already passed in 100% testing during the manufacturing process. Although 100% performance is expected, sample lot inspection is also carried out to prevent any possible accidental mixture of failed products with regular, satisfactory devices. The extra lot inspection not only confirms that all products meet all user requirements, but considers any other potential factors. Our lot inspection is based on MIL-STD-105D. 2. Reliability assurance tests To assure reliability, appropriate tests are performed periodically on each manufacturing lot if the user requires such a high level of examination. 10 Quality Assurance of IC Memorie Customer Claim (Failures, Information) Sales Dept. Sales Engineering Dept. Quality Assurance Dept. Manufacturing Dept. Failure Analysis Design Dept. Countermeasure Execution of Countermeasure Report Quality Assurance Dept. Follow-up and Confirmation of Countermeasure Execution Report Sales Dept. Sales Engineering Dept. Reply Customer Figure 4 Process Flowchart for Customer-Reported Failure 11