Jan Lange

A multistage mathematical programming based scheduling approach for the photolithography area in semiconductor manufacturing

Facilities for wafer fabrication are one of the most complex manufacturing systems. Typically, the bottleneck of such facilities is the photolithography area because of its highly expensive tools and complex resource constraints. In this research, a multistage mixed integer programming based optimization approach for planning of such an area is presented. Thereby, several existing process constraints like equipment dedications, resist allocation, vertical dedications, mask availability are taken into account on the basis of different granularity levels. Altogether eleven different optimization models are presented within four different decomposition stages. Thereby, objected goals are the maximization of throughput, the minimization of setup costs and a balancing of machine utilization. On the basis of real manufacturing data the benefit of the proposed approach is evaluated within a first prototype.

Time matters - acute stress response and glucocorticoid sensitivity in early multiple sclerosis.

OBJECTIVE Psychosocial stress has frequently been associated with disease activity and acute exacerbations in multiple sclerosis (MS). Despite this well established finding, strikingly little is known about the acute hypothalamic-pituitary-adrenal (HPA) and sympathetic-adrenal-medullary (SAM) stress response in MS. METHODS Twenty-six early relapsing-remitting MS (RRMS) patients and seventeen age- and sex-matched healthy control subjects (CS) took part in the Trier Social Stress Test (TSST), a well validated psycho-social laboratory stress protocol. Repeated blood samples were analyzed for stress-related cortisol and catecholamine levels as well as for glucocorticoid sensitivity (GCS) of target immune cells. Chronic and acute stress appraisals were assessed by self-report measures. RESULTS RRMS patients and CS did not differ in stress-related cortisol/catecholamine levels, GCS or stress appraisal in response to the TSST. However, cortisol release as well as GCS was strongly correlated with time since diagnosis but not with neurological disability. Patients with shorter disease duration (2-12 months) expressed a significantly higher cortisol stress response while MS patients with longer disease duration (14-36 months) showed a significantly diminished HPA response as well as lower post-stress GCS. DISCUSSION There is evidence for a time-dependent variability in the HPA stress system with an increased cortisol stress response in the first year after diagnosis along with a more blunted HPA stress response and a diminished GCS in subsequent disease stages. Data underscore the highly dynamic nature of HPA axis regulation in the MS disease process, which could possibly relate to compensatory mechanisms within a cytokine-HPA axis feedback circuit model.

Scheduling preventive maintenance tasks with synchronization constraints for human resources by a CP modeling approach

This paper presents an approach for scheduling different types of preventive maintenances (PMs) for a work center of a semiconductor manufacturing facility. The PM scheduling problem includes time-dependent synchronization constraints and is implemented in a constraint programming model. A mix of periodic and workload-specific maintenances is scheduled considering the synchronization to available engineers which have individual shift schedules and skills that define the range of feasible maintenances. This also comprises maintenances having process durations covering multiple shifts, which requires a continuous availability of sufficiently skilled engineers. Additionally to the PMs, also handling and maintaining of unscheduled downs is considered in the model. Multiple objectives are investigated and used for optimization and tested on realistic data. To compare the results an additional simulation model is built up.

Education and Genetic Risk Modulate Hippocampal Structure in Alzheimer’s Disease

Genetic and environmental protective factors and risks modulate brain structure and function in neurodegenerative diseases and their preclinical stages. We wanted to investigate whether the years of formal education, a proxy measure for cognitive reserve, would influence hippocampal structure in Alzheimer’s disease patients, and whether apolipoprotein Eε4 (APOE4) carrier status and a first-degree family history of the disease would change a possible association. Fifty-eight Alzheimer’s disease patients underwent 3T magnetic resonance imaging. We applied a cortical unfolding approach to investigate individual subregions of the medial temporal lobe. Among patients homozygous for the APOE4 genotype or carrying both APOE4 and family history risks, lower education was associated with a thinner cortex in multiple medial temporal regions, including the hippocampus. Our data suggest that the years of formal education and genetic risks interact in their influence on hippocampal structure in Alzheimer’s disease patients.

Improving flow line scheduling by upstream mixed integer resource allocation in a wafer test facility

The effort for scheduling real manufacturing systems is generally very high for mathematical as well as for simulation-based methods. Combining both methods is the key for solving complex scheduling problems. The paper introduces a special approach, where at first a static resource allocation problem is solved by mixed integer programming (MIP). Based on the resulting reduced dedication matrices, feasible schedules are then generated by a discrete event simulation (DES). Possible applications can be found in many parts of the semiconductor manufacturing process, for example in the wafer test. The investigated wafer test consists of two pronounced bottlenecks; each of it is formed as a workcenter with its own dedication matrix. After testing the method with practice oriented benchmarks, the benefits of the approach are shown on data derived directly from the semiconductor manufacturing process.

Scheduling maintenance tasks with time-dependent synchronization constraints by a CP modeling approach

Ensuring a high uptime for manufacturing machines is crucial for efficient and cost-effective production. In opposite, preventive maintenance tasks (PMs) are necessary to assure the reliability of manufacturing processes. This also prevents serious and unpredictable machine crashes, which affect uptime and process scheduling. However, PM tasks themselves lower the uptime and are to be smartly scheduled within their given domain considering some requirements as ensuring the availability of a sufficient number of qualified engineers for the concerning period. This work investigates a PM scheduling problem with time-dependent synchronization constraints for a lithography work center. For this, a constraint programming (CP) modeling approach including decomposition is used. Multiple objectives are considered. For example, the minimization of crew backup violations or scheduling PMs according to the work in process (WIP) for embedding upcoming PMs smoothly into the system work load. This minimizes negative effects on throughput and tardiness.

Introducing the virtual time based flow principle in a high-mix low-volume wafer test facility and exploring the behavior of its key performance indicators

In modern semiconductor manufacturing and primarily in high-mix-low-volume facilities it is increasingly important to ensure throughput and machine utilization requirements are met while satisfying tight goals in object tardiness at the same time. This is especially a challenge for the field of wafer test with its natural fluctuations and uncertainties of test times. A further important objective is the lowering of the work in process (WIP) for the purposes of minimizing costs held in the system and improving production predictability. For this, the Virtual Time Based Flow Principle (VTBFP) - a partly synchronized control strategy - is investigated in this paper. Tests are performed on a complex system, which is close to reality. As a result it is shown the benefits but also the limitations of the VTBFP approach.

Cognitive impairment and medial temporal lobe structure in young adults with a depressive episode

BACKGROUND Cognitive deficits are common in patients with a depressive episode although the predictors for their development and severity remain elusive. We investigated whether subjective and objective cognitive impairment in young depressed adults would be associated with cortical thinning in medial temporal subregions. METHODS High-resolution magnetic resonance imaging, cortical unfolding data analysis, and comprehensive assessments of subjective and objective cognitive abilities were performed on 27 young patients with a depressive episode (mean age: 29.0 ± 5.8 years) and 23 older participants without a history of a depressive disorder but amnestic mild cognitive impairment (68.5 ± 6.6 years) or normal cognition (65.2 ± 8.7 years). RESULTS Thickness reductions in parahippocampal, perirhinal and fusiform cortices were associated with subjective memory deficits only among young patients with a depressive episode and a measurable cognitive impairment. LIMITATIONS Long-term longitudinal data would be desirable to determine the trajectories of cognitive impairment associated with depression in patients with or without cortical structure changes. CONCLUSIONS The presence of clinically significant cognitive deficits in young people with a depressive episode may identify a patient population with extrahippocampal cortical thinning.

Family History of Alzheimer’s Disease and Subjective Memory Performance

People with a first-degree family history of Alzheimer’s disease are at an increased risk of developing dementia. Subjective memory impairment among individuals with no measurable cognitive deficits may also indicate elevated dementia risk. It remains unclear whether nondemented people with a positive family history of Alzheimer’s disease are more likely to experience cognitive deficits and whether such an association reflects underlying neuropathology. We therefore investigated subjective memory impairment and hippocampal cortical thickness in 40 healthy older adults and 35 patients with amnestic mild cognitive impairment. We found greater subjective memory impairment and left hemispheric hippocampal cortical thinning associated with a first-degree family history of Alzheimer’s disease in healthy older adults. This suggests that subjective memory impairment could reflect preclinical stage neurodegeneration among individuals with the family history risk factor.

Extrinsic and Intrinsic Help-Seeking Motivation in the Assessment of Cognitive Decline:

Diagnostic assessments for dementia include the evaluation of subjective memory impairment, dementia worries, or depressive symptoms. Data on the predictive value of these factors remain unclear, and varying help-seeking behavior may contribute to this finding. We investigate whether differentiating help-seeking motivation from other psychological factors associated with cognitive impairment would enhance the prediction of diagnostic outcomes in a memory clinic. We obtained information on help-seeking motivation from 171 patients who underwent routine diagnostic assessments. Utilizing a discriminant correspondence analysis, our results indicate that extrinsic motivation increases the likelihood of receiving a dementia diagnosis, whereas depression or the duration of deficits carries discriminatory information to further guide the differentiation of prodromal dementia. Recognizing motivational aspects of help-seeking behavior can complement the clinical evaluation of cognitive performance.